Over six decades of research on wild baboons and their close relatives (collectively, the African papionins) have uncovered substantial variation in their behavior and social systems. While most papionins form discrete social groups (single-level societies), a few others form small social units that are nested within larger supergroups (multi-level societies). These two systems are generally thought to be qualitatively distinct, but data from wild populations increasingly suggest that there may be areas of overlap. To quantify this potential gradient in social structure, a more systematic, comparative analysis is needed. Here, we constructed a database of behavioral and demographic records spanning 135 group-years, 28 social groups, 13 long-term field studies, and 11 species to quantify variation in grooming network structure and identify the individual and dyadic properties (e.g., kinship and social status effects) that underlie this variation. Consistent with accumulating field observations, the single-level species could be divided into two categories: cohesive and cliquish. Cohesive single-level networks were dense, kin-biased, and moderately rank-structured, while cliquish single-level networks were more differentiated, slightly more kin-biased, and strongly rank-structured. As expected, multi-level networks were very modular and shaped by females' ties to specific dominant males but varied in their kin biases. Taken together, these data suggest that in the African papionins i) kin and rank biases are widespread but vary in their strength; ii) male-centered subgroups are exclusive to multi-level systems; and iii) increases in network modularity can emerge in response to heightened nepotism and male-centered clustering.

Chromosomal rearrangements can play an important role in shaping patterns of genomic divergence and speciation, yet their evolutionary context is still largely unclear, as such events are rare and challenging to identify in recent evolutionary history. Geladas (Theropithecus gelada) provide a unique system to investigate this process, as a novel fission on chromosome 7 has resulted in polymorphic karyotypes among gelada populations. We integrate new cytogenetic data from ten animals together with whole-genome sequencing from 149 individuals spanning all three extant gelada populations (northern, central, and southern). We report additional cytogenetic evidence consistent with fixation of a chromosomal fission on chromosome 7 in northern geladas, and the absence of the fission in the central population. Phylogenetic analyses reveal that the southern lineage diverged first, prior to the split between northern and central geladas, strongly suggesting that the southern population retains the ancestral karyotype. Demographic modeling further refines the timing of the northern–central split, indicating a more recent divergence ( 160 thousand years ago) than previously estimated. We detect elevated genetic differentiation between the northern and central populations and reduced nucleotide diversity in the northern lineage on chromosome 7, which is consistent with historically suppressed recombination. These results suggest that the fission on chromosome 7 may have contributed to regional divergence by promoting recombination suppression in heterokaryotypes. By combining cytogenetics with population genomes, these findings highlight the role of chromosomal evolution in shaping localized genomic differentiation, and its potential for reinforcing reproductive isolation. Our results also suggest that northern geladas may represent an incipient species.

Mammals occupy diverse environments and face many physiological challenges, such as hypoxia. Several species, including certain human populations, have acclimated to high-altitude, chronically low-oxygen conditions. However, human high-altitude populations arose roughly 46,000 years ago, whereas other species, including primates like the gelada (Theropithecus gelada), have lived at elevations of 2,350-4,550m in the Ethiopian Highlands for at least 1 million years. Geladas exhibit a combination of morphological and physiological traits associated with altitude adaptation, including larger chest dimensions, reduced hemoglobin concentrations, and increased selection in hypoxia-related genes. To investigate the cellular mechanisms underlying hypoxia tolerance in geladas, we isolated dermal fibroblasts from ear biopsies collected from wild individuals (n = 25) in Ethiopia as part of the Simien Mountains Gelada Research Project. These cells were compared to dermal fibroblasts from sea-level humans (ATCC), and we also included fibroblasts from fat-tailed dwarf lemurs. Using an Amplex Red assay, we measured reactive oxygen species (ROS) production across species. Under baseline normoxic conditions (21% O 2 ), gelada fibroblasts generated 91% less ROS than human fibroblasts, suggesting a mechanism to limit hypoxia-induced cellular damage. Under hypoxic conditions (0.5% O 2 ), gelada fibroblasts produced 39% less ROS than humans (n=2) and 72% less than lemurs (n=1). We also assessed HIF1α accumulation by western blot following 6 hours of hypoxia (0.5% O 2 ) and found that gelada HIF1α accumulation was ~⅓ human levels, consistent with enhanced hypoxia tolerance. Future work will expand biological replication and incorporate additional comparison groups, including lowland baboons and high-altitude human populations. Funded by NSF 2022046 & NSF SBE-2010309. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

In humans, adenoviruses (AdVs) are frequently associated with respiratory illnesses, posing risks to children with developing immune systems and immunocompromised individuals. Outbreaks and epidemics are generally centred in close-contact settings, such as childcare facilities, and transmission occurs through faecal–oral and airborne pathways. AdVs have coevolved across the primate lineage, but very little is known about whether the early-life dynamics in non-human primates mirror those in humans. Here, we leverage longitudinal data collected on a population of geladas ( Theropithecus gelada ) in the Simien Mountains National Park, Ethiopia, to evaluate AdV dynamics across the gelada lifespan. We identified ten coding-complete AdV genomes representing seven unique simian adenovirus (SAdV) types, four of which are adequately different from the known ones to establish new species. We assessed behavioural and seasonal drivers of SAdV presence and richness across repeated faecal samples from known individuals. Contrary to our expectation that the highest risk would occur after the initiation of play behaviour in infancy (~6 months of age), when peer-to-peer transmission risk is expected to increase, SAdV likelihood was highest in infants under 6 months of age. Risk and richness declined over the lifespan, with very few adults infected, and higher minimum temperatures were weakly but significantly negatively associated with richness. Our results suggest that, unlike in humans, SAdV exposure occurs prior to the initiation of close-contact play behaviours and likely results from the close spatial proximity of conspecifics throughout the dependent period. Like AdVs in humans, SAdVs in geladas maintain low levels in adulthood, with early infections potentially conferring life-long immunity.

Intestivirids (order Crassvirales, family Intestiviridae), viruses that infect Bacteroidales bacteria in the mammalian gastrointestinal tract, have been identified as a highly abundant component of the healthy human virome that may shape patterns of human health and disease through direct action on the microbiome. While double-stranded DNA bacteriophages called crAssphages (Carjivirus communis) that infect bacteria in the Bacteroidales order have been identified in humans within the first month of life, the enormous variation in post-parturition infant environments and diets has inhibited a robust understanding of the physiological and environmental factors that govern acquisition patterns. We turned to a wild population of graminivorous nonhuman primates (geladas, Theropithecus gelada) under long-term study in the Simien Mountains National Park, Ethiopia, analysing faecal samples from infants and mothers in this population across the infancy period for richness and presence of crAssphage-like viruses (family Intestiviridae). Eight intestivirid genomes were identified based on terminal redundancy representing six unique variants (< 98% intergenomic similarity) closely related to the human crAssphage. The prevalence of intestivirids in gelada faecal samples begins to rise at about 10 months of age, peaks in the months surrounding weaning (~18 months), and somewhat decreases but maintains high levels into adulthood. We found a strong association between cumulative rainfall and intestivirid detection, with a higher likelihood accompanying wetter seasons with higher grass availability. In this population, the months prior to weaning have been found to be accompanied by a shift in the microbiome characterised by a decrease in glycan degrader Bacteroidales taxa and an increase in fermentative Bacteroidales taxa, and wetter seasons when the vast majority of the gelada diet comprises grasses are associated with an increase in fermentative Bacteroidales taxa. In the context of these microbiome shifts, our results suggest that the intestivirid-bacterial host relationship may interact with major developmental and seasonal dietary shifts in the mammalian host.

Animal signals typically convey reliable information, but deception can evolve when the sender and receiver have conflicting interests-especially in the context of mating. Here, we provide evidence from a Cercopithecine primate, the gelada (Theropithecus gelada), that females deceptively signal fertility when conception is unlikely, which functions as a counterstrategy in sexual conflict. In geladas, male takeovers are frequent and often lead to sexually selected infanticide, exacting high costs on lactating females. Using 14 years of demographic and hormone data from wild geladas in Ethiopia, we show that lactating females quickly resumed sexual swellings and mated with the new male following takeovers, but they took significantly longer to conceive than females resuming cycling at other times. Females that exhibited these post-takeover swellings were subsequently less likely to lose their infants to infanticide. Fecal hormone data revealed a surge in estrogens after takeovers, even among females with the youngest infants, suggesting that estrogens mediate both fertile ("true") and non-fertile ("false") swellings. These results support the idea that sexual swellings can deceptively blur fertility as an adaptive counterstrategy to infanticide.

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.

While strong maternal relationships have been linked with improved offspring survival in many mammals, maternal sociality appears to provide little protection against infanticidal males. Here, we evaluated whether maternal social integration predicts offspring survival to adulthood in geladas (Theropithecus gelada), a non-human primate that faces frequent alpha male takeovers coupled with high rates of infanticide. Mothers that formed stronger grooming relationships with their female and male groupmates showed higher offspring survival on average. However, when their infants experienced early-life takeovers and faced elevated infanticide risk, these survival advantages were weaker, delayed to juvenility and linked only to female-female grooming relationships. Thus, long-term social integration was not associated with reduced infanticide risk. Given this, we then examined whether females engaged in short-term social strategies that might provide more targeted protection against would-be male attackers. Following takeovers, females-particularly those with young, vulnerable infants-groomed males less frequently and prioritized deposed, protective males (i.e. their infants' presumed fathers) over new, potentially infanticidal males. Taken together, these data suggest that gelada mothers: (i) form long-term social relationships that might improve net offspring survival and (ii) implement short-term social strategies that might protect their offspring in ways that long-term relationships cannot.

Mammalian mothers provide behavioral and physiological signals that offspring use to calibrate development in relation to maternal resources and environmental cues. Infants respond selectively as they prioritize certain developmental systems over others, creating developmental tradeoffs between competing biological systems. Here, we investigate the influence of maternal capital (“investment capacity”) on the growth and development of their infants in wild olive baboons ( Papio anubis ) from Laikipia, Kenya. We posit that maternal capital is influenced by a mother’s own early life experiences (e.g., drought, maternal loss) and her current life experiences (e.g., dominance rank, food availability), and is signaled to offspring via maternal effort (i.e., nursing and carrying time) and glucocorticoids. We used behavioral data on 40 infants (43% female) in the first year of life to quantify maternal effort, infant play, and infant independence (i.e., frequency of infant departures from mother). We matched these behavioral data with maternal fecal glucocorticoid measures from lactating mothers, and infant growth measures assessed via photogrammetry. Signals of low maternal capital predicted lower rates of infant play, less behavioral independence, and slower growth. There was a negative relationship between the rate of social contact play and growth rate, indicating a developmental tradeoff. Males were more sensitive than females to some of the maternal signals measured in our study. These results add to a growing body of evidence demonstrating that maternal behavioral and physiological signals shape infant development. ### Competing Interest Statement The authors have declared no competing interest. U.S. National Science Foundation, https://ror.org/021nxhr62, BCS-1732172, GRFP-1841051 Leakey Foundation, https://ror.org/018kdpd27 Arizona State University, https://ror.org/03efmqc40

Mammalian pregnancy is characterized by a well-known suite of physiological changes that support fetal growth and development, thereby positively affecting both maternal and offspring fitness. However, mothers also experience trade-offs between current and future maternal reproductive success, and maternal responses to these trade-offs can result in mother-offspring fitness conflicts. Knowledge of the mechanisms through which these trade-offs operate, as well as the contexts in which they operate, is critical for understanding the evolution of reproduction. Historically, hormonal changes during pregnancy have been thought to play a pivotal role in these conflicts since they directly and indirectly influence maternal metabolism, immunity, fetal growth and other aspects of offspring development. However, recent research suggests that gut microbiota may also play an important role. Here, we create a foundation for exploring this role by constructing a mechanistic model linking changes in maternal hormones, immunity and metabolism during pregnancy to changes in the gut microbiota. We posit that marked changes in hormones alter maternal gut microbiome composition and function both directly and indirectly via impacts on the immune system. The gut microbiota then feeds back to influence maternal immunity and metabolism. We posit that these dynamics are likely to be involved in mediating maternal and offspring fitness as well as trade-offs in different aspects of maternal and offspring health and fitness during pregnancy. We also predict that the interactions we describe are likely to vary across populations in response to maternal environments. Moving forward, empirical studies that combine microbial functional data and maternal physiological data with health and fitness outcomes for both mothers and infants will allow us to test the evolutionary and fitness implications of the gestational microbiota, enriching our understanding of the ecology and evolution of reproductive physiology. Knowledge of the mechanisms through which trade-offs between fetal and maternal health operate during pregnancy is critical for understanding the evolution of reproduction. Because the gut microbiome interacts with host hormones, immunity, and metabolism, it can influence fetal and maternal biology and likely plays a role in these trade-offs. This review explores the potential avenues through which the microbiome interacts with fetal and maternal biology during pregnancy, and provides a vision for better exploring microbial impacts on the evolutionary dynamics of reproduction.