Search for: All records

Across mammals, fertility and offspring survival are often lowest at the beginning and end of females’ reproductive careers. However, extrinsic drivers of reproductive success—including infanticide by males—could stochastically obscure these expected age-related trends. Here, we modelled reproductive ageing trajectories in two cercopithecine primates that experience high rates of male infanticide: the chacma baboon (Papio ursinus) and the gelada (Theropithecus gelada). We found that middle-aged mothers generally achieved the shortest interbirth intervals in chacma baboons. By contrast, old gelada females often showed shorter interbirth intervals than their younger group-mates with one exception: the oldest females typically failed to produce additional offspring before their deaths. Infant survival peaked in middle-aged mothers in chacma baboons but in young mothers in geladas. While infant mortality linked with maternal death increased as mothers aged in both species, infanticide risk did not predictably shift with maternal age. Thus, infanticide patterns cannot explain the surprising young mother advantage observed in geladas. Instead, we argue that this could be a product of their graminivorous diets, which might remove some energetic constraints on early reproduction. In sum, our data suggest that reproductive ageing is widespread but may be differentially shaped by ecological pressures. 

Abstract Female social relationships are often shaped by the distribution of dietary resources. Socioecological models predict that females should form strict linear dominance hierarchies when resources are clumped and exhibit more egalitarian social structures when resources are evenly distributed. While many frugivores and omnivores indeed exhibit dominance hierarchies accompanied by differential resource access, many folivores deviate from the expected pattern and display dominance hierarchies despite evenly distributed resources. Among these outliers, geladas (Theropithecus gelada) present a conspicuous puzzle; females exhibit aggressive competition and strict dominance hierarchies despite feeding primarily on non-monopolizable grasses. However, these grasses become scarce in the dry season and geladas supplement their diet with underground storage organs that require relatively extensive energy to extract. We tested whether female dominance hierarchies provide differential access to underground storage organs by assessing how rank, season, and feeding context affect aggression in geladas under long-term study in the Simien Mountains National Park, Ethiopia. We found that the likelihood of receiving aggression was highest when feeding belowground and that the inverse relationship between rank and aggression was the most extreme while feeding belowground in the dry season. These results suggest that aggression in geladas revolves around belowground foods, which may mean that underground storage organs are an energetically central dietary component (despite being consumed less frequently than grasses), or that even “fallback” foods can influence feeding competition and social relationships. Further work should assess whether aggression in this context is directly associated with high-ranking usurpation of belowground foods from lower-ranking females following extraction. 

Abstract Female reproductive maturation is a critical life-history milestone, initiating an individual’s reproductive career. Studies in social mammals have often focused on how variables related to nutrition influence maturation age in females. However, parallel investigations have identified conspicuous male-mediated effects in which female maturation is sensitive to the presence and relatedness of males. Here, we evaluated whether the more “classic” socioecological variables (i.e., maternal rank, group size) predict maturation age in wild geladas—a primate species with known male-mediated effects on maturation and a grassy diet that is not expected to generate intense female competition. Females delayed maturation in the presence of their fathers and quickly matured when unrelated, dominant males arrived. Controlling for these male effects, however, higher-ranking daughters matured at earlier ages than lower-ranking daughters, suggesting an effect of within-group contest competition. However, contrary to predictions related to within-group scramble competition, females matured earliest in larger groups. We attribute this result to either: 1) a shift to “faster” development in response to the high infant mortality risk posed by larger groups; or 2) accelerated maturation triggered by brief, unobserved male visits. While earlier ages at maturation were indeed associated with earlier ages at first birth, these benefits were occasionally offset by male takeovers, which can delay successful reproduction via spontaneous abortion. In sum, rank-related effects on reproduction can still occur even when socioecological theory would predict otherwise, and males (and the risks they pose) may prompt female maturation even outside of successful takeovers. 

Abstract Neopterin, a product of activated white blood cells, is a marker of nonspecific inflammation that can capture variation in immune investment or disease-related immune activity and can be collected noninvasively in urine. Mounting studies in wildlife point to lifetime patterns in neopterin related to immune development, aging, and certain diseases, but rarely are studies able to assess whether neopterin can capture multiple concurrent dimensions of health and disease in a single system. We assessed the relationship between urinary neopterin stored on filter paper and multiple metrics of health and disease in wild geladas (Theropithecus gelada), primates endemic to the Ethiopian highlands. We tested whether neopterin captures age-related variation in inflammation arising from developing immunity in infancy and chronic inflammation in old age, inflammation related to intramuscular tapeworm infection, helminth-induced anti-inflammatory immunomodulation, and perturbations in the gastrointestinal microbiome. We found that neopterin had a U-shaped relationship with age, no association with larval tapeworm infection, a negative relationship with metrics related to gastrointestinal helminth infection, and a negative relationship with microbial diversity. Together with growing research on neopterin and specific diseases, our results demonstrate that urinary neopterin can be a powerful tool for assessing multiple dimensions of health and disease in wildlife. 

The cost–benefit ratio of group living is thought to vary with group size: individuals in ‘optimally sized’ groups should have higher fitness than individuals in groups that are either too large or too small. However, the relationship between group size and individual fitness has been difficult to establish for long-lived species where the number of groups studied is typically quite low. Here, we present evidence for optimal group size that maximizes female fitness in a population of geladas ( Theropithecus gelada ). Drawing on 14 years of demographic data, we found that females in small groups experienced the highest death rates, while females in mid-sized groups exhibited the highest reproductive performance. This group size effect on female reproductive performance was largely explained by variation in infant mortality (and, in particular, by infanticide from immigrant males) but not by variation in reproductive rates. Taken together, females in mid-sized groups are projected to attain optimal fitness due to conspecific infanticide and, potentially, predation. Our findings provide insight into how and why group size shapes fitness in long-lived species. 

Abstract Background Adaptive shifts in gut microbiome composition are one route by which animals adapt to seasonal changes in food availability and diet. However, outside of dietary shifts, other potential environmental drivers of gut microbial composition have rarely been investigated, particularly in organisms living in their natural environments. Results Here, we generated the largest wild nonhuman primate gut microbiome dataset to date to identify the environmental drivers of gut microbial diversity and function in 758 samples collected from wild Ethiopian geladas ( Theropithecus gelada ). Because geladas live in a cold, high-altitude environment and have a low-quality grass-based diet, they face extreme thermoregulatory and energetic constraints. We tested how proxies of food availability (rainfall) and thermoregulatory stress (temperature) predicted gut microbiome composition of geladas. The gelada gut microbiome composition covaried with rainfall and temperature in a pattern that suggests distinct responses to dietary and thermoregulatory challenges. Microbial changes were driven by differences in the main components of the diet across seasons: in rainier periods, the gut was dominated by cellulolytic/fermentative bacteria that specialized in digesting grass, while during dry periods the gut was dominated by bacteria that break down starches found in underground plant parts. Temperature had a comparatively smaller, but detectable, effect on the gut microbiome. During cold and dry periods, bacterial genes involved in energy, amino acid, and lipid metabolism increased, suggesting a stimulation of fermentation activity in the gut when thermoregulatory and nutritional stress co-occurred, and potentially helping geladas to maintain energy balance during challenging periods. Conclusion Together, these results shed light on the extent to which gut microbiota plasticity provides dietary and metabolic flexibility to the host, and might be a key factor to thriving in changing environments. On a longer evolutionary timescale, such metabolic flexibility provided by the gut microbiome may have also allowed members of Theropithecus to adopt a specialized diet, and colonize new high-altitude grassland habitats in East Africa.