Optimal foraging when regulating intake of multiple nutrients

Stoichiometric theory in optimal foraging strategy

Understanding how organisms make choices about what to eat is a fascinating puzzle explored in this study, which employs stoichiometric modeling and optimal foraging principles. The research delves into the intricate balance of nutrient intake with foraging strategies, investigating quality and quantity-based food selection through mathematical models. The stoichiometric models in this study, encompassing producers and a grazer, unveils the dynamics of decision-making processes, introducing fixed and variable energetic foraging costs. Analysis reveals cell quota-dependent predation behaviors, elucidating biological phenomena such as "compensatory foraging behaviors" and the "stoichiometric extinction effect". The Marginal Value Theorem quantifies food selection, highlighting the profitability of prey items and emphasizing its role in optimizing foraging strategies in predator-prey dynamics. The environmental factors like light and nutrient availability prove pivotal in shaping optimal foraging strategies, with numerical results from a multi-species model contributing to a comprehensive understanding of the intricate interplay between organisms and their environment.

How adolescents learn to build social bonds: A developmental computational account of social explore-exploit decision-making

Building social bonds is a critical task of adolescence that affords opportunities for learning, identity formation, and social support. Failing to develop close relationships in adolescence hinders adult interpersonal functioning and contributes to problems such as loneliness and depression. During adolescence, increased reward sensitivity and greater social flexibility both contribute to healthy social development, yet we lack a clear theory of how these processes interact to support social functioning. Here, we propose synthesizing these two literatures using a computational reinforcement learning framework that recasts how adolescents pursue and learn from social rewards as a social explore-exploit problem. To become socially skilled, adolescents must balance both their efforts to form individual bonds within specific groups and manage memberships across multiple groups to maximize access to social resources. We draw on insights from sociological studies on social capital in collective networks and neurocognitive research on foraging and cooperation to describe the social explore-exploit dilemma faced by adolescents navigating a modern world with increasing access to diverse resources and group memberships. Our account provides important new directions for examining the dynamics of adolescent behavior in social groups and understanding how social value computations can support positive relationships into adulthood.

Macronutrient regulation in nymphs of the two-spotted cricket, Gryllus bimaculatus (Orthoptera: Gryllidae)

Crickets have been extensively studied in recent insect nutritional research, but it remains largely unexplored how they balance the intake of multiple nutrients. Here, we used the nutritional geometry framework to examine the behavioural and physiological regulation of dietary protein and carbohydrate in nymphs of the two-spotted cricket, Gryllus bimaculatus (Orthoptera: Gryllidae). Growth, intake, utilization efficiencies, and body composition were measured from the eighth instar nymphs that received either food pairs or single foods with differing protein and carbohydrate content. When food choices were available, crickets preferentially selected a carbohydrate-biased protein:carbohydrate (P:C) ratio of 1:1.74. During this nutrient selection, carbohydrate intake was more tightly regulated than protein intake. When confined to nutritionally imbalanced foods, crickets adopted a nutrient balancing strategy that maximized the nutrient intake regardless of the nutrient imbalance, reflecting their omnivorous feeding habit. Intake was significantly reduced when crickets were confined to the most carbohydrate-biased food (P:C = 1:5). When nutrients were ingested in excess of the requirements, the post-ingestive utilization efficiencies of these nutrients were down-regulated, thereby buffering the impacts of nutrient imbalances on body nutrient composition. Crickets reared on the most carbohydrate-biased food (P:C = 1:5) suffered delayed development and reduced growth. Our data provide the most accurate description of nutrient regulation in G. bimaculatus and lay the foundation for further nutritional research in this omnivorous insect.

Increased intake of tree forage by moose is associated with intake of crops rich in nonstructural carbohydrates

Animals representing a wide range of taxonomic groups are known to select specific food combinations to achieve a nutritionally balanced diet. The nutrient balancing hypothesis suggests that, when given the opportunity, animals select foods to achieve a particular target nutrient balance, and that balancing occurs between meals and between days. For wild ruminants who inhabit landscapes dominated by human land use, nutritionally imbalanced diets can result from ingesting agricultural crops rich in starch and sugar (nonstructural carbohydrates [NCs]), which can be provided to them by people as supplementary feeds. Here, we test the nutrient balancing hypothesis by assessing potential effects that the ingestion of such crops by Alces alces (moose) may have on forage intake. We predicted that moose compensate for an imbalanced intake of excess NC by selecting tree forage with macro-nutritional content better suited for their rumen microbiome during wintertime. We applied DNA metabarcoding to identify plants in fecal and rumen content from the same moose during winter in Sweden. We found that the concentration of NC-rich crops in feces predicted the presence of Picea abies (Norway spruce) in rumen samples. The finding is consistent with the prediction that moose use tree forage as a nutritionally complementary resource to balance their intake of NC-rich foods, and that they ingested P. abies in particular (normally a forage rarely eaten by moose) because it was the most readily available tree. Our finding sheds new light on the foraging behavior of a model species in herbivore ecology, and on how habitat alterations by humans may change the behavior of wildlife.

Nutritional ecology of a prototypical generalist predator, the red fox (Vulpes vulpes)

Generalist species, which exploit a wide range of food resources, are expected to be able to combine available resources as to attain their specific macronutrient ratio (percentage of caloric intake of protein, lipids and carbohydrates). Among mammalian predators, the red fox Vulpes vulpes is a widespread, opportunistic forager: its diet has been largely studied, outlining wide variation according to geographic and climatic factors. We aimed to check if, throughout the species' European range, diets vary widely in macronutrient composition or foxes can combine complementary foods to gain the same nutrient intake. First, we assessed fox's intake target in the framework of nutritional geometry. Secondly, we aimed to highlight the effects of unbalanced diets on fox density, which was assumed as a proxy for Darwinian fitness, as assessed in five areas of the western Italian Alps. Unexpectedly, the target macronutrient ratio of the fox (52.4% protein-, 38.7% lipid- and 8.9% carbohydrate energy) was consistent with that of hypercarnivores, such as wolves and felids, except for carbohydrate intakes in urban and rural habitats. The inverse relation between density and the deviation of observed macronutrient ratios from the intake target suggests that fox capability of surviving in a wide range of habitats may not be exempt from fitness costs and that nutrient availability should be regarded among the biotic factors affecting animal abundance and distribution.

'Dust you shall eat': The complex nutritional and functional considerations underlying a simple diet

Animals assimilate macronutrients and mineral nutrients in specific quantities and ratios to maximise fitness. To achieve this, animals must ingest different foods that contain the needed nutrients or facilitate the digestion of those nutrients. We explored how these multidimensional considerations affect the desert isopods (Hemilepistus reaumuri) curious food selection, using field and laboratory experiments. Wild isopods consumed three-fold more macronutrient-poor biological soil crust (BSC) than plant litter. Isopods tightly regulated macronutrient and calcium intake, but not phosphorus when eating the two natural foods and when artificial calcium and phosphorus sources substituted the BSC. Despite the equivalent calcium ingestion, isopods performed better when eating BSC compared to artificial foods. Isopods that consumed BSC sterilised by gamma-radiation ate more but grew slower than isopods that ate live BSC, implying that ingested microorganisms facilitate litter digestion. Our work highlights the need to reveal the multifaceted considerations that affect food-selection when exploring trophic-interactions.

Stoichiometric microplastics models in natural and laboratory environments

Microplastics pose a severe threat to marine ecosystems; however, relevant mathematical modeling and analysis are lacking. This paper formulates two stoichiometric producer-grazer models to investigate the interactive effects of microplastics, nutrients, and light on population dynamics under different settings. One model incorporates optimal microplastic uptake and foraging behavior based on nutrient availability for natural settings, while the other model does not include foraging in laboratory settings. We establish the well-posedness of the models and examine their long-term behaviors. Our results reveal that in natural environments, producers and grazers exhibit higher sensitivity to microplastics, and the system may demonstrate bistability or tristability. Moreover, the influences of microplastics, nutrients, and light intensity are highly intertwined. The presence of microplastics amplifies the constraints on grazer growth related to food quality and quantity imposed by extreme light intensities, while elevated phosphorus input enhances the system's resistance to intense light conditions. Furthermore, higher environmental microplastic levels do not always imply elevated microplastic body burdens in organisms, as organisms are also influenced by nutrients and light. We also find that grazers are more vulnerable to microplastics, compared to producers. If producers can utilize microplastics for growth, the system displays significantly greater resilience to microplastics.

The paradox of plant preference: The malaria vectors Anopheles gambiae and Anopheles coluzzii select suboptimal food sources for their survival and reproduction

Anopheles gambiae and Anopheles coluzzii mosquitoes, two major malaria vectors in sub-Saharan Africa, exhibit selectivity among plant species as potential food sources. However, it remains unclear if their preference aligns with optimal nutrient intake and survival. Following an extensive screening of the effects of 31 plant species on An. coluzzii in Burkina Faso, we selected three species for their contrasting effects on mosquito survival, namely Ixora coccinea, Caesalpinia pulcherrima, and Combretum indicum. We assessed the sugar content of these plants and their impact on mosquito fructose positivity, survival, and insemination rate, using Anopheles coluzzii and Anopheles gambiae, with glucose 5% and water as controls. Plants displayed varying sugar content and differentially affected the survival, sugar intake, and insemination rate of mosquitoes. All three plants were more attractive to mosquitoes than controls, with An. gambiae being more responsive than An. coluzzii. Notably, C. indicum was the most attractive but had the lowest sugar content and offered the lowest survival, insemination rate, and fructose positivity. Our findings unveil a performance-preference mismatch in An. coluzzii and An. gambiae regarding plant food sources. Several possible reasons for this negative correlation between performance and preference are discussed.

Food preferences and nutrient composition in captive Southern brown howler monkeys, Alouatta guariba clamitans

Studies of food preferences in captive primates have so far mainly been restricted to frugivorous species. It was therefore the aim of the present study to assess the occurrence of spontaneous food preferences in a mainly folivorous primate, the captive Southern brown howler monkey, and to analyze whether these preferences correlate with nutrient composition. Using a two-alternative choice test, we presented ten male and five female adult Alouatta guariba clamitans with all possible binary combinations of ten types of food that are part of their diet in captivity and recorded their choice behavior. We found the howler monkeys to display the following rank order of preference: banana > mango > watermelon > papaya > beetroot > apple > pear > orange > cucumber > tomato. This preference ranking significantly and positively correlated with the total carbohydrate content and with the sucrose content of the food items. We also found significant positive correlations between the food preference ranking and the content of the minerals copper and magnesium. Male and female howler monkeys did not differ significantly in their food preference rankings. These results suggest this howler monkeys under human care are not opportunistic, but selective feeders with regard to maximizing their net gain of energy as only the content of carbohydrates, but not the contents of total energy, proteins, or lipids significantly correlated with the displayed food preferences. Thus, the food preferences of this primate are similar to those reported in several species of frugivorous primates tested with cultivated fruits and vegetables.

Alanine metabolism mediates energy allocation of the brown planthopper to adapt to resistant rice

INTRODUCTION

During the adaptation to host plant resistance, herbivorous insects faced the challenge of overcoming plant defenses while ensuring their own development and reproductive success. To achieve this, a strategic allocation of energy resources for detoxification and ecological fitness maintenance became essential.

OBJECTIVE

This study aimed to elucidate the intricate energy allocation mechanisms involved in herbivore adaptation that are currently poorly understood.

METHODS

The rice Oryza sativa and its monophagous pest, the brown planthopper (BPH), Nilaparvata lugens were used as a model system. An integrated analysis of metabolomes and transcriptomes from different BPH populations were conducted to identify the biomarkers. RNA interference of key genes and exogenous injection of key metabolites were performed to validate the function of biomarkers.

RESULTS

We found that alanine was one of the key biomarkers of BPH adaptation to resistant rice variety IR36. We also found that alanine flow determined the adaptation of BPH to IR36 rice. The alanine aminotransferase (ALT)-mediated alanine transfer to pyruvate was necessary and sufficient for the adaptation. This pathway may be conserved, at least to some extent, in BPH adaptation to multiple rice cultivars with different resistance genes. More importantly, ALT-mediated alanine metabolism is the foundation of downstream energy resource allocation for the adaptation. The adapted BPH population exhibited a significantly higher level of energy reserves in the fat body and ovary when fed with IR36 rice, compared to the unadapted population. This rendered the elevated detoxification in the adapted BPH and their ecological fitness recovery.

CONCLUSION

Overall, our findings demonstrated the crucial role of ALT-mediated alanine metabolism in energy allocation during the adaptation to resistant rice in BPH. This will provide novel knowledge regarding the co-evolutionary mechanisms between herbivores and their host plants.

Parrot Dietary Habits and Consumption of Alternate Foodstuffs

Inappropriate diets cause many of the health problems commonly reported in parrots by psittaculturists and veterinarians. The dietary management of captive parrots would benefit from information derived from studies of dietary habits of wild parrots; however, it is unclear how complete this body of knowledge is at this time. Documentation of parrots' dietary habits appears to have grown dramatically over the past century. Reports of parrots consuming a number of foodstuffs beyond the reproductive parts of plants (alternate foodstuffs) have increased. The extent of alternate foodstuffs in parrot diets is currently unknown. We used Google search engines (ie, Scholar, Videos, Images) to determine how well psittaciform dietary habits have been studied to date and to quantify reports of alternate foodstuffs consumption among genera of Psittaciformes. We found that the dietary habits of over 43% of parrot species are poorly resolved. The dietary habits of 71.5% of parrot species classified by the International Union for Conservation of Nature as at risk of extinction are not well resolved. Parrots' consumption of alternate foodstuffs occurred at the following rates at the genus level: 91.2% foliage, 76.9% terrestrial invertebrates and fine earthen materials, 74.7% wood, 44% pure minerals, 34.1% vertebrates (9.9% dung), 29.7% sap, 19.8% roots, 17.6% charcoal, 18.7% epiphytes, 16.5% coarse earthen materials, 8.8% algae, and 6.6% aquatic invertebrates. Of these reports, 79.1% involved observations of wild parrots. Many parrot species may be more omnivorous than previously realized. Alternate foodstuffs are generally absent from current veterinary-based dietary recommendations for captive parrots. Future studies are needed to determine whether providing alternate foodstuffs to captive parrots can be used as a means to improve their diets and thus their health, welfare, and reproductive success.

Fitness of Nutrition Regulation in a Caterpillar Pest Mythimna separata (Walker): Insights from the Geometric Framework

In nature, plants can contain variable nutrients depending upon the species, tissue, and developmental stage. Insect herbivores may regulate their nutrient intake behaviorally and physio- logically when encountering different foods. This study examined the nutritional regulation of the oriental armyworm, Mythimna separata, for the first time. In one experiment, we allowed the cater-pillars to choose between two nutritionally balanced but complementary diets. The caterpillars did not randomly consume the paired foods, but instead chose between the nutritionally balanced but complementary diets. This intake behavior was found to change with their developmental stages. Furthermore, the nutrient concentrations in food significantly impacted the insect's performance. In the other experiment, caterpillars were given one of eleven diets that reflected the different nutrient conditions in the field. The results showed that proteins were significantly associated with developmental time and fecundity. For example, by consuming protein-biased food, the caterpillars developed faster and produced more eggs. In contrast, carbohydrates were more strongly linked to lipid accumulation, and caterpillars accumulated more lipids when consuming the carbohydrate-biased food. Moreover, the caterpillars were also found to actively regulate their intake of proteins and carbohydrates based on food quality and to physiologically prepare for subsequent life stages. These findings enhance our understanding of how M. separata feeds and responds to different nutritional environments in the field, which could have implications for managing insect herbivores in agricultural settings.

Towards an integrated understanding of dietary phenotypes

Diet and nutrition comprise a complex, multi-faceted interface between animal biology and food environments. With accumulating information on the many facets of this association arises a need for systems-based approaches that integrate dietary components and their links with ecology, feeding, post-ingestive processes and the functional and ecological consequences of these interactions. We briefly show how a modelling approach, nutritional geometry, has used the experimental control afforded in laboratory studies to begin to unravel these links. Laboratory studies, however, have limited ability to establish whether and how the feeding and physiological mechanisms interface with realistic ecological environments. We next provide an overview of observational field studies of free-ranging primates that have examined this, producing largely correlative data suggesting that similar feeding mechanisms operate in the wild as in the laboratory. Significant challenges remain, however, in establishing causal links between feeding, resource variation and physiological processes in the wild. We end with a more detailed account of two studies of temperate primates that have capitalized on the discrete variation provided by seasonal environments to strengthen causal inference in field studies and link patterns of intake to dynamics of nutrient processing. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Accelerometry reveals limits to use of an energy-saving anthropogenic food source by a threatened species: A case of Carnaby's cockatoos (Zanda latirostris) and canola

The use of anthropogenic resources is becoming increasingly common as species adapt to human-induced environmental changes, but their use can expose species to new risks. Understanding how animals exploit these resources is important for guiding conservation management, particularly where species are threatened. The introduction of canola cropping to breeding areas of endangered Carnaby's cockatoo (Zanda latirostris) has been attributed to an increase in the birds' reproductive success; however, the seed may be protein-limiting for nestling growth and its use by cockatoos has been implicated in the emergence of a new disease. We used high-resolution accelerometer-capable GPS tags to track eight birds. Accelerometer data were used to calculate overall dynamic body acceleration (ODBA), a proxy for energy expenditure, and to identify and quantify canola and native vegetation foraging behaviours. We used linear mixed models to determine which factors affected patterns of resource use and to determine whether, and to what extent, canola use was associated with reduced energetic and movement costs. We then compared the energetic content of canola seed and native food sources to inform patterns of behaviour and habitat use revealed by our tracking data. Use of canola was associated with reduced movement costs and energy expenditure. However, there was an apparent reluctance to increase foraging on canola above a threshold of time, even when conditions reduced time available to utilise native food sources. While anthropogenic resources may appear to improve population trends in some cases, careful investigations of patterns of resource use are necessary to guide appropriate conservation management efforts. For Carnaby's cockatoos, conservation efforts should focus on retention, protection and expansion of native food sources.

Towards a mechanistic understanding of the impacts of nitrogen deposition on producer-consumer interactions

Nitrogen (N) deposition has increased substantially since the second half of the 20th century due to human activities. This increase of reactive N into the biosphere has major implications for ecosystem functioning, including primary production, soil and water chemistry and producer community structure and diversity. Increased N deposition is also linked to the decline of insects observed over recent decades. However, we currently lack a mechanistic understanding of the effects of high N deposition on individual fitness, species richness and community structure of both invertebrate and vertebrate consumers. Here, we review the effects of N deposition on producer-consumer interactions, focusing on five existing ecological frameworks: C:N:P ecological stoichiometry, trace element ecological stoichiometry, nutritional geometry, essential micronutrients and allelochemicals. We link reported N deposition-mediated changes in producer quality to life-history strategies and traits of consumers, to gain a mechanistic understanding of the direction of response in consumers. We conclude that high N deposition influences producer quality via eutrophication and acidification pathways. This makes oligotrophic poorly buffered ecosystems most vulnerable to significant changes in producer quality. Changes in producer quality between the reviewed frameworks are often interlinked, complicating predictions of the effects of high N deposition on producer quality. The degree and direction of fitness responses of consumers to changes in producer quality varies among species but can be explained by differences in life-history traits and strategies, particularly those affecting species nutrient intake regulation, mobility, relative growth rate, host-plant specialisation, ontogeny and physiology. To increase our understanding of the effects of N deposition on these complex mechanisms, the inclusion of life-history traits of consumer species in future study designs is pivotal. Based on the reviewed literature, we formulate five hypotheses on the mechanisms underlying the effects of high N deposition on consumers, by linking effects of nutritional ecological frameworks to life-history strategies. Importantly, we expect that N-deposition-mediated changes in producer quality will result in a net decrease in consumer community as well as functional diversity. Moreover, we anticipate an increased risk of outbreak events of a small subset of generalist species, with concomitant declines in a multitude of specialist species. Overall, linking ecological frameworks with consumer life-history strategies provides a mechanistic understanding of the impacts of high N deposition on producer-consumer interactions, which can inform management towards more effective mitigation strategies.

Assessment of the diet of the critically endangered northern hairy-nosed wombat (Lasiorhinus krefftii) using DNA metabarcoding

Invasive buffel grass (Cenchrus ciliaris) is considered a threat to the critically endangered northern hairy-nosed wombat (Lasiorhinus krefftii; NHW). Buffel grass outcompetes native grasses, reducing availability of native food items for NHW, and causes more intense fires due to the large volumes of dead matter it produces. Previous studies suggested buffel grass was increasing in the diet; however, the diet of the NHW has not been reassessed for over two decades and was limited to Epping Forest National Park, with the population at Richard Underwood Nature Refuge having never been assessed. The recently released 2022 Recovery Action Plan for the species outlined objectives to assist its conservation and recommended the impact of buffel grass on the species' diet be investigated. This study aimed to determine: (1) which plant species are being consumed by the NHW; (2) the differences in the diet between sites; (3) differences between seasons; and (4) the abundance of buffel grass in the diet. The diet was assessed using DNA metabarcoding of scat samples collected from both sites from winter 2020 to spring 2021. Site and season significantly affected the diet of the NHW. Buffel grass dominated the diet and has increased in the diet since past assessments. The findings of this study will support population and habitat management of the critically endangered NHW. Enhanced knowledge of dietary items consumed at both sites will also assist efforts to locate additional sites suitable for translocation.

Intake of caffeine containing sugar diet remodels gut microbiota and perturbs Drosophila melanogaster immunity and lifespan

The diet-microbiome-immunity axis is one among the many arms that draw up the "we are what we intake" proclamation. As such, studies on the effect of food and beverage intake on the gut environment and microbiome and on modulating immunological responses and the host's susceptibility to pathogens are on the rise. A typical accompaniment in different sustenance we consume on daily basis is the trimethylxanthine alkaloid caffeine. Being a chief component in our regular aliment, a better understanding of the effect of caffeine containing food and beverages on our gut-microbiome-immunity axis and henceforth on our health is much needed. In this study, we shed more light on the effect of oral consumption of caffeine supplemented sugar diet on the gut environment, specifically on the gut microbiota, innate immunity and host susceptibility to pathogens using the Drosophila melanogaster model organism. Our findings reveal that the oral intake of a dose-specific caffeine containing sucrose/agarose sugar diet causes a significant alteration within the fly gut milieu demarcated by microbial dysbiosis and an elevation in the production of reactive oxygen species and expression of immune-deficiency (Imd) pathway-dependent antimicrobial peptide genes. The oral intake of caffeine containing sucrose/agarose sugar diet also renders the flies more susceptible to bacterial infection and shortens their lifespan in both infection and non-infection settings. Our findings set forth additional insight into the potentiality of diet to alter the gut milieu and highlight the importance of dietary control on health.

Mapping the nutritional landscape in the yellow mealworm: testing the nutrient-mediated life-history trade-offs

Animals must acquire an ideal amount and balance of macronutrients to optimize their performance, health and fitness. The nutritional landscape provides an integrative framework for analysing how animal phenotypes are associated with multiple nutritional components. Here, we applied this powerful approach to examine how the intake of protein and carbohydrate affects nutrient acquisition and performance in the yellow mealworm (Tenebrio molitor) reared on one of 42 synthetic foods varying in protein and carbohydrate content. Tenebrio molitor larvae increased their food consumption rate in response to nutrient dilution, but this increase was not sufficient to fully compensate for the dilution. Diluting the food nutrient content with cellulose reduced the efficiency of post-ingestive nutrient utilization, further restricting macronutrient acquisition. Tenebrio molitor larvae utilized macronutrients most efficiently at a protein to carbohydrate (P:C) ratio of 1.77:1, but became less efficient at imbalanced P:C ratios. Survivorship was high at high protein intake and fell with decreasing protein intake. Pupal mass and growth rate exhibited a bell-shaped landscape, with the nutritional optima being located around protein-biased P:C ratios of 1.99:1 to 2.03:1 and 1.66:1 to 2.86:1, respectively. The nutritional optimum for development time was also identified at high P:C ratios (1.66:1  to 5.86:1). Unlike these performance traits, lipid content was maximized at carbohydrate-biased P:C ratios of 1:3.88 to 1:3.06. When given a food choice, T. molitor larvae self-composed a slightly carbohydrate-biased P:C ratio of 1:1.24, which lies between the P:C ratios that maximize performance and lipid content. Our findings indicate the occurrence of a nutrient-mediated trade-off between performance and energy storage in this insect.

Temperature- and Diet-Induced Plasticity of Growth and Digestive Enzymes Activity in Spongy Moth Larvae

Temperature and food quality are the most important environmental factors determining the performance of herbivorous insects. The objective of our study was to evaluate the responses of the spongy moth (formerly known as the gypsy moth) [Lymantria dispar L. (Lepidoptera: Erebidae)] to simultaneous variation in these two factors. From hatching to the fourth instar, larvae were exposed to three temperatures (19 °C, 23 °C, and 28 °C) and fed four artificial diets that differed in protein (P) and carbohydrate (C) content. Within each temperature regime, the effects of the nutrient content (P+C) and ratio (P:C) on development duration, larval mass, growth rate, and activities of digestive proteases, carbohydrases, and lipase were examined. It was found that temperature and food quality had a significant effect on the fitness-related traits and digestive physiology of the larvae. The greatest mass and highest growth rate were obtained at 28 °C on a high-protein low-carbohydrate diet. A homeostatic increase in activity was observed for total protease, trypsin, and amylase in response to low substrate levels in the diet. A significant modulation of overall enzyme activities in response to 28 °C was detected only with a low diet quality. A decrease in the nutrient content and P:C ratio only affected the coordination of enzyme activities at 28 °C, as indicated by the significantly altered correlation matrices. Multiple linear regression analysis showed that variation in fitness traits in response to different rearing conditions could be explained by variation in digestion. Our results contribute to the understanding of the role of digestive enzymes in post-ingestive nutrient balancing.

Nutrigonometry IV: Thales' theorem to measure the rules of dietary compromise in animals

Diet specialists and generalists face a common challenge: they must regulate the intake and balance of nutrients to achieve a target diet for optimum nutrition. When optimum nutrition is unattainable, organisms must cope with dietary imbalances and trade-off surplus and deficits of nutrients that ensue. Animals achieve this through compensatory rules that dictate how to cope with nutrient imbalances, known as 'rules of compromise'. Understanding the patterns of the rules of compromise can provide invaluable insights into animal physiology and behaviour, and shed light into the evolution of diet specialisation. However, we lack an analytical method for quantitative comparisons of the rules of compromise within and between species. Here, I present a new analytical method that uses Thales' theorem as foundation, and that enables fast comparisons of the rules of compromise within and between species. I then apply the method on three landmark datasets to show how the method enables us to gain insights into how animals with different diet specialisation cope with nutrient imbalances. The method opens new avenues of research to understand how animals cope with nutrient imbalances in comparative nutrition.

Nutrient intake and its possible drivers in free-ranging European brown bears (Ursus arctos arctos)

The dietary nutrient profile has metabolic significance and possibly contributes to species' foraging behavior. The brown bear (Ursus arctos) was used as a model species for which dietary ingredient and nutrient concentrations as well as nutrient ratios were determined annually, seasonally and per reproductive class. Brown bears had a vertebrate- and ant-dominated diet in spring and early summer and a berry-dominated diet in fall, which translated into protein-rich and carbohydrate-rich diets, respectively. Fiber concentrations appeared constant over time and averaged at 25% of dry matter intake. Dietary ingredient proportions differed between reproductive classes; however, these differences did not translate into a difference in dietary nutrient concentrations, suggesting that bears manage to maintain similar nutrient profiles with selection of different ingredients. In terms of nutrient ratios, the dietary protein to non-protein ratio, considered optimal at around 0.2 (on metabolizable energy basis), averaged around 0.2 in this study in fall and around 0.8 in spring and summer. We introduced the minimal non-fat to fat ratio necessary for efficient maintenance metabolism. This ratio varied across seasons but never fell beneath the theoretically estimated minimum to ensure metabolic efficiency. This population thus managed to ingest diets that never exerted a lack of glucogenic substrate, suggesting that metabolic efficiency may either be a driver of active diet selection or that natural resources available to bears did not constitute a constraint in this respect. Given the considerable proportion of fiber in the diet of brown bears, the relevance of this nutrient and its role in foraging behavior might be underestimated.

Environmental safety of second and third generation bioplastics in the context of the circular economy

Bioplastics derived from organic materials other than crude oil are often suggested as sustainable solutions for tackling end-of-life plastic waste, but little is known of their ecotoxicity to aquatic species. Here, we investigated the ecotoxicity of second and third generation bioplastics toward the freshwater zooplankton Daphnia magna. In acute toxicity tests (48 h), survival was impacted at high concentrations (g.L^-1 range), within the range of salinity-induced toxicity. Macroalgae-derived bioplastic induced hormetic responses under chronic exposure (21 d). Most biological traits were enhanced from 0.06 to 0.25 g.L^-1 (reproduction rate, body length, width, apical spine, protein concentration), while most of these traits returned to controls level at 0.5 g.L^-1. Phenol-oxidase activity, indicative of immune function, was enhanced only at the lowest concentration (0.06 g.L^-1). We hypothesise these suggested health benefits were due to assimilation of carbon derived from the macroalgae-based bioplastic as food. Polymer identity was confirmed by infra-red spectroscopy. Chemical analysis of each bioplastic revealed low metal abundance whilst non target exploration of organic compounds revealed trace amounts of phthalates and flame retardants. The macroalgae-bioplastic disintegrated completely in compost and biodegraded up to 86 % in aqueous medium. All bioplastics acidified the test medium. In conclusion, the tested bioplastics were classified as environmentally safe. Nonetheless, a reasonable end-of-life management of these safer-by-design materials is advised to ensure the absence of harmful effects at high concentrations, depending on the receiving environment.

Multiple interacting factors affect seed predation in an African savanna small mammal community

Multiple factors affect seed predation, including seed traits, habitat type, seed predator community composition, predation risk, and seasonality. How all these factors and their interactions simultaneously influence seed predation has rarely been tested experimentally in situ. Here, we assessed the relative contribution of the factors driving seed predation in an African savanna rodent community, comprising six ecologically similar species. We first conducted seed preference tests under semicaptive conditions to determine which seed trait (size, shell hardness, nutritional content) influenced seed predation. Then we performed in situ experiments to establish whether rodent community composition (diversity and abundance), seed type, habitat type, seasonality, predation risk, and their interactions affected seed predation. Semicaptive experiments showed that rodents preferred smaller, lighter seeds, containing relatively high water content. In situ experiments showed that predation risk was an important factor influencing seed predation, with rodents removing considerably more seeds in areas where predation risk was lower. Habitat type also affected seed predation, but its effects were strongly linked to predation risk. In areas where predation risk was higher, rodents removed more seeds in more heterogeneous habitats, whereas in areas where predation risk was lower, rodents removed more seeds in less heterogeneous habitats. Seasonality was the least influential factor shaping seed predation. Rodents removed more seeds in winter compared to other seasons, but only in areas where predation risk was low. We provide experimental evidence for a multifaceted approach to understanding the relative contribution of the different factors driving variation in seed predation in natural communities and show that these factors are likely hierarchically arranged.

Integrating isotopic and nutritional niches reveals multiple dimensions of individual diet specialisation in a marine apex predator

Dietary specialisations are important determinants of ecological structure, particularly in species with high per-capita trophic influence like marine apex predators. These species are, however, among the most challenging in which to establish spatiotemporally integrated diets. We introduce a novel integration of stable isotopes with a multidimensional nutritional niche framework that addresses the challenges of establishing spatiotemporally integrated nutritional niches in wild populations, and apply the framework to explore individual diet specialisation in a marine apex predator, the white shark Carcharodon carcharias. Sequential tooth files were sampled from juvenile white sharks to establish individual isotopic (δ-space; δ¹³ C, δ¹⁵ N, δ³⁴ S) niche specialisation. Bayesian mixing models were then used to reveal individual-level prey (p-space) specialisation, and further combined with nutritional geometry models to quantify the nutritional (N-space) dimensions of individual specialisation, and their relationships to prey use. Isotopic and mixing model analyses indicated juvenile white sharks as individual specialists within a broader, generalist, population niche. Individual sharks differed in their consumption of several important mesopredator species, which suggested among-individual variance in trophic roles in either pelagic or benthic food webs. However, variation in nutrient intakes was small and not consistently correlated with differences in prey use, suggesting white sharks as nutritional specialists and that individuals could use functionally and nutritionally different prey as complementary means to achieve a common nutritional goal. We identify how degrees of individual specialisation can differ between niche spaces (δ-, p- or N-space), the physiological and ecological implications of this, and argue that integrating nutrition can provide stronger, mechanistic links between diet specialisation and its intrinsic (fitness/performance) and extrinsic (ecological) outcomes. Our time-integrated framework is adaptable for examining the nutritional consequences and drivers of food use variation at the individual, population or species level.

The Effect of Pollen Diet Composition and Quantity on Diapause Survival and Performance in an Annual Pollinator (Bombus Impatiens)

Most pollination services are provided by annual bees that go through a winter diapause, during which they are exposed to extreme temperatures, pathogens, and starvation. The ability of bees to successfully face these stressors during diapause and subsequently initiate a nest depends on their overall nutritional state and an adequate preparatory diet. Here, we used queens of the common eastern bumble bee, Bombus impatiens, to examine how pollen diets varying in their protein to lipid ratio and total nutrient amounts affected queen performance during and after diapause. We compared diapause survival and reproductive performance post-diapause across different diets and found that queen survival was highest when pollen had a nutritional ratio of approximately 5:1 (protein to lipid). This diet is significantly enriched in proteins compared to the pollen fed to bumble bees in the lab (1:1) or commonly available in agricultural landscapes. Altering the quantity of macronutrients within this ratio did not improve survival or performance. Our results emphasize the importance of adequate nutrition for diapause performance in bees with annual life cycles and the importance of providing annual bees with floral provisioning based on their individual nutritional targets.

Electrolytes on the prairie: How urine-like additions of Na and K shape the dynamics of a grassland food web

The electrolytes Na and K both function to maintain water balance and membrane potential. However, these elements work differently in plants-where K is the primary electrolyte-than in animals-where ATPases require a balanced supply of Na and K. Here, we use monthly factorial additions of Na and K to simulate bovine urine inputs and explore how these electrolytes ramify through a prairie food web. Against a seasonal trend of increasing grass biomass and decreasing water and elemental tissue concentrations, +K and +Na plots boosted water content and, when added together, plant biomass. Compared to control plots, +Na and +K plots increased element concentrations in above-ground plant tissue early in summer and decreased them in September. Simultaneously, invertebrate abundance on Na and K additions were sequentially higher and lower than control plots from June to September and were most suppressed when grass was most nutrient rich. K was the more effective plant electrolyte, but Na frequently promoted similar changes in grass ionomes. The soluble/leachable ions of Na and K showed significant ability to shape plant growth, water content, and the 15-element ionome, with consequences for higher trophic levels. Grasslands with high inputs of Na and K-via large mammal grazers or coastal aerosol deposition-likely enhance the ability of plants to adjust their above-ground ionomes, with dramatic consequences for the distribution of invertebrate consumers.

Within-population variation in body size plasticity in response to combined nutritional and thermal stress is partially independent from variation in development time

Ongoing climate change has forced animals to face changing thermal and nutritional environments. Animals can adjust to such combinations of stressors via plasticity. Body size is a key trait influencing organismal fitness, and plasticity in this trait in response to nutritional and thermal conditions varies among genetically diverse, locally adapted populations. The standing genetic variation within a population can also influence the extent of body size plasticity. We generated near-isogenic lines from a newly collected population of Drosophila melanogaster at the mid-point of east coast Australia and assayed body size for all lines in combinations of thermal and nutritional stress. We found that isogenic lines showed distinct underlying patterns of body size plasticity in response to temperature and nutrition that were often different from the overall population response. We then tested whether plasticity in development time could explain, and therefore regulate, variation in body size to these combinations of environmental conditions. We selected five genotypes that showed the greatest variation in response to combined thermal and nutritional stress and assessed the correlation between response of developmental time and body size. While we found significant genetic variation in development time plasticity, it was a poor predictor of body size among genotypes. Our results therefore suggest that multiple developmental pathways could generate genetic variation in body size plasticity. Our study emphasizes the need to better understand genetic variation in plasticity within a population, which will help determine the potential for populations to adapt to ongoing environmental change.

Go for Lipids! Food Preferences and Nutrient Composition in Zoo-Housed White-Faced Sakis, Pithecia pithecia

The role that single nutrients may play for food choices in nonhuman primates is not fully understood. White-faced sakis (Pithecia pithecia) are unusual among frugivorous primates as they do not serve as seed dispersers but rather exploit the seeds they consume, presumably for their high contents of lipids and proteins. Therefore, we assessed the occurrence of spontaneous food preferences in zoo-housed white-faced sakis and analyzed whether these preferences correlate with nutrient composition. Using a two-alternative choice test, we repeatedly presented three female and two male sakis with all possible binary combinations of 15 types of food that are part of their diet under human care, and found them to display the following rank order of preference: peanut > hazelnut > avocado > melon > egg > apple > mealworms > beetroot > carrot > cucumber > cabbage > tomato > sweet potato > broccoli > eggplant. This preference ranking significantly and positively correlated with the total energy content of the food items. However, we found the strongest positive correlation among the three macronutrients providing metabolic energy between the sakis’ food preferences and lipid content. This is remarkable as all other primate species tested so far using this method displayed the strongest correlation with carbohydrates instead. Together with our finding that the sakis significantly preferred foods high in mono-unsaturated fatty acids, the building blocks of lipids, these results support the notion that white-faced sakis exploit the lipids contained in seeds to meet their requirements of metabolic energy.

Nutrigonometry III: curvature, area and differences between performance landscapes

Nutrition is one of the underlying factors necessary for the expression of life-histories and fitness across the tree of life. In recent decades, the geometric framework (GF) has become a powerful framework to obtain biological insights through the construction of multidimensional performance landscapes. However, to date, many properties of these multidimensional landscapes have remained inaccessible due to our lack of mathematical and statistical frameworks for GF analysis. This has limited our ability to understand, describe and estimate parameters which may contain useful biological information from GF multidimensional performance landscapes. Here, we propose a new model to investigate the curvature of GF multidimensional landscapes by calculating the parameters from differential geometry known as Gaussian and mean curvatures. We also estimate the surface area of multidimensional performance landscapes as a way to measure landscape deviations from flat. We applied the models to a landmark dataset in the field, where we also validate the assumptions required for the calculations of curvature. In particular, we showed that linear models perform as well as other models used in GF data, enabling landscapes to be approximated by quadratic polynomials. We then introduced the Hausdorff distance as a metric to compare the similarity of multidimensional landscapes.

Diverse diets and low-fiber, low-tannin foraging preferences: Foraging criteria of Tibetan macaques (Macaca thibetana) at low altitude in Huangshan

Nutrient composition and food availability determine food choices and foraging strategies of animals, while altitude and geographical location affect species distribution and food availability. Tibetan macaques (Macaca thibetana) have sophisticated foraging strategies as the largest species in Macaca. They are important in understanding the ecological evolution of the entire genus. However, the mechanism of food selection in Tibetan macaques at low altitudes remains unclear. In this study, we researched a wild Tibetan macaques group (Tianhu Mountain Group, 29 individuals) living in a low-altitude area around Mt. Huangshan, Anhui Province, China. We used instantaneous scan sampling to observe these macaques' foraging behavior from September 2020 to August 2021. We recorded the dietary composition and food availability, compared the nutrient content of staple food and non-food items, and analyzed the role of key nutrients in food selection. We found that Tibetan macaques forage on 111 plants belonging to 93 genera and 55 families. The food types included fruits (52.5%), mature leaves (17.0%), bamboo shoots (14.4%), young leaves (6.3%), flowers (4.5%), others (2.1%), stems (1.9%), and tender shoots (1.3%). Tibetan macaques forage for a maximum of 76 plant species during spring. However, dietary diversity was highest during summer (H' = 3.052). Monthly fruit consumption was positively correlated with food availability. Staple foods are lower in fiber, tannin, and water than non-foods. In addition, the time spent foraging for specific foods was negatively correlated with the fiber and tannin content of the food. The results showed that Tibetan macaques' foraging plant species and food types were diverse, and their foraging strategies varied seasonally. Our findings confirmed the effect of nutrients on food choice in Tibetan macaques. We highlighted the important role of fiber and tannin in their food choices and suggested that the foraging behavior of Tibetan macaques is highly flexible and adaptive.

Dietary Choice Reshapes Metabolism in Drosophila by Affecting Consumption of Macronutrients

The precise regulation of metabolism and feeding behavior is important for preventing the development of metabolic diseases. Here we examine the effects on Drosophila metabolism of dietary choice. These changes are predicted to be dependent on both the quantity and quality of the chosen diet. Using a geometric framework for both no-choice and two-choice conditions, we found that feeding decisions led to higher glucose and trehalose levels but lower triglycerides pools. The feeding regimens had similar strategies for macronutrient balancing, and both maximized hemolymph glucose and glycogen content under low protein intake. In addition, the flies showed significant differences in the way they regulated trehalose and triglyceride levels in response to carbohydrate and protein consumption between choice and no-choice nutrition. Under choice conditions, trehalose and triglyceride levels were maximized at the lowest protein and carbohydrate consumption. Thus, we suggest that these changes in carbohydrate and lipid metabolism are caused by differences in the macronutrients consumed by flies. Food choice elicits rapid metabolic changes to maintain energy homeostasis. These results contribute to our understanding of how metabolism is regulated by the revealed nutrient variation in response to food decisions.

Host-Specific larval lepidopteran mortality to pathogenic Serratia mediated by poor diet

Insect guts often harbor an abundance of bacteria. Many of these members are commensal, but some may emerge as opportunistic pathogens when the host is under stress. In this study, we evaluated how dietary nutritional concentration mediates a shift from commensal to pathogenic, and if host species influences those interactions. We used the lepidopterans (Noctuidae) fall armyworm (Spodoptera frugiperda), beet armyworm (Spodoptera exigua), and corn earworm (Helicoverpa zea) as hosts and a Serratia strain initially isolated from healthy fall armyworm. Diet concentration was altered by bulk reduction in nutritional content with dilution using cellulose. Our experiments revealed that low nutrient diet increased mortality from Serratia for beet armyworm and corn earworm. However, for fall armyworm, little mortality was observed in any of the diet combinations. Dietary nutrition and oral inoculation with Serratia did not change the expression of two antimicrobial peptides in fall and beet armyworm, suggesting that other mechanisms that mediate mortality were involved. Our results have implications for how pathogens may persist as commensals in the digestive tract of insects. These findings also suggest that diet plays a very important role in the switch from commensal to pathogen. Finally, our data indicate that the host response to changing conditions is critical in determining if a pathogen may overtake its host and that these three lepidopteran species have different responses to opportunistic enteric pathogens.

Sustained changes in digestive physiology and microbiome across sequential generations of zebrafish fed different diets

Alterations to ratios of protein and fiber in an organism's diet have been shown to structurally and functionally alter its individual digestive physiology. However, it is unclear how these dietary changes may affect phenotypic changes across generations. We utilized feeding trials, morphological analyses, enzyme activities, and 16S rRNA sequencing of the gut microbiome of zebrafish (Danio rerio) to determine how variations to fiber and protein concentrations, kept consistent across sequential generations, affect phenotypic changes. Our results show that Parental (P) and first generation (F₁) fish did not differ from each other in terms of their intestine length, intestine mass, enzyme activity levels, and microbial community composition for any of the three experimental diets (high-protein/low-fiber, moderate-protein/fiber, and low-protein/high-fiber). However, each of the three experimental diets for the P and F₁ fish, as well as the ancestral diet fish, did have distinct microbial community structure from one another. This indicates that there is a strong dietary effect on digestive physiology and gut microbial community and that these effects are consistent when the diet is kept homogenous across generations.

Optimal foraging strategy to balance mixed diet by generalist consumers: a simulation model

Animals of a wide range of taxonomic groups mix various food sources to achieve a nutritionally balanced diet. The strategies they adopt to balance multiple nutrients depend on their availability in the environment. Behavioural and physiological adaptations to forage for nutrient-differing food sources have rarely been investigated in respect to nutrient availability in the environment. We developed a simulation model to explore the strategy consumers should adopt in response to the abundance of two nutritionally complementary food types. Results show that (1) consumers should invest more effort in detecting the scarce resource; (2) there is an optimized negative relationship between effort foragers should allocate to find the two types of food; (3) consumers should exhibit higher selectivity when the proportion of food types in the habitat deviates from their optimal ratio in the diet. These findings have important implications for pest control using predators that benefit from plant-based food supplements.

Quantity versus quality: Effects of diet protein-carbohydrate ratios and amounts on insect herbivore gene expression

Dietary protein and digestible carbohydrates are two key macronutrients for insect herbivores, but the amounts and ratios of these two macronutrients in plant vegetative tissues can be highly variable. Typically, insect herbivores regulate their protein-carbohydrate intake by feeding selectively on nutritionally complementary plant tissues, but this may not always be possible. Interestingly, lab experiments consistently demonstrate that performance - especially growth and survival - does not vary greatly when caterpillars and nymphal grasshoppers are reared on diets that differ in their protein-carbohydrate content. This suggests insect herbivores employ post-ingestive physiological mechanisms to compensate for variation in diet protein-carbohydrate profile. However, the molecular mechanisms that underlie this compensation are not well understood. Here we explore, for the first time in an insect herbivore, the transcriptional effects of two dietary factors: protein-to-carbohydrate ratio (p:c) and total macronutrient (p + c) content. Specifically, we reared Helicoverpa zea caterpillars on three diets that varied in diet p:c ratio and one diet that varied in total p + c concentration, all within an ecologically-relevant range. We observed two key findings. Caterpillars reared on diets with elevated total p + c content showed large differences in gene expression. In contrast, only small differences in gene expression were observed when caterpillars were reared on diets with different p:c ratios (spanning from protein-biased to carbohydrate-biased). The invariable expression of many metabolic genes across these variable diets suggests that H. zea caterpillars employ a strategy of constitutive expression to deal with protein-carbohydrate imbalances rather than diet-specific changes. This is further supported by two findings. First, few genes were uniquely associated with feeding on a protein- and carbohydrate-biased diet. Second, many differentially-expressed genes were shared across protein-biased, carbohydrate-biased, and concentrated diet treatments. Our study provides insights into the post-ingestive physiological mechanisms insect herbivores employ to regulate protein-carbohydrate intake. Most notably, it suggests that H. zea, and perhaps other generalist species, use similar post-ingestive mechanisms to deal with protein-carbohydrate imbalances - regardless of the direction of the imbalance.

An integrative approach to dietary balance across the life course

Animals require specific blends of nutrients that vary across the life course and with circumstances, e.g., health and activity levels. Underpinning and complicating these requirements is that individual traits may be optimized on different dietary compositions leading to nutrition-mediated trade-offs among outcomes. Additionally, the food environment may constrain which nutrient mixtures are achievable. Natural selection has equipped animals for solving such multi-dimensional, dynamic challenges of nutrition, but little is understood about the details and their theoretical and practical implications. We present an integrative framework, nutritional geometry, which models complex nutritional interactions in the context of multiple nutrients and across levels of biological organization (e.g., cellular, individual, and population) and levels of analysis (e.g., mechanistic, developmental, ecological, and evolutionary). The framework is generalizable across different situations and taxa. We illustrate this using examples spanning insects to primates and settings (laboratory, and the wild), and demonstrate its relevance for human health.

B-Vitamins Influence the Consumption of Macronutrients in Honey Bees

Insects require dietary sources of B-vitamins, but relatively little is known about whether they regulate B-vitamin intake in the same way they regulate other nutrients. Honey bees meet their B-vitamin requirements mainly from the pollen they collect. Employing the geometric framework for nutrition, we found that honey bees actively regulate their vitamin intake following Bertrand's rule. We fed bees with a diet of essential amino acids (EAAs) and carbohydrate (C) to identify how the addition of B-vitamins affected the regulation of these macronutrients. In our experiments, honey bees preferred vitamins in concentrations comparable to those found in honey bee food (pollen, beebread, and royal jelly). Honey bees actively regulated niacin around an optimal value. Supplementing honey bee diets with B-vitamins influenced the amount of EAAs and carbohydrate ingested differently depending on the type of the vitamin. The impact of these vitamins was observed over the course of seven days where honey bees' mortality increased on diets of low and medium folic acid concentrations. This study provides insights into honey bee food intake regulation and the feeding preferences and sets the basis for future studies considering B-vitamins in honey bees diets.

Assassin snails (Anentome helena) as a biological model for exploring the effects of individual specialisation within generalist predators

Quantifying feeding behaviour of generalist predators at the population and individual levels is crucial for understanding the structure and functioning of food webs. Individual predator/consumer feeding niches can be significantly narrower than that of the population across animal taxa. In such species, the population of a generalist predator becomes essentially an ensemble of specialist individuals and this often highly affects the dynamics of the prey-predator interactions. Currently, few experimental systems exist that are both easily technically manipulated in a lab and are reliable to accurately assess effects of individual specialisation within generalist predators. Here we argue that a freshwater predaceous snail, Anentome helena (also known as an ‘assassin snail’), is a convenient and reliable experimental system to study feeding of a generalist predator on multiple food types which exhibits well-pronounced specialisation of foraging individuals. Using A. helena we experimentally test: (i) how relative prey abundances in the environment affect the feeding patterns, (ii) whether the feeding patterns are consistent over the duration of the experimental period, and (iii) compare the feeding niche breadth of individuals to that of the laboratory population. By offering four different prey snail species, at a range of relative abundances, we show that there are consistent patterns in feeding. Importantly, the consumption of each prey was independent of the relative abundance at which they were present. Individual predators showed selectivity to a particular prey, i.e. the population of assassin snails seems to be formed of individuals that specialise on different prey. Our findings would contribute to the recent revision and the ongoing debate on the classification of predator species into generalists and specialists.

Balancing nutrients in a toxic environment: the challenge of eating

Insect herbivores can regulate their food intake by mixing food sources with different nutrient content, but face the resulting challenge of ingesting various plant secondary metabolites. How insects deal with toxins in a complex nutrient environment is unclear. Here we investigated the influence of a classic plant secondary metabolite, allyl glucosinolate (sinigrin), and its hydrolyzed product allyl isothiocyanate (AITC), on the development of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) when fed on diets with different protein-to-carbohydrate (p : c) ratios. We also examined the effects of these toxins on larval biochemistry, by chemically analyzing the frass produced by insects feeding on the different diets. As expected, AITC had a greater negative effect than sinigrin on H. armigera life-history traits. However, AITC at low concentration appeared to have a positive effect on some traits. Both sinigrin and AITC-induced detoxification activity in the gut, and the reaction was related to diet protein concentration. High-protein diets can provide the required free amino acid, especially cysteine, needed for the detoxification process. The nutrient content of the diet influences how plant secondary metabolites are handled, and the use of artificial diets in experiments investigating the metabolic fate of plant secondary compounds needs to be carefully evaluated.

Sugar addicted in the city: impact of urbanisation on food choice and diet composition of the Eurasian red squirrel (Sciurus vulgaris)

Urban wildlife faces a great variety of human-induced habitat alterations, among others changes in resource availability and composition, often resulting in serious declines in biodiversity. Nevertheless, Eurasian red squirrels (Sciurus vulgaris) occur in high densities in urban areas and seem to benefit from supplementary feeding. However, we still lack knowledge about consequences of urbanisation on mammalian foraging behaviour and nutrient intake. Thus, we investigated body mass, food choice and diet composition in squirrels from an urban core area versus a forest population in a cafeteria experiment. Urban individuals were lower in initial body mass and condition, but consumed significantly more g and kJ per day and significantly gained weight over the course of the experiment (around 2 weeks); nevertheless, the difference in body mass and condition persisted. All squirrels preferred hazelnuts, but urban squirrels had a wider dietary range and consumed more non-natural food items. Both groups prioritised fat and there was no difference in protein intake. Urban squirrels though had a significantly higher sugar intake, mainly by eating biscuits. Our results demonstrate clear effects of urbanisation on foraging behaviour and preferences, which has the potential for nutritional mismatch or negative side effects due to consumption of non-natural food items. Our findings show that highly supplemented urban core fragments might not serve as adequate refuge for wildlife.

Behavioural and physiological regulation of protein and carbohydrates in mealworm larvae: A geometric analysis

Protein-carbohydrate regulation in the larvae of the mealworm beetle (Tenebrio molitor L.) was analyzed using the Geometric Framework for nutrition. In this study, the ingestive and post-ingestive responses were measured from T. molitor larvae that were subjected to choice and no-choice experiments. In the choice experiment, T. moitor larvae were simultaneously presented with one of two protein-biased foods (p35:c7 or p28:c5.6) and one of two carbohydrate-biased foods (p7:c35 or p5.6:c28). T. molitor larvae selected protein and carbohydrate in a ratio close to 1:1 over the first 15 days since the start of the experiment (days 0-15), but exhibited preference for carbohydrate-biased food over the next 15 days. The average protein:carbohydrate ratio selected over days 0-30 was 1:1.24. In the no-choice experiment, T. molitor larvae were restricted to one of seven foods with different protein and carbohydrate content (p0:c42, p7:c35, p14:c28, p21:c21, p28:c14, p35:c7, or p42:c0). On the p0:c42 food, consumption was greatly suppressed and no larvae completed their development. Across a range of these foods except p0:c42, T. molitor larvae consistently over-ate the surplus nutrient in the foods and showed a pattern of nutrient balancing similar to that previously described for other nutritional generalists. Despite having consumed substantially different amounts and ratios of macronutrients as larvae, T. molitor pupae in the no-choice food treatments had similar body nutrient composition, suggesting the presence of strong homeostatic regulation for body nutrient growth. Larval survivorship was significantly lower on two extremely imbalanced foods (p7:c35 and p42:0) than on more balanced foods. T. molitor larvae reared on p7:c35 suffered reduced biomass growth and delayed development compared with those on foods with higher protein content.

The proximate sources of genetic variation in body size plasticity: The relative contributions of feeding behaviour and development in Drosophila melanogaster

Body size is a key life-history trait that influences many aspects of an animal's biology and is shaped by a variety of factors, both genetic and environmental. While we know that locally-adapted populations differ in the extent to which body size responds plastically to environmental conditions like diet, we have a limited understanding of what causes these differences. We hypothesized that populations could differ in the way body size responds to nutrition either by modulating growth rate, development time, feeding rate, or a combination of the above. Using three locally-adapted populations of Drosophila melanogaster from along the east coast of Australia, we investigated body size plasticity across five different diets. We then assessed how these populations differed in feeding behaviour and developmental timing on each of the diets. We observed population-specific plastic responses to nutrition for body size and feeding rate, but not development time. However, differences in feeding rate did not fully explain the differences in the way body size responded to diet. Thus, we conclude that body size variation in locally-adapted populations is shaped by a combination of growth rate and feeding behaviour. This paves the way for further studies that explore how differences in the regulation of the genetic pathways that control feeding behaviour and growth rate contribute to population-specific responses of body size to diet.

Assessing the nutritional consequences of switching foraging behavior in wood bison

Diet is one of the most common traits used to organize species of animals into niches. For ruminant herbivores, the breadth and uniqueness of their dietary niche are placed on a spectrum from browsers that consume woody (i.e., browse) and herbaceous (i.e., forbs) plants, to grazers with graminoid-rich diets. However, seasonal changes in plant availability and quality can lead to switching of their dietary niche, even within species. In this study, we examined whether a population of wood bison (Bison bison athabascae) in northeast Alberta, Canada, seasonally switched their foraging behavior, and if so, whether this was associated with changes in nutrient acquisition. We hypothesized that bison should switch foraging behaviors from grazing in the winter when standing, dead graminoids are the only foliar plants readily available to browsing during spring and summer as nutritious and digestible foliar parts of browse and forbs become available. If bison are switching foraging strategy to maximize protein consumption, then there should be a corresponding shift in the nutritional niche. Alternatively, if bison are eating different plants, but consuming similar amounts of nutrients, then bison are switching their dietary niche to maintain a particular nutrient composition. We found wood bison were grazers in the winter and spring, but switch to a browsing during summer. However, only winter nutrient consumption of consumed plants differed significantly among seasons. Between spring and summer, bison maintained a specific nutritional composition in their diet despite compositional differences in the consumed plants. Our evidence suggests that bison are selecting plants to maintain a target macronutrient composition. We posit that herbivore's can and will switch their dietary niche to maintain a target nutrient composition.

Complex relationship between amino acids, fitness and food intake in Bombus terrestris

The ratio of amino acids to carbohydrates (AA:C) that bumble bees consume has been reported to affect their survival. However, it is unknown how dietary AA:C ratio affects other bumble bee fitness traits (e.g., fecundity, condition) and possible trade-offs between them. Moreover, while individual AAs affect phenotype in many species, the effects of AA blend on bumble bee fitness and food intake are unclear. We test how the AA:C ratio that bumble bees (Bombus terrestris) consume affects their condition (abdomen lipid and dry mass), survival following food removal, and ovarian activation. We then compare ovarian activation and food intake in bees fed identical AA:C ratios, but where the blend of AAs in diets differ, i.e., diets contained the same 10 AAs in an equimolar ratio or in the same ratio as in bee collected pollen. We found that AA:C ratio did not significantly affect survival following food removal or ovarian activation; however, high AA intake increased body mass, which is positively correlated with multiple fitness traits in bumble bees. AA blend (i.e., equimolar versus pollen) did not significantly affect overall ovarian activation or consumption of each experimental diet. However, there was an interaction between AA mix and dietary AA:C ratio affecting survival during the feeding experiment, and signs that there may have been weak, interactive effects of AA mix and AA:C ratio on food consumption. These results suggest that the effect of total AA intake on bumble bee phenotype may depend on the blend of individual AAs in experimental diets. We suggest that research exploring how AA blend affects bumble bee performance and dietary intake is warranted, and highlight that comparing research on bee nutrition is complicated by even subtle variation in experimental diet composition.

The importance of environmental microbes for Drosophila melanogaster during seasonal macronutrient variability

Experiments manipulating the nutritional environment and the associated microbiome of animals have demonstrated their importance for key fitness components. However, there is little information on how macronutrient composition and bacterial communities in natural food sources vary across seasons in nature and on how these factors affect the fitness components of insects. In this study, diet samples from an orchard compost heap, which is a natural habitat for many Drosophila species and other arthropods, were collected over 9 months covering all seasons in a temperate climate. We developed D. melanogaster on diet samples and investigated stress resistance and life-history traits as well as the microbial community of flies and compost. Nutrient and microbial community analysis of the diet samples showed marked differences in macronutrient composition and microbial community across seasons. However, except for the duration of development on these diet samples and Critical Thermal maximum, fly stress resistance and life-history traits were unaffected. The resulting differences in the fly microbial community were also more stable and less diverse than the microbial community of the diet samples. Our study suggests that when D. melanogaster are exposed to a vastly varying nutritional environment with a rich, diverse microbial community, the detrimental consequences of an unfavourable macronutrient composition are offset by the complex interactions between microbes and nutrients.

Macronutrient balancing in free-ranging populations of moose

At northern latitudes, large spatial and temporal variation in the nutritional composition of available foods poses challenges to wild herbivores trying to satisfy their nutrient requirements. Studies conducted in mostly captive settings have shown that animals from a variety of taxonomic groups deal with this challenge by adjusting the amounts and proportions of available food combinations to achieve a target nutrient balance. In this study, we used proportions-based nutritional geometry to analyze the nutritional composition of rumen samples collected in winter from 481 moose (Alces alces) in southern Sweden and examine whether free-ranging moose show comparable patterns of nutrient balancing. Our main hypothesis was that wild moose actively regulate their rumen nutrient composition to offset ecologically imposed variation in the nutritional composition of available foods. To test this, we assessed the macronutritional composition (protein, carbohydrates, and lipids) of rumen contents and commonly eaten foods, including supplementary feed, across populations with contrasting winter diets, spanning an area of approximately 10,000 km2. Our results suggest that moose balanced the macronutrient composition of their rumen, with the rumen contents having consistently similar proportional relationship between protein and nonstructural carbohydrates, despite differences in available (and eaten) foods. Furthermore, we found that rumen macronutrient balance was tightly related to ingested levels of dietary fiber (cellulose and hemicellulose), such that the greater the fiber content, the less protein was present in the rumen compared with nonstructural carbohydrates. Our results also suggest that moose benefit from access to a greater variety of trees, shrubs, herbs, and grasses, which provides them with a larger nutritional space to maneuver within. Our findings provide novel theoretical insights into a model species for ungulate nutritional ecology, while also generating data of direct relevance to wildlife and forest management, such as silvicultural or supplementary feeding practices.