Responses to nutritional challenges in ant colonies

Effects of macro- and micro-nutrients on momentary and season-long feeding responses by select species of ants

Few studies have investigated the relative contribution of specific nutrients to momentary and season-long foraging responses by ants. Using western carpenter ants, Camponotus modoc, and European fire ants, Myrmica rubra, as model species, we: (1) tested preferential consumption of various macro- and micro-nutrients; (2) compared consumption of preferred macro-nutrients; (3) investigated seasonal shifts (late May to mid-September) in nutrient preferences; and (4) tested whether nutrient preferences of C. modoc and M. rubra pertain to black garden ants, Lasius niger, and thatching ants, Formica aserva. In laboratory and field experiments, we measured nutrient consumption by weighing Eppendorf tubes containing aqueous nutrient solutions before and after feeding by ants. Laboratory colonies of C. modoc favored nitrogenous urea and essential amino acids (EAAs), whereas M. rubra colonies favored sucrose. Field colonies of C. modoc and M. rubra preferentially consumed EAAs and sucrose, respectively, with no sustained shift in preferred macro-nutrient over the course of the foraging season. The presence of a less preferred macro-nutrient in a nutrient blend did not diminish the blend's 'appeal' to foraging ants. Sucrose and EAAs singly and in combination were equally consumed by L. niger, whereas F. aserva preferred EAAs. Baits containing both sucrose and EAAs were consistently consumed by the ants studied in this project and should be considered for pest ant control.

Harvester ant colonies differ in collective behavioural plasticity to regulate water loss

Collective behavioural plasticity allows ant colonies to adjust to changing conditions. The red harvester ant (Pogonomyrmex barbatus), a desert seed-eating species, regulates foraging activity in response to water stress. Foraging ants lose water to evaporation. Reducing foraging activity in dry conditions sacrifices food intake but conserves water. Within a year, some colonies tend to reduce foraging on dry days while others do not. We examined whether these differences among colonies in collective behavioural plasticity persist from year to year. Colonies live 20-30 years with a single queen who produces successive cohorts of workers which live only a year. The humidity level at which all colonies tend to reduce foraging varies from year to year. Longitudinal observations of 95 colonies over 5 years between 2016 and 2021 showed that differences among colonies, in how they regulate foraging activity in response to day-to-day changes in humidity, persist across years. Approximately 40% of colonies consistently reduced foraging activity, year after year, on days with low daily maximum relative humidity; approximately 20% of colonies never did, foraging as much or more on dry days as on humid days. This variation among colonies may allow evolutionary rescue from drought due to climate change.

Effects of diesel exhaust particles on the health and survival of the buff-tailed bumblebee Bombus terrestris after acute and chronic oral exposure

The diesel-powered transportation sector is a major producer of environmental pollution in the form of micro- and nanoscale diesel exhaust particles (DEP). Pollinators, such as wild bees, may inhale DEP or ingest it orally through plant nectar. However, if these insects are adversely affected by DEP is largely unknown. To investigate potential health threats of DEP to pollinators, we exposed individuals of Bombus terrestris to different concentrations of DEP. We analysed the polycyclic aromatic hydrocarbons (PAH) content of DEP since these are known to elicit adverse effects on invertebrates. We investigated the dose-dependent effects of those well-characterized DEP on survival and fat body content, as a proxy for the insects' health condition, in acute and chronic oral exposure experiments. Acute oral exposure to DEP showed no dose-dependent effects on survival or fat body content of B. terrestris. However, we could show dose-dependent effects after chronic oral exposure with high doses of DEP where significantly increased mortality was observed. Further, there was no dose-dependent effect of DEP on the fat body content after exposure. Our results give insights into how the accumulation of high concentrations of DEP e.g., near heavily trafficked sites, can influence insect pollinators' health and survival.

Protein-rich diet decreases survival, but does not alter reproduction, in fertile ant workers

Aging is associated with diverse molecular processes such as oxidative damage, decrease in immunocompetence, or increase in epigenetic abnormalities, mutations, and inflammations. Many of these processes are linked to nutrient-sensing signalling pathways, suggesting that diet plays a critical role in the aging process. In fact, the protein content in the diet can affect both longevity and fecundity, but often in opposite directions. In many solitary organisms, protein-rich diets dramatically shorten lifespan, but increase egg production. We used the ant Temnothorax rugatulus to investigate the effect of the protein to carbohydrate ratio in the diet on the survival and fecundity of fertile workers. We fed colonies either a moderately high-carbohydrate or high-protein diet (1:2 and 2:1 respectively) and monitored worker survival and egg production over 9 weeks. The protein-rich diet did not alter the ability of workers to lay eggs, but reduced worker survival, suggesting that consuming large amounts of protein may shorten lifespan in fertile ant workers without promoting reproduction. Our study shows for the first time that a protein-rich diet reduces the overall fitness of fertile workers.

Floral and Bird Excreta Semiochemicals Attract Western Carpenter Ants

Ant colonies have vast and diverse nutritional needs but forager ants have limited mobility to meet these needs. Forager ants would accrue significant energy savings if they were able to sense and orient toward odor plumes of both carbohydrate and protein food sources. Moreover, if worker ants, like other flightless insects, had reduced olfactory acuity, they would not recognize the specific odor signatures of diverse carbohydrate and protein sources, but they may be able to orient toward those odorants that are shared between (macronutrient) food sources. Using the Western carpenter ant, Camponotus modoc, as a model species, we tested the hypotheses that (1) food sources rich in carbohydrates (aphid honeydew, floral nectar) and rich in proteins (bird excreta, house mouse carrion, cow liver infested or not with fly maggots) all prompt long-distance, anemotactic attraction of worker ants, and (2) attraction of ants to plant inflorescences (fireweed, Chamaenerion angustifolium; thimbleberry, Rubus parviflorus; and hardhack, Spiraea douglasii) is mediated by shared floral odorants. In moving-air Y-tube olfactometer bioassays, ants were attracted to two of four carbohydrate sources (thimbleberry and fireweed), and one of four protein sources (bird excreta). Headspace volatiles of these three attractive sources were analyzed by gas chromatography-mass spectrometry, and synthetic odor blends of thimbleberry (7 components), fireweed (23 components), and bird excreta (38 components) were prepared. In Y-tube olfactometer bioassays, synthetic blends of thimbleberry and fireweed but not of bird excreta attracted ants, indicating that only the two floral blends contained all essential attractants. A blend of components shared between thimbleberry and fireweed was not attractive to ants. Our data support the conclusion that C. modoc worker ants can sense and orient toward both carbohydrate and protein food sources. As ants were selective in their responses to carbohydrate and protein resources, it seems that they can discern between specific food odor profiles and that they have good, rather than poor, olfactory acuity.

Effect of queen number on colony-level nutrient regulation, food collection and performance in two polygynous ant species

There is growing appreciation for how social interactions influence animal foraging behavior, especially with respect to key nutrients. Ants, given their eusocial nature and ability to be reared and manipulated in the laboratory, offer unique opportunities to explore how social interactions influence nutrient regulation and related processes. At the colony-level, ants simultaneously regulate their protein and carbohydrate intake; a regulation tied to the presence of larvae. However, even though 45% of the approximately 10,000 ant species are polygynous, we know little about the influence of queen number on colony-level foraging behavior and performance. Here we explored the direct effects of queen number on colony-level protein-carbohydrate regulation, food collection, survival, and brood production in two polygynous ant species (Nylanderia fulva and Solenopsis invicta). For both species we conducted choice and no-choice experiments using small experimental colonoids (20 workers) with 0, 1, or 2 queens. Both species regulated their relative intake of protein and carbohydrate around a P1:C2 mark. However, only N. fulva responded to the addition of queens, increasing overall food collection, biasing intake towards carbohydrates, and over-collecting imbalanced foods. N. fulva also exhibited reduced survival and reproduction on protein-biased foods. In contrast, S. invicta showed no response to queen number and reduced food collection on the protein-biased diet while maintaining high survival and reproduction. Our results demonstrate the potential for queens of some ant species to impact colony-level foraging and performance, with interspecific variation likely being shaped by differences in life history traits.

All sugars ain't sweet: selection of particular mono-, di- and trisaccharides by western carpenter ants and European fire ants

Ants select sustained carbohydrate resources, such as aphid honeydew, based on many factors including sugar type, volume and concentration. We tested the hypotheses (H1-H3) that western carpenter ants, Camponotus modoc, seek honeydew excretions from Cinara splendens aphids based solely on the presence of sugar constituents (H1), prefer sugar solutions containing aphid-specific sugars (H2) and preferentially seek sugar solutions with higher sugar content (H3). We further tested the hypothesis (H4) that workers of both Ca. modoc and European fire ants, Myrmica rubra, selectively consume particular mono-, di- and trisaccharides. In choice bioassays with entire ant colonies, sugar constituents in honeydew (but not aphid-specific sugar) as well as sugar concentration affected foraging decisions by Ca. modoc. Both Ca. modoc and M. rubra foragers preferred fructose to other monosaccharides (xylose, glucose) and sucrose to other disaccharides (maltose, melibiose, trehalose). Conversely, when offered a choice between the aphid-specific trisaccharides raffinose and melezitose, Ca. modoc and M. rubra favoured raffinose and melezitose, respectively. Testing the favourite mono-, di- and trisaccharide head-to-head, both ant species favoured sucrose. While both sugar type and sugar concentration are the ultimate cause for consumption by foraging ants, strong recruitment of nest-mates to superior sources is probably the major proximate cause.

Hierarchical networks of food exchange in the black garden ant Lasius niger

In most eusocial insects, the division of labor results in relatively few individuals foraging for the entire colony. Thus, the survival of the colony depends on its efficiency in meeting the nutritional needs of all its members. Here, we characterize the network topology of a eusocial insect to understand the role and centrality of each caste in this network during the process of food dissemination. We constructed trophallaxis networks from 34 food-exchange experiments in black garden ants (Lasius niger). We tested the influence of brood and colony size on (i) global indices at the network level (i.e., efficiency, resilience, centralization, and modularity) and (ii) individual values (i.e., degree, strength, betweenness, and the clustering coefficient). Network resilience, the ratio between global efficiency and centralization, was stable with colony size but increased in the presence of broods, presumably in response to the nutritional needs of larvae. Individual metrics highlighted the major role of foragers in food dissemination. In addition, a hierarchical clustering analysis suggested that some domestics acted as intermediaries between foragers and other domestics. Networks appeared to be hierarchical rather than random or centralized exclusively around foragers. Finally, our results suggested that networks emerging from social insect interactions can improve group performance and thus colony fitness.

Best be(e) on low fat: linking nutrient perception, regulation and fitness

Preventing malnutrition through consuming nutritionally appropriate resources represents a challenge for foraging animals. This is due to often high variation in the nutritional quality of available resources. Foragers consequently need to evaluate different food sources. However, even the same food source can provide a plethora of nutritional and non-nutritional cues, which could serve for quality assessment. We show that bumblebees, Bombus terrestris, overcome this challenge by relying on lipids as nutritional cue when selecting pollen. The bees 'prioritised' lipid perception in learning experiments and avoided lipid consumption in feeding experiments, which supported survival and reproduction. In contrast, survival and reproduction were severely reduced by increased lipid contents. Our study highlights the importance of fat regulation for pollen foraging bumblebees. It also reveals that nutrient perception, nutrient regulation and reproductive fitness can be linked, which represents an effective strategy enabling quick foraging decisions that prevent malnutrition and maximise fitness.

Bumblebees adjust protein and lipid collection rules to the presence of brood

Animals have evolved foraging strategies to acquire blends of nutrients that maximize fitness traits. In social insects, nutrient regulation is complicated by the fact that few individuals, the foragers, must address the divergent nutritional needs of all colony members simultaneously, including other workers, the reproductives, and the brood. Here we used 3D nutritional geometry design to examine how bumblebee workers regulate their collection of 3 major macronutrients in the presence and absence of brood. We provided small colonies artificial nectars (liquid diets) and pollens (solid diets) varying in their compositions of proteins, lipids, and carbohydrates during 2 weeks. Colonies given a choice between nutritionally complementary diets self-selected foods to reach a target ratio of 71% proteins, 6% carbohydrates, and 23% lipids, irrespective of the presence of brood. When confined to a single nutritionally imbalanced solid diet, colonies without brood regulated lipid collection and over-collected protein relative to this target ratio, whereas colonies with brood regulated both lipid and protein collection. This brood effect on the regulation of nutrient collection by workers suggests that protein levels are critical for larval development. Our results highlight the importance of considering bee nutrition as a multidimensional phenomenon to better assess the effects of environmental impoverishment and malnutrition on population declines.

Using nutritional geometry to define the fundamental macronutrient niche of the widespread invasive ant Monomorium pharaonis

The emerging field of nutritional geometry (NG) provides powerful new approaches to test whether and how organisms prioritize specific nutritional blends when consuming chemically complex foods. NG approaches can thus help move beyond food-level estimates of diet breadth to predict invasive success, for instance by revealing narrow nutritional niches if broad diets are actually composed of nutritionally similar foods. We used two NG paradigms to provide different, but complementary insights into nutrient regulation strategies and test a hypothesis of extreme nutritional generalism in colony propagules of the globally distributed invasive ant Monomorium pharaonis. First, in two dimensions (protein:carbohydrates; P:C), M. pharaonis colonies consistently defended a slightly carbohydrate-biased intake target, while using a generalist equal-distance strategy of collectively overharvesting both protein and carbohydrates to reach this target when confined to imbalanced P:C diets. Second, a recently developed right-angled mixture triangle method enabled us to define the fundamental niche breadth in three dimensions (protein:carbohydrates:lipid, P:C:L). We found that colonies navigated the P:C:L landscape, in part, to mediate a tradeoff between worker survival (maximized on high-carbohydrate diets) and brood production (maximized on high-protein diets). Colonies further appeared unable to avoid this tradeoff by consuming extra lipids when the other nutrients were limiting. Colonies also did not rely on nutrient regulation inside their nests, as they did not hoard or scatter fractions of harvested diets to adjust the nutritional blends they consumed. These complementary NG approaches highlight that even the most successful invasive species with broad fundamental macronutrient niches must navigate complex multidimensional nutritional landscapes to acquire limiting macronutrients and overcome developmental constraints as small propagules.

Effect of temperature on survival and cuticular composition of three different ant species

Climatic factors, such as temperature variation, interfere with the survival of insects. To respond to these variations, insects have some specific characteristics. These include water content of the body, thickness of the lipid layer, as well as the qualitative and quantitative characteristics of cuticular chemical components. This study hypothesizes that different ant species respond to temperature changes in different ways and that such differences may be associated with cuticle hydrocarbons (CHCs) and fatty acids. As model ant species, Atta sexdens, Odontomachus bauri and Ectatomma brunneum were used for experimental analyses. Ants were submitted to a water bath for 5 h at different temperatures, and their CHCs and fatty acids were identified and quantified, followed by correlating these chemical compounds with temperature variations and the survival. Temperatures below 30 °C did not affect the survival of the three species. E. brunneum had a higher percentage of survival at temperatures above 30 °C. O. bauri was the most sensitive species with 100% mortality at 40 °C. Survival was found to be unrelated to any of the identified fatty acids. However, CHCs underwent significant quantitative and qualitative variation, as shown by an increased percentage of CHCs with longer chain length of linear alkanes at temperatures above 30 °C. These increase enables these ants to maintain the integrity of their cuticle and survive at temperatures above 30 °C. It can be concluded that the forager ants studied respond differently to temperature variation and that changes in the conformation of CHCs are in line with the ecological characteristics of the different studied species because, they vary in terms of diurnal/nocturnal foraging and types of environments foraged. Among the three species, E. brunneum foragers were found to be more active under adverse conditions and more tolerant to temperature variation with the correspondingly appropriate changes in CHCs composition.

Individual crop loads provide local control for collective food intake in ant colonies

Nutritional regulation by ants emerges from a distributed process: food is collected by a small fraction of workers, stored within the crops of individuals, and spread via local ant-to-ant interactions. The precise individual-level underpinnings of this collective regulation have remained unclear mainly due to difficulties in measuring food within ants' crops. Here we image fluorescent liquid food in individually tagged Camponotus sanctus ants and track the real-time food flow from foragers to their gradually satiating colonies. We show how the feedback between colony satiation level and food inflow is mediated by individual crop loads; specifically, the crop loads of recipient ants control food flow rates, while those of foragers regulate the frequency of foraging-trips. Interestingly, these effects do not rise from pure physical limitations of crop capacity. Our findings suggest that the emergence of food intake regulation does not require individual foragers to assess the global state of the colony.

Onset of fights and mutual assessment in ant founding queens

In animals, the progress and outcome of contests can be influenced by an individual's own condition, their opponent's condition or a combination of the two. The use of chemical information to assess the quality of rivals has been underestimated despite its central role in the regulation of social interactions in many taxa. Here, we studied pairwise contests between founding queens of the ant Lasius niger to investigate whether the decision to engage in agonistic interactions relies on self-assessment or mutual assessment. Queens modulated their aggressive behaviours depending on both their own status and their opponent's status. We found no influence of lipid stores or size on the onset of fights. However, differences in cuticular chemical signatures linked to fertility status accurately predicted the probability of behaving aggressively in pairs. Our study thus suggests that ant queens could rely on mutual assessment via chemical cues to make informed decisions about fight initiation.

Parsing the life-shortening effects of dietary protein: effects of individual amino acids

High-protein diets shorten lifespan in many organisms. Is it because protein digestion is energetically costly or because the final products (the amino acids) are harmful? To answer this question while circumventing the life-history trade-off between reproduction and longevity, we fed sterile ant workers on diets based on whole proteins or free amino acids. We found that (i) free amino acids shortened lifespan even more than proteins; (ii) the higher the amino acid-to-carbohydrate ratio, the shorter ants lived and the lower their lipid reserves; (iii) for the same amino acid-to-carbohydrate ratio, ants eating free amino acids had more lipid reserves than those eating whole proteins; and (iv) on whole protein diets, ants seem to regulate food intake by prioritizing sugar, while on free amino acid diets, they seem to prioritize amino acids. To test the effect of the amino acid profile, we tested diets containing proportions of each amino acid that matched the ant's exome; surprisingly, longevity was unaffected by this change. We further tested diets with all amino acids under-represented except one, finding that methionine, serine, threonine and phenylalanine are especially harmful. All together, our results show certain amino acids are key elements behind the high-protein diet reduction in lifespan.

Resistance to nutritional stress in ants: when being fat is advantageous

In ants, nutrient acquisition for the whole colony relies on a minority of workers, the foragers, which are often old and lean. Some studies have shown that the link between age, physiology and foraging activity is more flexible than once thought, especially in response to colony or environmental perturbations. This great plasticity offers the intriguing possibility to disentangle the effect of age, behaviour and physiology on the ants' abilities to cope with nutritional stresses. In this paper, we first looked at the capacity of groups of foragers and inner-nest workers to resist starvation and macronutrient imbalance. Second, we investigated whether behavioural task reversion modified the tolerance to nutritional stresses and by extension, changed mortality rate. We found that inner-nest workers live longer than foragers under nutritional stresses but not under optimal conditions. The reversion from foraging to inner-nest activities is followed by an increase in fat content and longevity. Finally, we demonstrated that changes in fat content associated with behavioural transition are highly flexible and strongly correlated to tolerance of nutritional stress. Our results have considerable implications for our understanding of the population dynamics of social insects under adverse nutritional conditions.