Sex differences in energy expenditure in non-human primates

Science-based suggestions to save the world's rarest primate species Nomascus hainanus

Conservation practices for extremely small populations must be grounded in solid science to prevent extinction. Hainan gibbon (Nomascus hainanus) is the world's rarest primate species; however, insufficient data on its habitat suitability and genetic status impede evidence-based decisions for habitat restoration. Here, we conducted a comprehensive analysis of Hainan gibbons' energy intake and expenditure, reproductive parameters, and genetic diversity based on field research (March 2021 to December 2022) and long-term historical data (2003 to 2024). By comparing our results with those of captive gibbons and other free-feeding captive primates, we found that Hainan gibbons can obtain sufficient energy for growth and reproduction in their existing habitats. Furthermore, we identified an additional D-loop haplotype indicating that the current population is more genetically diverse than previously thought. However, recently formed adult male-female pairs are increasingly related, signaling a high risk for inbreeding within this small population. Based on these findings, we highlight an urgent need to expand available habitat by building corridors.

Roadside Dining: The Collective Movement Behavior of Sulawesi Moor Macaques in a Provisioning Context

How group-living primates come to a consensus about navigating their environment is a result of their decision-making processes. Although decision-making has been examined in several primate taxa, it remains underexplored for primates living in anthropogenic landscapes. To shed light on consensus decision-making and flexibility in this process, we examined collective movement behavior in a group of wild moor macaques (Macaca maura) experiencing a risk-reward tradeoff as a result of roadside provisioning within Bantimurung Bulusaraung National Park in South Sulawesi, Indonesia. Our goal was to determine whether individual characteristics (e.g., sex, dominance rank, and/or social network centrality) predict the likelihood of initiating a collective movement and if the opportunity to receive food provisions along the road alters these patterns. Using the all-occurrences method, we recorded the location, time, and identity of initiators and followers of each collective movement observed from April to June 2023 (N = 61). We used conditional logistic regression models to examine which individual characteristics predicted initiation overall and based on two destination categories: forest- and road-directed collective movements. Initiation was distributed amongst most of the group, indicating a partially-shared decision-making style. Overall, adult males were more likely to initiate collective movements than adult females. However, for collective movements directed toward the risky roadside, dominance, rather than sex, was a better predictor of initiation, with higher ranked individuals being more likely to initiate collective movements. Examining the decision-making processes in this species through collective movements can provide insight into how primates come to a consensus and the extent to which anthropogenic factors shape these processes. By shedding light on how moor macaques navigate the risk-reward tradeoff at this site, our results can also inform the management of human-macaque interfaces.

Sex-specific efficacy of deslorelin in downregulating reproductive activity in the grey mouse lemur (Microcebus murinus)

Deslorelin is a GnRH agonist used in veterinary medicine to temporarily inhibit reproduction in domestic animals and is sometimes tested in captive species in zoo to control population or tame aggressive behaviours in males. However, some studies have revealed the inefficacy of deslorelin specifically in males, contrary to females that follow a classic long-term inhibition of the reproductive hypothalamic-pituitary axis through sexual steroid negative feedback. We implanted 5 males and 6 females grey mouse lemurs (Microcebus murinus), long-day breeders that display a complete inhibition of the reproductive system during winter, at the end of the short-day period, a few weeks before the breeding season. Contrary to females, which exhibited a classic inhibitory response to deslorelin, males' testosterone levels increased as well as their testis size, which suggests a sex-specific sensitivity to the negative feedback of sexual steroids before the mating period. We propose that this sex-imbalance is related to the different life-history of males as opposed to females concerning reproductive tasks and behaviour.

Glucocorticoids and behavior in non-human primates: A meta-analytic approach to unveil potential coping mechanisms

Glucocorticoids (GCs) mediate responses to energetic and psychosocial challenges and are associated with behavioral adjustments that form part of an adaptive vertebrate stress response. GCs and behavior can indirectly influence each other, leading to either an intensification or attenuation of stress responses. Exploring these GC-behavior relationships may offer insights into the beneficial aspects of behavior and help identify coping mechanisms that potentially enhance individual fitness. We conducted a systematic review of the relationship between GCs and several behavioral traits, as described in the literature on captive and wild primates, and evaluated the effect of different categorical factors on these relationships using a meta-analytic approach. According to the type of behavior, we grouped statistical measures into affiliative, agonistic, anxiety-like, and foraging domains which were further differentiated into behavioral subgroups. We also categorized measures based on setting, method, sex and age of individuals, and sample matrix involved in each primary study. Overall, we found that some affiliative and foraging behaviors are associated with lower GC levels, while agonistic and anxiety-like behaviors are linked to higher GC levels. Specifically, non-sexual affiliation and energetically inexpensive activities were negatively related to GCs. In contrast, inter- and intragroup aggression, noncommunicative and self-directed behaviors, and energetically expensive activities were positively related to GCs. By demonstrating how certain social, ecological and intrinsic factors affect the GC-behavior relationships, our study helps elucidate the contexts that may alleviate or intensify the stress responses in non-human primates.

Leadership and the finder's advantage in mantled howler monkeys

Leading collective movements and arriving first at feeding sites may improve food acquisition. Specifically, the first individual to discover and exploit a feeding site may gain a feeding advantage known as the "finder's advantage." The aim of this research was to verify if the probability of leading group movements to feeding sites in mantled howler monkeys (Alouatta palliata) varied by sex and reproductive status, and whether finders had higher foraging success than followers. We studied 18 adult individuals from two groups in La Flor de Catemaco over a year (978 h), and sampled group movements (n = 211) and foraging behavior (n = 215 feeding episodes). Gestating females were leaders and finders of group movements to feeding sites more often than expected but were also replaced in the leading position more frequently than individuals of other sex/reproductive states. Feeding behavior was not influenced by the order of arrival at feeding sites per se, but gestating females had higher food intake rate, bite rate, and feeding time when arriving earlier (i.e., occupying front group positions) than later. Therefore, leadership and the finder's advantage occur in this species and are probably employed by gestating females to maintain their energetic condition.

Activity budget and gut microbiota stability and flexibility across reproductive states in wild capuchin monkeys in a seasonal tropical dry forest

BACKGROUND

Energy demands associated with pregnancy and lactation are significant forces in mammalian evolution. To mitigate increased energy costs associated with reproduction, female mammals have evolved behavioural and physiological responses. Some species alter activity to conserve energy during pregnancy and lactation, while others experience changes in metabolism and fat deposition. Restructuring of gut microbiota with shifting reproductive states may also help females increase the energy gained from foods, especially during pregnancy. The goal of this study was to examine the relationships among behaviour, gut microbiota composition, and reproductive state in a wild, non-human primate to better understand reproductive ecology. We combined life history data with > 13,000 behavioural scans and 298 fecal samples collected longitudinally across multiple years from 33 white-faced capuchin monkey (Cebus imitator) females. We sequenced the V4 region of the 16S rRNA gene and used the DADA2 pipeline to analyze microbial diversity. We used PICRUSt2 to assess putative functions.

RESULTS

Reproductive state explained some variation in activity, but overall resting behaviours were relatively stable across pregnancy and lactation. Foraging was less frequent among females in the early stage of nursing compared to the cycling stage, though otherwise remained at comparable levels. Maximum temperature was a strong, significantly positive predictor of resting, while social dominance had a small but significantly negative effect on resting. Ecological variables such as available fruit biomass and rainfall had a small but significantly positive effects on measures of foraging time. Gut microbial community structure, including richness, alpha diversity, and beta diversity remained stable across the reproductive cycle. In pairwise comparisons, pregnant females exhibited increased relative abundances of multiple microbial ASVs, suggesting small changes in relation to reproductive state. Reproductive state was not linked to differential abundance of putative metabolic pathways.

CONCLUSIONS

Previous data suggest that activity budget and the gut microbiome shifts considerably during reproduction. The present study finds that both activity and gut microbial communities are less associated with reproduction compared to other predictors, including ecological contexts. This suggests that behavioural flexibility and gut microbial community plasticity is contrained by ecological factors in this population. These data contribute to a broader understanding of plasticity and stability in response to physiological shifts associated with mammalian reproduction.

Reward maximization assessed using a sequential patch depletion task in a large sample of heterogeneous stock rats

Choice behavior requires animals to evaluate both short- and long-term advantages and disadvantages of all potential alternatives. Impulsive choice is traditionally measured in laboratory tasks by utilizing delay discounting (DD), a paradigm that offers a choice between a smaller immediate reward, or a larger more delayed reward. This study tested a large sample of Heterogeneous Stock (HS) male (n = 896) and female (n = 898) rats, part of a larger genetic study, to investigate whether measures of reward maximization overlapped with traditional models of delay discounting via the patch depletion model using a Sequential Patch Depletion procedure. In this task, rats were offered a concurrent choice between two water "patches" and could elect to "stay" in the current patch or "leave" for an alternative patch. Staying in the current patch resulted in decreasing subsequent reward magnitudes, whereas the choice to leave a patch was followed by a delay and a resetting to the maximum reward magnitude. Based on the delay in a given session, different visit durations were necessary to obtain the maximum number of rewards. Visit duration may be analogous to an indifference point in traditional DD tasks. Males and females did not significantly differ on traditional measures of DD (e.g. delay gradient; AUC). When examining measures of patch utilization, females made fewer patch changes at all delays and spent more time in the patch before leaving for the alternative patch compared to males. Consistent with this, there was some evidence that females deviated from reward maximization more than males. However, when controlling for body weight, females had a higher normalized rate of reinforcement than males. Measures of reward maximization were only weakly associated with traditional DD measures and may represent distinctive underlying processes. Taken together, females performance differed from males with regard to reward maximization that were not observed utilizing traditional measures of DD, suggesting that the patch depletion model was more sensitive to modest sex differences when compared to traditional DD measures in a large sample of HS rats.

Quantifying allo-grooming in wild chacma baboons (Papio ursinus) using tri-axial acceleration data and machine learning

Quantification of activity budgets is pivotal for understanding how animals respond to changes in their environment. Social grooming is a key activity that underpins various social processes with consequences for health and fitness. Traditional methods use direct (focal) observations to calculate grooming rates, providing systematic but sparse data. Accelerometers, in contrast, can quantify activity budgets continuously but have not been used to quantify social grooming. We test whether grooming can be accurately identified using machine learning (random forest model) trained on labelled acceleration data from wild chacma baboons (Papio ursinus). We successfully identified giving and receiving grooming with high precision (81% and 91%) and recall (87% and 79%). Giving grooming was associated with a distinct rhythmical signal along the surge axis. Receiving grooming had similar acceleration signals to resting, and thus was more difficult to assign. We applied our machine learning model to n = 680 collar data days from n = 12 baboons and found that grooming rates obtained from accelerometers were significantly and positively correlated with direct observation rates for giving but not receiving grooming. The ability to collect continuous grooming data in wild populations will allow researchers to re-examine and expand upon long-standing questions regarding the formation and function of grooming bonds.

Home alone: A population neuroscience investigation of brain morphology substrates

As a social species, ready exchange with peers is a pivotal asset - our "social capital". Yet, single-person households have come to pervade metropolitan cities worldwide, with unknown consequences in the long run. Here, we systematically explore the morphological manifestations associated with singular living in ∼40,000 UK Biobank participants. The uncovered population-level signature spotlights the highly associative default mode network, in addition to findings such as in the amygdala central, cortical and corticoamygdaloid nuclei groups, as well as the hippocampal fimbria and dentate gyrus. Both positive effects, equating to greater gray matter volume associated with living alone, and negative effects, which can be interpreted as greater gray matter associations with not living alone, were found across the cortex and subcortical structures Sex-stratified analyses revealed male-specific neural substrates, including somatomotor, saliency and visual systems, while female-specific neural substrates centered on the dorsomedial prefrontal cortex. In line with our demographic profiling results, the discovered neural pattern of living alone is potentially linked to alcohol and tobacco consumption, anxiety, sleep quality as well as daily TV watching. The persistent trend for solitary living will require new answers from public-health decision makers. SIGNIFICANCE STATEMENT: Living alone has profound consequences for mental and physical health. Despite this, there has been a rapid increase in single-person households worldwide, with the long-term consequences yet unknown. In the largest study of its kind, we investigate how the objective lack of everyday social interaction, through living alone, manifests in the brain. Our population neuroscience approach uncovered a gray matter signature that converged on the 'default network', alongside targeted subcortical, sex and demographic profiling analyses. The human urge for social relationships is highlighted by the evolving COVID-19 pandemic. Better understanding of how social isolation relates to the brain will influence health and social policy decision-making of pandemic planning, as well as social interventions in light of global shifts in houseful structures.

A systematic review of sex differences in rough and tumble play across non-human mammals

Abstract

It is widely believed that juvenile male mammals typically engage in higher rates of rough and tumble play (RTP) than do females, in preparation for adult roles involving intense physical competition between males. The consistency of this sex difference across diverse mammalian species has, however, not yet been systematically investigated, limiting our current understanding of its possible adaptive function. This review uses narrative synthesis to (i) evaluate the ubiquity of male-biased RTP across non-human mammals, (ii) identify patterns of variation within and between taxonomic groups, and (iii) propose possible predictors of variation in these differences, including methodological and socio-ecological factors, for investigation by future studies. We find that most species studied do exhibit higher rates or RTP in males than females, while female-biased RTP is rare. Sex differences are smaller and less consistent than expected, with many studies finding similar rates of RTP in males and females. We identify multiple potential socio-ecological predictors of variation in sex differences in RTP, such as intrasexual competition and dietary niche. However, variation is not strongly phylogenetically patterned, suggesting that methodological and environmental factors, such as sample size and play partner availability, are important to consider in future comparative analyses.

Significance statement

Rough and tumble play (RTP) is thought to be vital for developing physical skills necessary for aggressive competition in adulthood, explaining an apparently widespread sex difference in RTP in mammals whereby immature males are more likely to engage in this behaviour than females. However, no prior study has systematically investigated the extent to which a male bias in RTP is consistent across diverse mammalian species. We find that although RTP is commonly male biased, findings were highly variable both within- and between-species, and equal participation in RTP by males and females is more common than widely assumed. Our review suggests several potential predictors of variation in sex differences in RTP, particularly levels of intrasexual competition in both males and females. However, our findings also suggest the importance of considering methodological in addition to socio-ecological factors for future research.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Variability in energy expenditure is much greater in males than females

In mammals, trait variation is often reported to be greater among males than females. However, to date, mainly only morphological traits have been studied. Energy expenditure represents the metabolic costs of multiple physical, physiological, and behavioral traits. Energy expenditure could exhibit particularly high greater male variation through a cumulative effect if those traits mostly exhibit greater male variation, or a lack of greater male variation if many of them do not. Sex differences in energy expenditure variation have been little explored. We analyzed a large database on energy expenditure in adult humans (1494 males and 3108 females) to investigate whether humans have evolved sex differences in the degree of interindividual variation in energy expenditure. We found that, even when statistically comparing males and females of the same age, height, and body composition, there is much more variation in total, activity, and basal energy expenditure among males. However, with aging, variation in total energy expenditure decreases, and because this happens more rapidly in males, the magnitude of greater male variation, though still large, is attenuated in older age groups. Considerably greater male variation in both total and activity energy expenditure could be explained by greater male variation in levels of daily activity. The considerably greater male variation in basal energy expenditure is remarkable and may be explained, at least in part, by greater male variation in the size of energy-demanding organs. If energy expenditure is a trait that is of indirect interest to females when choosing a sexual partner, this would suggest that energy expenditure is under sexual selection. However, we present a novel energetics model demonstrating that it is also possible that females have been under stabilizing selection pressure for an intermediate basal energy expenditure to maximize energy available for reproduction.

Social complexity as a driving force of gut microbiota exchange among conspecific hosts in non-human primates

The emergent concept of the social microbiome implies a view of a highly connected biological world, in which microbial interchange across organisms may be influenced by social and ecological connections occurring at different levels of biological organization. We explore this idea reviewing evidence of whether increasing social complexity in primate societies is associated with both higher diversity and greater similarity in the composition of the gut microbiota. By proposing a series of predictions regarding such relationship, we evaluate the existence of a link between gut microbiota and primate social behavior. Overall, we find that enough empirical evidence already supports these predictions. Nonetheless, we conclude that studies with the necessary, sufficient, explicit, and available evidence are still scarce. Therefore, we reflect on the benefit of founding future analyses on the utility of social complexity as a theoretical framework.

Stone tools improve diet quality in wild monkeys

Tool use is a fundamental feature of human evolution. Stone tools are in the archaeological record from 3.4 Ma, even before Homo,¹ and the use of stone tools probably predated the split between hominins and panins.² Using tools (hereafter, tooling cf Fragaszy and Mangalam³) is hypothesized to have improved hominins' foraging efficiency or access to high-quality foods.^4-7 This hypothesis is supported if feeding with tools positively contributes to diet quality in extant non-human primates or if foraging efficiency is increased by tooling. However, the contribution of tooling to non-human primates' foraging success has never been investigated through a direct analysis of nutritional ecology.^8,9 We used multi-dimensional nutritional geometry to analyze energy and macronutrients (nonstructural carbohydrates, lipids, and protein) in the diets of wild capuchin monkeys (Sapajus libidinous) that routinely crack palm nuts with stone hammers.^10,11 We show that eating nuts obtained through tooling helps monkeys to achieve more consistent dietary intakes. Tooling increased the net energy gain by 50% and decreased the proportion of fiber ingested by 7%. Tooling also increased the daily non-protein energy intake. By contrast, protein intake remained constant across foraging days, suggesting a pattern of macronutrient regulation called protein prioritization, which is also found in contemporary humans.^8,9 In addition, tooling reduced dispersion in the ratio of protein to non-protein energy, suggesting a role in macronutrient balancing. Our findings suggest that tooling prior to tool making could have substantially increased the nutritional security of ancestral hominins, sowing the seeds for cultural development.^5,7 VIDEO ABSTRACT.

Free hand hitting of stone-like objects in wild gorillas

The earliest stone tool types, sharp flakes knapped from stone cores, are assumed to have played a crucial role in human cognitive evolution. Flaked stone tools have been observed to be accidentally produced when wild monkeys use handheld stones as tools. Holding a stone core in hand and hitting it with another in the absence of flaking, free hand hitting, has been considered a requirement for producing sharp stone flakes by hitting stone on stone, free hand percussion. We report on five observations of free hand hitting behavior in two wild western gorillas, using stone-like objects (pieces of termite mound). Gorillas are therefore the second non-human lineage primate showing free-hand hitting behavior in the wild, and ours is the first report for free hand hitting behavior in wild apes. This study helps to shed light on the morphofunctional and cognitive requirements for the emergence of stone tool production as it shows that a prerequisite for free hand percussion (namely, free hand hitting) is part of the spontaneous behavioral repertoire of one of humans' closest relatives (gorillas). However, the ability to combine free hand hitting with the force, precision, and accuracy needed to facilitate conchoidal fracture in free hand percussion may still have been a critical watershed for hominin evolution.

Energetics and Water Flux in the Subterranean Rodent Family Bathyergidae

The doubly labeled water (DLW) technique and indirect calorimetry enable measurement of an animal’s daily energy expenditure (DEE, kJ/day), resting metabolic rate (RMR, kJ/d), sustained metabolic scope (SusMS), body fat content (BF, %) as well as water turnover (WTO, ml/day), and water economy index (ml/kJ). Small mammals have been the primary focus of many of the DLW studies to date. From large multi-species analyses of the energetics and water flux of aboveground small mammals, well-defined trends have been observed. These trends mainly refer to an adaptive advantage for lower RMR, DEE, SusMS, WTO and WEI in more ariddwelling animals to increase water and energy savings under low and unpredictable resource availability. The study of the subterranean rodent family Bathyergidae (African mole-rats) has been of particular interest with regards to field metabolic rate and metabolic studies. Although a great deal of research has been conducted on the Bathyergidae, a complete overview and multi-species analysis of the energetics and water flux of this family is lacking. Consequently, we assessed DEE, RMR, SusMS, BF, WTO and WEI across several different species of bathyergids from various climatic regions, and compared these to the established patterns of energetics and water flux for aboveground rodents. There was notable variation across the Bathyergidae inhabiting areas with different aridities, often contrary to the variations observed in above-ground species. These include increased DEE and WEI in arid-dwelling bathyergid species. While the climate was not a clear factor when predicting the SusMS of a bathyergid species, rather the degree of group living was a strong driver of SusMS, with solitary species possessing the highest SusMS compared to the socially living species. We conclude that the constraints of the underground lifestyle and the consequent spectrum of social behaviors possessed by the family Bathyergidae are most likely to be more crucial to their energetics and water flux than their habitat; however other important unstudied factors may still be at play. More so, this study provides evidence that often unreported parameters, measured through use of the DLW technique (such as BF and WEI) can enable species to be identified that might be at particular risk to climate change.

All-You-Can-Eat: Influence of Proximity to Maize Gardens on the Wild Diet and the Forest Activities of the Sebitoli Chimpanzee Community in Kibale National Park

Simple Summary

Understanding the resilience of primate populations to the threat of agricultural expansion is critical for effective conservation. Based on individual monitoring from morning to evening of wild chimpanzees in and around a protected area, we showed that the availability of maize at the forest edge had little effect on their activity budget by less resting and no impact on their wild diet and energy expenditure. In this area, large, caloric wild fruits are available year-round, and we observed no behavioral or dietary changes regarding wild resource availability either. Thus, the chimpanzees consume maize opportunistically as a bonus treat in their diet, and the presence of this nutritious resource does not seem to affect their role in seed dispersal and forest regeneration.

Abstract

Frugivorous primates have developed several strategies to deal with wild fruit scarcity, such as modifying their activity budget or enlarging their diet. Agricultural expansion threatens primate habitats and populations (e.g., disease transmission, agrochemical exposure), but it also increases crop feeding opportunities. We aimed at understanding whether maize presence close to the natural habitat of chimpanzees, a threatened species, would lead to significant behavioral modifications. We monitored 20 chimpanzees over 37 months in Kibale National Park, Uganda, with maize gardens at the forest edge. Based on focal nest-to-nest data, we analyzed their diet, activity budget, and energy balance depending on wild fruit and maize availability. We found that the Sebitoli area is a highly nutritive habitat for chimpanzees, with large and caloric wild fruits available all year long. The chimpanzees opportunistically consume maize and exploit it by resting less during maize season. However, no significant variation was found in daily paths and energy expenditures according to maize availability. No behavioral or energy modification was observed regarding wild resources either. Despite the availability of nutritious domestic resources, chimpanzees still exploit wild fruits and do not limit their movements. Thus, their contribution to seed dispersal and forest regeneration in this area is not affected.

The energetics of uniquely human subsistence strategies

[Figure: see text].

Parental reactions to a dying marmoset infant: conditional investment by the mother, but not the father

The reproductive costs of cooperatively breeding callitrichid mothers are remarkable, and they have to rely on fathers and other group members to raise their offspring. Consequently, maternal responsiveness to and investment in infants tends to be conditional, and especially sensitive to infant cues and signals of vigour. Since fathers do not bear the same excessive reproductive costs, their threshold to invest in a dying immature may be lower than in mothers. We present an anecdotal report of reactions of a first-time breeding pair of captive common marmosets (Callithrix jacchus) to their dying infant. We found a male bias in all interactions with the dying infant that did not show typical cues of infant vigour. These results show that the dying infant elicited more investment by the father than the mother. Because of this conditional maternal investment, infants of cooperatively breeding primates may be under selection to advertise their viability, in particular to their mothers.

Costs of male infanticide for female capuchins: When does an adaptive male reproductive strategy become costly for females and detrimental to population viability?

OBJECTIVES

Infanticide in white-faced capuchins (Cebus capucinus imitator) typically occurs in association with alpha male replacements (AMRs). Although infanticide is likely adaptive for males, it imposes costs on females that are difficult to quantify without long-term demographic data. Here we investigate effects of AMRs and infanticide on female reproductive success and how these costs affect capuchin groups. We investigate (1) effects of AMR frequency on the production of surviving infants; (2) energetic and (3) temporal "opportunity costs" of infant loss; and (4) how AMR frequency impacts capuchin group sizes.

MATERIALS AND METHODS

We censused six groups (7-33 years/group, 74 adult females). We modeled surviving infant production in relation to AMR. We estimated a female's energy requirements for lost infants and the temporal cost relative to the median reproductive window. We simulated how varying AMR rates would affect future capuchin group sizes.

RESULTS

Females exposed to more frequent AMR tended to produce fewer surviving offspring. We estimate the average lost infant requires approximately 33% additional energy intake for its mother and represents 10% of the average reproductive opportunity window available to females. Simulated populations remain viable at the observed rate of AMR occurrence but decrease in size at even slightly higher rates.

DISCUSSION

While infanticide is adaptive for males, for females it affects lifetime reproductive success and imposes energetic and opportunity costs. Although capuchin populations have evolved with AMRs and infanticide, small increases in AMR frequency may lead to population decline/extinction. Infanticide likely plays a large role in population maintenance for capuchins.

The development and maintenance of sex differences in dietary breadth and complexity in Bornean orangutans

ABSTRACT

Orangutans show a pronounced sexual dimorphism, with flanged males (i.e., males with fully grown secondary sexual characteristics) reaching twice the size of adult females. Furthermore, adult orangutans show sex-specific dispersal and activity patterns. This study investigates sex differences in adult foraging behavior and sheds light on how these differences develop in immatures. We analyzed 11 years of feeding data on ten adult female, seven flanged male, and 14 immature Bornean orangutans (Pongo pygmaeus wurmbii) at Tuanan in Central Kalimantan, Indonesia. We found that the diets of the adult females were significantly broader and required more processing steps before ingestion than the diets of flanged males. We also found evidence for a similar difference in overall diet repertoire sizes. For the immatures, we found that whereas females reached 100% of their mothers' diet spectrum size by the age of weaning, males reached only around 80%. From the age of 4 years on (i.e., years before being weaned) females had significantly broader daily diets than males. We found no difference in daily or overall diet processing intensity of immature males and females but found preliminary evidence that immature males included fewer items of their mother's diet in their own diets that were processing-intensive. Overall, our results suggest that by eating a broader variety and more complex to process food items, female orangutans go to greater lengths to achieve a balanced diet than males do. These behavioral differences are not just apparent in adult foraging behavior but also reflected in immature development from an early age on.

SIGNIFICANCE STATEMENT

In many species, males and females have different nutritional needs and are thus expected to show sex-specific foraging behavior. Sex differences in several aspects of foraging behavior have been found in various species, but it remains largely unclear when and how those develop during ontogeny, which is especially relevant for long-lived altricial species that learn foraging skills over many years. In our study, we analyzed a cross-sectional and longitudinal data set containing more than 750,000 feeding events of adult and immature Bornean orangutans (Pongo pygmaeus wurmbii). We found that adult females had significantly broader and more complex diets than males. We also found that these differences started to develop during infancy, suggesting that immature orangutans prepare for their sex-specific foraging niches long before those become physiologically relevant while they are still in constant association with their mothers and before being frequently exposed to other role models.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00265-021-03014-3.

Aggression, glucocorticoids, and the chronic costs of status competition for wild male chimpanzees

Across vertebrates, high social status affords preferential access to resources, and is expected to correlate positively with health and longevity. Increasing evidence, however, suggests that although dominant females generally enjoy reduced exposure to physiological and psychosocial stressors, dominant males do not. Here we test the hypothesis that costly mating competition by high-ranking males results in chronic, potentially harmful elevations in glucocorticoid production. We examined urinary glucocorticoids (n = 8029 samples) in a 20-year longitudinal study of wild male chimpanzees (n = 20 adults) in the Kanyawara community of Kibale National Park, Uganda. We tested whether glucocorticoid production was associated with dominance rank in the long term, and with mating competition and dominance instability in the short term. Using mixed models, we found that both male aggression and glucocorticoid excretion increased when the dominance hierarchy was unstable, and when parous females were sexually available. Glucocorticoid excretion was positively associated with male rank in stable and unstable hierarchies, and in mating and non-mating contexts. Glucorticoids increased with both giving and receiving aggression, but giving aggression was the primary mechanism linking elevated glucocorticoids with high rank. Glucocorticoids also increased with age. Together these results show that investment in male-male competition increases cumulative exposure to glucocorticoids, suggesting a long-term tradeoff with health that may constrain the ability to maintain high status across the life course. Our data suggest that the relationship between social rank and glucocorticoid production often differs in males and females owing to sex differences in the operation of sexual selection.

Variation in primate decision-making under uncertainty and the roots of human economic behaviour

Uncertainty is a ubiquitous component of human economic behaviour, yet people can vary in their preferences for risk across populations, individuals and different points in time. As uncertainty also characterizes many aspects of animal decision-making, comparative research can help evaluate different potential mechanisms that generate this variation, including the role of biological differences or maturational change versus cultural learning, as well as identify human-unique components of economic decision-making. Here, we examine decision-making under risk across non-human primates, our closest relatives. We first review theoretical approaches and current methods for understanding decision-making in animals. We then assess the current evidence for variation in animal preferences between species and populations, between individuals based on personality, sex and age, and finally, between different contexts and individual states. We then use these primate data to evaluate the processes that can shape human decision-making strategies and identify the primate foundations of human economic behaviour. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

Variation of Gut Microbiome in Free-Ranging Female Tibetan Macaques (Macaca thibetana) across Different Reproductive States

The gut microbiome is expected to adapt to the varying energetic and nutritional pressures in females of different reproductive states. Changes in the gut microbiome may lead to varying nutrient utilizing efficiency in pregnant and lactating female primates. In this study, we examined variation in the gut bacterial community composition of wild female Tibetan macaques (Macaca thibetana) across different reproductive states (cycling, pregnancy and lactation). Fecal samples (n = 25) were collected from ten adult females harvested across different reproductive states. Gut microbial community composition and potential functions were assessed using 16 S rRNA gene sequences. We found significant changes in gut bacterial taxonomic composition, structure and their potential functions in different reproductive states of our study species. In particular, the relative abundance of Proteobacteria increased significantly during pregnancy and lactation. In addition, the relative abundance of Succinivibrionaceae and Succinivibrio (Succinivibrionaceae) were overrepresented in pregnant females, whereas Bifidobacteriaceae and Bifidobacterium (Bifidobacteriaceae) were overrepresented in lactating females. Furthermore, the relative abundance of predicted functional genes of several metabolic pathways related to host's energy and nutrition, such as metabolism of carbohydrates, cofactors and vitamins, glycans and other amino acids, were enriched in pregnancy and lactation. Our findings suggest that changes in the gut microbiome may play an important role in meeting the energetic needs of pregnant and lactating Tibetan macaques. Future studies of the "microbial reproductive ecology" of primates that incorporate food availability, reproductive seasonality, female reproductive physiology and gut inflammation are warranted.

Demography, life-history trade-offs, and the gastrointestinal virome of wild chimpanzees

In humans, senescence increases susceptibility to viral infection. However, comparative data on viral infection in free-living non-human primates-even in our closest living relatives, chimpanzees and bonobos (Pan troglodytes and P. paniscus)-are relatively scarce, thereby constraining an evolutionary understanding of age-related patterns of viral infection. We investigated a population of wild eastern chimpanzees (P. t. schweinfurthii), using metagenomics to characterize viromes (full viral communities) in the faeces of 42 sexually mature chimpanzees (22 males, 20 females) from the Kanyawara and Ngogo communities of Kibale National Park, Uganda. We identified 12 viruses from at least four viral families possessing genomes of both single-stranded RNA and single-stranded DNA. Faecal viromes of both sexes varied with chimpanzee age, but viral richness increased with age only in males. This effect was largely due to three viruses, salivirus, porprismacovirus and chimpanzee stool-associated RNA virus (chisavirus), which occurred most frequently in samples from older males. This finding is consistent with the hypothesis that selection on males for early-life reproduction compromises investment in somatic maintenance, which has delayed consequences for health later in life, in this case reflected in viral infection and/or shedding. Faecal viromes are therefore useful for studying processes related to the divergent reproductive strategies of males and females, ageing, and sex differences in longevity. This article is part of the theme issue 'Evolution of the primate ageing process'.

Sex-Specific Response to Caloric Restriction After Reproductive Investment in Microcebus murinus: An Integrative Approach

In seasonal environments, males and females usually maintain high metabolic activity during the whole summer season, exhausting their energy reserves. In the global warming context, unpredictability of food availability during summer could dramatically challenge the energy budget of individuals. Therefore, one can predict that resilience to environmental stress would be dramatically endangered during summer. Here, we hypothesized that females could have greater capacity to survive harsh conditions than males, considering the temporal shift in their respective reproductive energy investment, which can challenge them differently, as well as enhanced flexibility in females' physiological regulation. We tackled this question on the gray mouse lemur (Microcebus murinus), focusing on the late summer period, after the reproductive effort. We monitored six males and six females before and after a 2-weeks 60% caloric restriction (CR), measuring different physiological and cellular parameters in an integrative and comparative multiscale approach. Before CR, females were heavier than males and mostly characterized by high levels of energy expenditure, a more energetic mitochondrial profile and a downregulation of blood antioxidants. We observed a similar energy balance between sexes due to CR, with a decrease in metabolic activity over time only in males. Oxidative damage to DNA was also reduced by different pathways between sexes, which may reflect variability in their physiological status and life-history traits at the end of summer. Finally, females' mitochondria seemed to exhibit greater flexibility and greater metabolic potential than males in response to CR. Our results showed strong differences between males and females in response to food shortage during late summer, underlining the necessity to consider sex as a factor for population dynamics in climate change models.

Dominance rank but not body size influences female reproductive success in mountain gorillas

According to life history theory, natural selection has shaped trade-offs for allocating energy among growth, reproduction and maintenance to maximize individual fitness. In social mammals body size and dominance rank are two key variables believed to influence female reproductive success. However, few studies have examined these variables together, particularly in long-lived species. Previous studies found that female dominance rank correlates with reproductive success in mountain gorillas (Gorilla beringei beringei), which is surprising given they have weak dominance relationships and experience seemingly low levels of feeding competition. It is not currently known whether this relationship is primarily driven by a positive correlation between rank and body size. We used the non-invasive parallel laser method to measure two body size variables (back breadth and body length) of 34 wild adult female mountain gorillas, together with long-term dominance and demography data to investigate the interrelationships among body size, dominance rank and two measures of female reproductive success (inter-birth interval N = 29 and infant mortality N = 64). Using linear mixed models, we found no support for body size to be significantly correlated with dominance rank or female reproductive success. Higher-ranking females had significantly shorter inter-birth intervals than lower-ranking ones, but dominance rank was not significantly correlated with infant mortality. Our results suggest that female dominance rank is primarily determined by factors other than linear body dimensions and that high rank provides benefits even in species with weak dominance relationships and abundant year-round food resources. Future studies should focus on the mechanisms behind heterogeneity in female body size in relation to trade-offs in allocating energy to growth, maintenance and lifetime reproductive success.

Fruit-feeding and activity patterns of mountain gorillas (Gorilla beringei beringei) in Bwindi Impenetrable National Park, Uganda

OBJECTIVES

Availability of fruit is an important factor influencing variation in great ape foraging strategies and activity patterns. This study aims to quantify how frugivory influences activity budgets across age-sex classes of mountain gorillas in Bwindi Impenetrable National Park, Uganda.

MATERIALS AND METHODS

Daily proportions of fruit-feeding and activity budgets were calculated using 6 years of observational data on four habituated groups. We fitted generalized linear mixed models to test for age-sex differences in the amount of fruit-feeding, and to test whether these factors influence the proportion of time spent feeding, resting, and traveling.

RESULTS

Bwindi mountain gorillas spent on average 15% of feeding time consuming fruit, with monthly variation ranging from 0 to 70%. Greater amounts of fruit-feeding were associated with more time feeding and traveling, and less time resting. Immatures tended to spend more feeding time on fruit than adults, but less overall time feeding and more time traveling. There were no significant differences in the amount of fruit-feeding and overall feeding time between adult females and silverback males, despite differences in body size.

DISCUSSION

This study confirms that gorillas are frugivorous, and only the Virunga mountain gorilla population can be characterized as highly folivorous. Along with other frugivorous great apes, Bwindi mountain gorillas alter their activity patterns in response to varying amounts of fruit in their diet. A better understanding of how variable ecological conditions can drive diversity even within a subspecies has important implications for understanding relationships between ecology, body size, and foraging strategies in great apes.

Exploring individual variation in associative learning abilities through an operant conditioning task in wild baboons

Cognitive abilities underpin many of the behavioural decisions of animals. However, we still have very little understanding of how and why cognitive abilities vary between individuals of the same species in wild populations. In this study, we assessed the associative learning abilities of wild chacma baboons (Papio ursinus) across two troops in Namibia with a simple operant conditioning task. We evaluated the ability of individuals to correctly associate a particular colour of corn kernels with a distasteful flavour through repeated presentations of two small piles of corn dyed different colours, one of which had been treated with a non-toxic bitter substance. We also assessed whether individual variation in learning ability was associated with particular phenotypic traits (sex, social rank and neophilia) and states (age and prior vigilance). We found no evidence of learning the association either within each trial or across trials, nor any variation based on individuals' phenotypes. This appeared to be due to a high tolerance for bitter foods leading to similar acceptance of both palatable and unpalatable kernels. Earlier avoidance of the bitter kernels during pilot trials suggests this higher tolerance may have been largely driven by a drought during the experiments. Overall, our findings highlight the potential influence of current environmental challenges associated with conducting cognitive tests of animals in the wild.

10,000 social brains: Sex differentiation in human brain anatomy

In human and nonhuman primates, sex differences typically explain much interindividual variability. Male and female behaviors may have played unique roles in the likely coevolution of increasing brain volume and more complex social dynamics. To explore possible divergence in social brain morphology between men and women living in different social environments, we applied probabilistic generative modeling to ~10,000 UK Biobank participants. We observed strong volume effects especially in the limbic system but also in regions of the sensory, intermediate, and higher association networks. Sex-specific brain volume effects in the limbic system were linked to the frequency and intensity of social contact, such as indexed by loneliness, household size, and social support. Across the processing hierarchy of neural networks, different conditions for social interplay may resonate in and be influenced by brain anatomy in sex-dependent ways.

Non-invasive estimation of the costs of feeding competition in a neotropical primate

A key goal in behavioral ecology is to investigate the factors influencing the access to food resources and energetic condition of females, which are strong predictors of their reproductive success. We aimed to investigate how ecological factors, social factors, and reproductive state are associated with energetic condition in a wild neotropical primate using non-invasive measures. We first assessed and compared urinary C-peptide levels (uCP), the presence of urinary ketones (uKet), and behaviorally assessed energy balance (bEB) in female white-faced capuchin monkeys (Cebus imitator) living in Santa Rosa, Costa Rica. Then, we assessed how these measures were associated with feeding competition, dominance rank, and reproductive state. As predicted, uCP and bEB were positively associated with each other, and bEB was negatively associated with uKet. However, we did not find a relationship between uCP and uKet. Females showed lower uCP and bEB values during periods of intense feeding competition, but this relationship was not dependent on dominance rank. Furthermore, rank was not directly associated with uCP and bEB. Urinary ketones, on the other hand, were only produced in the most adverse conditions: by low-ranking, lactating females during periods of intense feeding competition. Behavioral strategies are assumed to maximize reproductive success and not energetic condition per se, which might explain why rank was not generally associated with energetic condition in our study population. This highlights the importance of considering potential differences between reproductive success and proxies of reproductive success, such as energetic condition or food intake, when investigating predictions of socioecological models.

Investigating the relationship between corticosterone and glucose in a reptile

The glucocorticoid hormone corticosterone (CORT) has classically been used in ecophysiological studies as a proxy for stress and energy mobilization, but rarely are CORT and the energy metabolites themselves concurrently measured. To examine CORT's role in mobilizing glucose in a wild reptile, we conducted two studies. The first study measured natural baseline and stress-induced blood-borne CORT and glucose levels in snakes during spring emergence and again when snakes return to the denning sites in autumn. The second study manipulated the hypothalamic-pituitary-adrenal (HPA) axis in male snakes in the autumn by taking a baseline blood sample, then subjecting individuals to one of five treatments (no injection, saline, CORT, adrenocorticotropin hormone and metyrapone). Subsequent samples were taken at 30 and 60 min. In both studies, we found that glucose levels do increase with acute stress, but that the relationship was not directly related to CORT elevation. In the second study, we found that none of the HPA axis manipulations directly affected blood glucose levels, further indicating that CORT may play a complex but not direct role in glucose mobilization in snakes. This study highlights the need for testing mechanisms in wild organisms by combining in situ observations with manipulative studies.

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

Seasonal variation of energy expenditure in Japanese macaques (Macaca fuscata)

Animals living in seasonal environments must adapt to a wide variation of temperature changes which requires flexible adjustments of time budget and metabolic processes for efficient thermoregulation. The Japanese macaque (Macaca fuscata) is one of only a handful of nonhuman primate species that experience seasonal climates over a wide temperature range. We used behavior observations, accelerometer sensors and the doubly-labelled water (DLW) method to measure activity and total daily energy expenditure (TDEE) of M. fuscata housed in captivity but exposed to natural seasonal variations at day lengths ranging from 10 to 12 h and temperature ranging from 0° to 32°C. Although overall activity was significantly lower in winter compared to summer and autumn, we found no effect of temperature on day-time activity. However nocturnal inactivity and mean length of sleeping bouts significantly increased along a gradient of decreasing temperatures from summer through winter, suggesting the importance of adaptive behavioral thermoregulation in this species. Energy expenditure that was unaccounted for by Basal Metabolic Rate (BMR) and physical activity i.e. expended through diet-induced thermogenesis or thermoregulation was between 14% and 32%. This residual energy expenditure differed between summer/autumn and winter and was relatively consistent across individuals (approximately 5-8% higher in winter). The percentage of body fat and residual energy expenditure were negatively correlated, supporting that fat storage was higher when less energy was required for thermoregulation. Our results suggest that physiological mechanisms like behavioral and autonomic thermoregulation enable M. fuscata to adapt to wide fluctuations in environmental conditions which provide insights into the evolutionary adaptations of nonhuman primates in seasonal climate.

Muscles provide an internal water reserve for reproduction

The use of fat to support the energy needs of reproduction (i.e. capital breeding) has been studied in a diversity of taxa. However, despite reproductive output (i.e. young or eggs) being approximately 70% water, little is known about the availability of internal resources to accommodate the hydric demands of reproduction. Recent research suggests that dehydration increases the catabolism of muscle as a means of maintaining water balance. Accordingly, we investigated the interactive effects of reproductive investment and water deprivation on catabolism and reproductive output in female Children's pythons (Antaresia childreni). Both reproductive and non-reproductive females were either provided water ad libitum or were water-deprived for three weeks at the time when reproductive females were gravid. We found that water-deprived reproductive females had, in general, greater body mass loss, epaxial muscle loss, plasma osmolality and plasma uric acid concentrations relative to the other groups. Furthermore, water-deprived females had similar clutch sizes compared with females with access to water, but produced lighter eggs and lower total clutch masses. Our results provide the first evidence that selective protein catabolism can be used to support water demands during reproduction, and, as a result, these findings extend the capital breeding concept to non-energetic resources.

Alpha male status and availability of conceptive females are associated with high glucocorticoid concentrations in high-ranking male rhesus macaques (Macaca mulatta) during the mating season

The relationship between male mating opportunities, stress, and glucocorticoid concentrations is complicated by the fact that physiological stress and glucocorticoid concentrations can be influenced by dominance rank, group size, and the stability of the male dominance hierarchy, along with ecological factors. We studied the three highest-ranking males in nine different social groups within the same free-ranging population of rhesus macaques on Cayo Santiago, Puerto Rico, during the mating season, to examine variation in glucocorticoid concentrations in relation to number of females that conceived each month, alpha status, number of adult males in a group, and male rank hierarchy stability. We found that glucocorticoid concentrations were highest in the early mating season period when more females conceived in each group and declined linearly as the mating season progressed and the number of conceptive females decreased. Alpha males had significantly higher mean monthly glucocorticoid concentrations than other high-ranking males throughout the study period. Male age, number of adult males in a group, and hierarchy stability were not significantly associated with glucocorticoid concentrations. Our findings suggest that alpha males may experience significantly higher levels of physiological stress than their immediate subordinates and that this stress coincides with the period of the mating season when most conceptions occur.

Behavioural Variability in Macaques and Langurs of the Western Ghats, India

Studies that compare differences in the behavioural variability across species and genera are rare among south Asian primates. Such studies are important for understanding within-group feeding competition in primates as interindividual difference in frequency of behaviour is a good indicator of feeding competition. We compared the variability in individual activities of lion-tailed macaques, bonnet macaques, Nilgiri langurs, and black-footed grey langurs. Both macaque species showed variability in their activities, with bonnet macaques showing higher variability than lion-tailed macaques. This indicated higher within-group competition in bonnet macaques than in lion-tailed macaques. Folivorous Nilgiri langurs and black-footed grey langurs did not show variability in any behaviour, indicating weak or negligible within-group competition. Except in the bonnet macaque, the interindividual differences in activities in the other species were potentially due to the differences between lactating and non-lactating females. We observed that langurs were less variable than macaques, indicating higher within-group competition in macaques than in langurs. We also observed a higher frequency of aggressive interactions during feeding among macaques than langurs, substantiating higher within-group competition in macaques than in langurs. We further discuss the different possibilities of within-group contest and scramble competition in these species.

Daily energy expenditure in the face of predation: hedgehog energetics in rural landscapes

Failure to balance daily energy expenditure (DEE) with energy intake can have an impact on survival and reproduction, and therefore on the persistence of populations. Here we study the DEE of the European hedgehog (Erinaceus europaeus), which is declining in the UK. We hypothesise that there is a gradient of suitable habitat for hedgehogs in rural areas, which is a result of fewer food resources, a higher risk from predation by badgers (Meles meles) and colder ambient temperatures, as distance to the nearest building increases. We used the doubly labelled water method to obtain 44 measurements of DEE from hedgehogs on four predominately arable sites, to determine the energetic costs associated with proximity to buildings, on sites with and without badgers. The mean±s.e.m. DEE was 508.9±34.8 kJ day^-1 DEE increased the further a hedgehog was from buildings during the study, possibly as they ranged larger distances on arable land, supporting the hypothesis that hedgehogs select villages owing to the lower energy demands in comparison to arable farmland. Hedgehogs had an approximately 30% lower DEE on sites with badgers. We speculate that on badger-occupied sites, hedgehogs may restrict movement and foraging in response to a threat from predation and thus have reduced DEE. Therefore, hedgehogs may also seek refuge in villages where the perceived threat of predation is lower and foraging is unrestricted. In a broader context, we demonstrate that individual differences in DEE can aid in understanding habitat selection in a patchily distributed species.