Marie J. E. Charpentier

Previous correlational studies in wild mandrills suggest that males may show greater socio-sexual interest toward less feminine-looking females, challenging the expectation that facial femininity enhances attractiveness. However, whether facial femininity itself causally influences male response remains unknown, as natural faces vary simultaneously in many visual and social attributes. We tested this question in semi-captive male mandrills using paired looking-time assays with synthetic female faces generated by a generative artificial intelligence (AI) model trained on mandrill face photographs. This approach allowed us to manipulate facial femininity along a calibrated axis while holding identity and other visual features constant. We also used the same framework to animate static portraits, testing whether dynamic presentation increases attention to facial stimuli. Contrary to predictions from previous field observations, males showed no detectable preference for either more feminine or less feminine female faces. By contrast, they looked longer at animated rather than static versions of the same faces. These findings suggest that facial femininity alone is insufficient to elicit a consistent visual preference in male mandrills under controlled conditions, whereas motion enhances the salience of conspecific facial stimuli. More broadly, our study shows how calibrated generative-AI stimuli can experimentally isolate socio-sexual signal components that are difficult to disentangle observationally.

In primates, most evidence linking sociality to fitness comes from studies of female–female bonds, whereas the consequences of heterosexual relationships remain understudied. This is particularly true in species such as mandrills (Mandrillus sphinx), where male group tenure is short, and interactions with females can range from affiliative to coercive. Using 13 years of behavioural and life-history data on 76 adult females, we show that grooming exchanged with all males has contrasting fitness consequences depending on its direction. Grooming given to males tended to improve female survival and was associated with higher juvenile offspring survival, whereas grooming received from males reduced female survival. These opposing effects suggest that heterosexual grooming interactions may entail both benefits and costs for females, depending on whether females provide or receive affiliation. Finally, agonistic interactions between females and males showed modest evidence of negative associations with juvenile survival, whereas male–female spatial proximity had no detectable effect overall. Our results show that even transient male–female associations can have lasting consequences for fitness and that these effects depend critically on the direction and timing of specific interactions rather than on global measures of sociality.

Whether non-human animals can experience complex emotions like jealousy remains debated. In humans, jealousy is particularly salient in sibships, where children often disrupt affiliative parent–sibling interactions. Jealousy could similarly mediate competitive interactions among siblings in animals. Here, we examine the context, targets and function of sibling interference in mother–offspring grooming interactions in wild chacma baboons. We test the jealousy hypothesis proposing that juveniles interfere to disrupt parent–sibling affiliation without immediate reward, alongside two alternatives stating that interference instead aims at interacting with the mother or sibling. Our results support the jealousy hypothesis, with patterns strikingly echoing observations on children. First, juveniles approach their mother more often when she is grooming a sibling than when she is socially available. Second, they preferentially target same-sex and younger siblings, as well as ‘maternal favourites’ that receive a disproportionate share of maternal grooming. Third, interference is more successful at disrupting mother–sibling grooming than at obtaining attention from either the mother or the sibling. This study introduces a novel framework to study the expression of jealousy and the emotional mechanisms underpinning family conflicts in animals, thereby opening perspectives in the growing field of the ecology of emotions.

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 observations in the field, 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 (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. SIGNIFICANCE STATEMENT: What forces explain variation in primate societies? While kinship and dominance shape the social lives of many of our close relatives, it is unclear how their effects differ across species. Using a new database comprising decades of field research, we found that baboons and their close relatives fell into three general patterns: one in which groups were cohesive, kin-biased, and moderately rank-biased, another in which groups were more cliquish and nepotistic, and a third in which groups were divided into clusters centered on dominant males. Distinct primate societies may thus reflect differences in the strength of females’ nepotistic biases and the degree of males’ social influence.

Mammalian species with slow life histories invest heavily in offspring care to meet offspring nutritional and developmental requirements, typically at significant costs to mothers. While maternal care has been extensively studied, understanding the mechanisms driving variation in mother‒offspring relationships during key offspring developmental periods require more comparative data from natural populations. Using eight years of behavioral data, we analyzed mother-offspring interactions in 68 infants born to 46 mothers in wild mandrills (Mandrillus sphinx), spanning multiple birth cohorts and the entire first year of infant life, a critical period for individual social and physical development. We found that mothers dynamically adjusted caregiving behavior as infants aged, reducing physical contact while promoting spatial independence and social integration. Maternal traits, such as age, social rank, and reproductive history, shaped maternal phenotypes: high-ranking mothers promoted infant socialization while reducing carrying, and older, multiparous mothers invested more in grooming and physical contact than younger, inexperienced females. Previous infant loss predicted reduced maternal aggression, potentially due to fewer immatures to care for or behavioral adjustments aimed at improving offspring survival. Finally, mothers fostered closer bonds with their daughters while encouraging their sons’ independence, possibly resulting in more frequent tantrums observed in males. This sex-biased pattern likely reflects preparation for contrasting life histories between the sexes in this species. By combining extensive longitudinal observations with fine-scale, individual analyses, our study emphasizes the dynamic and multifaceted nature of early mother-offspring interactions and their evolutionary implications in long-lived mammals.

In monotocous mammals, most individuals experience the birth of a younger sibling. This period may induce losses in maternal care and can be physiologically, energetically and emotionally challenging for the older sibling, yet has rarely been studied in wild primates. We used behavioural data collected from a natural population of mandrills to investigate changes in maternal care and mother–juvenile relationship throughout the transition to siblinghood (TTS), by comparing juveniles who recently experienced the birth of a younger sibling, to juveniles who did not. We found that the TTS was associated with an abrupt cessation of the weaning process for the juvenile, and to a decrease in maternal affiliation. Juveniles’ reactions were sex-specific, as males associated less with their mother, while females tended to groom their mother more often after the birth of their sibling. Despite the substantial loss of maternal care, juveniles did not show an increase in conflict or anxiety-related behaviours. This study contributes to explain why short interbirth intervals often pose a risk to juveniles’ survival in monotocous primates. Our results contrast existing studies and further highlight the importance of examining the TTS in species and populations with various life histories and ecologies.

Birth is a fundamental event in the life of animals, including our own species. More reports of wild non-human primate births and stillbirths are thus needed to better understand the evolutionary pressures shaping parturition behaviors in our lineage. In diurnal non-human primates, births generally occur at night, when individuals are resting. Consequently, they are difficult to observe in the wild and most of the current knowledge regarding perinatal behaviors comes from rare daytime births. Information about stillbirths is even rarer and their proximate causes are generally unknown. Here, we present detailed observations of a daytime birth of a stillborn wild mandrill (Mandrillus sphinx). During this event, which lasted an entire day, we recorded the behaviors of the parturient female ad libitum, using video recordings and photos. The 5-year-old female was primiparous and of low dominance rank. The length of her pregnancy was shorter than usual and the partum phase was extremely long compared to other birth reports in non-human primates. The female disappeared shortly after this event and was assumed to have died. We discuss the possible causes of this stillbirth including the infant’s presentation at birth and maternal inexperience.

Social animals are particularly exposed to infectious diseases. Pathogen-driven selection pressures have thus favoured the evolution of behavioural adaptations to decrease transmission risk such as the avoidance of contagious individuals. Yet, such strategies deprive individuals of valuable social interactions, generating a cost–benefit trade-off between pathogen avoidance and social opportunities. Recent studies revealed that hosts differ in these behavioural defences, but the determinants driving such inter-individual variation remain understudied. Using 6 years of behavioural and parasite data on a large natural population of mandrills (Mandrillus sphinx), we showed that, when parasite prevalence was high in the population, females avoided grooming their conspecifics' peri-anal region (PAR), where contagious gastro-intestinal parasites accumulate. Females varied, however, in their propensity to avoid this risky body region: across years, some females consistently avoided grooming it, while others did not. Interestingly, hygienic females (i.e. those avoiding the PAR) were less parasitized than non-hygienic females. Finally, age, dominance rank and grooming frequency did not influence a female's hygiene, but both mother–daughter and maternal half-sisters exhibited similar hygienic levels, whereas paternal half-sisters and non-kin dyads did not, suggesting a social transmission of this behaviour. Our study emphasizes that the social inheritance of hygiene may structure behavioural resistance to pathogens in host populations with potential consequences on the dynamics of infectious diseases.