The reproductive conflict hypothesis suggests menopause is rare in nature because it is only evolutionarily favoured in specific dispersal and mating systems. In social groups with local mating, shared resource competition and female-biased dispersal, an increase in a breeding female's relatedness to her fellow community members with age is expected to favour late-life reproductive cessation as a response to intergenerational reproductive competition. Here, we use observational and genomic data from the Ngogo chimpanzee community to characterize kinship dynamics and investigate the potential role of reproductive conflict in explaining a recent report of menopause in this community. We first find that, as predicted by simple models, the combination of female dispersal and local breeding leads to age-specific increases in relatedness between female and male community members. Next, we use the observed kinship dynamics in inclusive fitness formulae to test whether reproductive cessation might have been selected for in chimpanzee females. We find that kinship dynamics measured within subgroups of the community, where competition is presumably most intense, favour the evolution of menopause beginning around age 40. This is consistent with patterns of age-related fertility declines observed in Ngogo, suggesting reproductive conflict may have contributed to the evolution of chimpanzee post-reproductive lifespans.

In humans and non-human primates, male aggression and physical violence are common strategies in the struggle over power and are efficient in exerting control over individuals and groups. In contrast, our close relative, the bonobo (Pan paniscus) is often described as an exceptionally peaceful primate due to the lack of lethal aggression or infanticide and the tendency for individuals to reconcile after conflicts. Nonetheless, rates of male aggression are high1, but, atypically for primates, bonobo females are observed to confront males without support from others. Despite female exogamy, forming coalitions of unrelated females in response to male aggression appears to be a common strategy, mostly involving charging or chasing and, in some cases, escalating to physical attacks2,3,4. Here, we report on a violent coalitionary attack by resident females against an adult male in a well-studied group of wild bonobos habituated to observation, detailing participants' violent actions and the victim's responses. The assault involved a fraction of the group, while almost everybody was present, and bystanders, including some close maternal kin, did not support the victim. Our observations detail a rare behavior that is not easy to reconcile with the assumed peaceful nature of bonobo society, but which contributes to evolutionary models of aggression5.

Inbreeding (reproduction between relatives) often decreases the fitness of offspring and is thus expected to lead to the evolution of inbreeding avoidance strategies. Chimpanzees (Pan troglodytes) are expected to avoid inbreeding as they are long-lived, invest heavily in offspring and may encounter adult, opposite sex kin frequently, especially in populations where both males and females commonly remain in the group in which they were born (bisexual philopatry). However, it is unclear whether substantial bisexual philopatry has been a feature of chimpanzees' evolutionary history or whether it is a result of recent anthropogenic interference, as the only groups for which it has been documented are significantly impacted by human encroachment and experience notable rates of potentially unsustainable inbreeding. Here we use 14 years of observational data and a large genomic dataset of 256 481 loci sequenced from 459 individuals to document dispersal and inbreeding dynamics in an eastern chimpanzee (P. t. schweinfurthii) community with low levels of anthropogenic disturbance. We document the first case of substantial bisexual philopatry in a relatively undisturbed chimpanzee community and show that, despite an increased inbreeding risk incurred by females who do not disperse before reaching reproductive age, natal females were still able to avoid producing inbred offspring.

How populations adapt to their environment is a fundamental question in biology. Yet, we know surprisingly little about this process, especially for endangered species, such as nonhuman great apes. Chimpanzees, our closest living relatives, are particularly notable because they inhabit diverse habitats, from rainforest to woodland-savannah. Whether genetic adaptation facilitates such habitat diversity remains unknown, despite it having wide implications for evolutionary biology and conservation. By using newly sequenced exomes from 828 wild chimpanzees (388 postfiltering), we found evidence of fine-scale genetic adaptation to habitat, with signatures of positive selection in forest chimpanzees in the same genes underlying adaptation to malaria in humans. This work demonstrates the power of noninvasive samples to reveal genetic adaptations in endangered populations and highlights the importance of adaptive genetic diversity for chimpanzees.

Mechanisms of inheritance remain poorly defined for many fitness-mediating traits, especially in long-lived animals with protracted development. Using 6,123 urinary samples from 170 wild chimpanzees, we examined the contributions of genetics, non-genetic maternal effects, and shared community effects on variation in cortisol levels, an established predictor of survival in long-lived primates. Despite evidence for consistent individual variation in cortisol levels across years, between-group effects were more influential and made an overwhelming contribution to variation in this trait. Focusing on within-group variation, non-genetic maternal effects accounted for 8% of the individual differences in average cortisol levels, significantly more than that attributable to genetic factors, which was indistinguishable from zero. These maternal effects are consistent with a primary role of a shared environment in shaping physiology. For chimpanzees, and perhaps other species with long life histories, community and maternal effects appear more relevant than genetic inheritance in shaping key physiological traits.

Reproductive inequality, or reproductive skew, drives natural selection, but has been difficult to assess, particularly for males in species with promiscuous mating and slow life histories, such as bonobos ( Pan paniscus ) and chimpanzees ( Pan troglodytes ). Although bonobos are often portrayed as more egalitarian than chimpanzees, genetic studies have found high male reproductive skew in bonobos. Here, we discuss mechanisms likely to affect male reproductive skew in Pan , then re-examine skew patterns using paternity data from published work and new data from the Kokolopori Bonobo Reserve, Democratic Republic of Congo and Gombe National Park, Tanzania. Using the multinomial index ( M ), we found considerable overlap in skew between the species, but the highest skew occurred among bonobos. Additionally, for two of three bonobo communities, but no chimpanzee communities, the highest ranking male had greater siring success than predicted by priority-of-access. Thus, an expanded dataset covering a broader demographic range confirms that bonobos have high male reproductive skew. Detailed comparison of data from Pan highlights that reproductive skew models should consider male–male dynamics including the effect of between-group competition on incentives for reproductive concessions, but also female grouping patterns and factors related to male–female dynamics including the expression of female choice. This article is part of the theme issue ‘Evolutionary ecology of inequality’.

One way mammalian mothers support offspring is by intervening on their behalf when they receive conspecific aggression. Maternal intervention protects offspring and facilitates mother–offspring rank correlation in several female-philopatric species. We tested the hypothesis that maternal intervention during development similarly facilitates mother–offspring rank correlation in one of our two male-philopatric closest living relatives: bonobos, Pan paniscus, for whom male ranks have been described as matrifocal but mechanisms of status transmission are unclear. We predicted mothers would intervene at higher rates in bonobos compared to chimpanzees, Pan troglodytes, where sons earn adult ranks independently. We expected this difference would be especially pronounced for sons as they reached reproductive age (∼8 years) and when aggressors were adult males, which female bonobos, but not female chimpanzees, often dominate. However, for both sons and daughters of all ages (0.5–16 years) and against aggressors of varying age and sex, bonobo mothers in Kokolopori, DRC (N=22 pairs, 210 conflicts) intervened less frequently than did chimpanzee mothers at Ngogo, Uganda (N=66 pairs, 221 conflicts), doing so in 8% versus 49% of all conflicts. These differences persisted regardless of the severity of aggression offspring received and the distress they demonstrated. Our results provide no support for our hypothesis that conflict intervention facilitates mother–son rank correlation in bonobos. Nor are patterns explained by additional alternative hypotheses related to species variation in costs of retaliation, quality of female–female relationships or infanticide risk. We introduce two new hypotheses: (1) that success in aggressive competition is less important for male bonobos than for male chimpanzees and (2) proclivities for support are heightened in chimpanzees compared to bonobos because of increased ingroup protectiveness in chimpanzees. The latter hypothesis arises from our unexpected, preliminary observation that nonmother bonobo bystanders intervened less often than nonmother chimpanzee bystanders during these events.

Studies of the evolutionary relationships among gorilla populations using autosomal and mitochondrial sequences suggest that male-mediated gene flow may have been important in the past, but data on the Y-chromosomal relationships among the gorilla subspecies are limited. Here, we genotyped blood and noninvasively collected fecal samples from 12 captives and 257 wild male gorillas of known origin representing all four subspecies (Gorilla gorilla gorilla, G. g. diehli, G. beringei beringei, and G. b. graueri) at 10 Y-linked microsatellite loci resulting in 102 unique Y-haplotypes for 224 individuals. We found that western lowland gorilla (G. g. gorilla) haplotypes were consistently more diverse than any other subspecies for all measures of diversity and comprised several genetically distinct groups. However, these did not correspond to geographical proximity and some closely related haplotypes were found several hundred kilometers apart. Similarly, our broad sampling of eastern gorillas revealed that mountain (G. b. beringei) and Grauer's (G. b. graueri) gorilla Y-chromosomal haplotypes did not form distinct clusters. These observations suggest structure in the ancestral population with subsequent mixing of differentiated haplotypes by male dispersal for western lowland gorillas, and postisolation migration or incomplete lineage sorting due to short divergence times for eastern gorillas.

Knowledge on the population history of endangered species is critical for conservation, but whole-genome data on chimpanzees (Pan troglodytes) is geographically sparse. Here, we produced the first non-invasive geolocalized catalog of genomic diversity by capturing chromosome 21 from 828 non-invasive samples collected at 48 sampling sites across Africa. The four recognized subspecies show clear genetic differentiation correlating with known barriers, while previously undescribed genetic exchange suggests that these have been permeable on a local scale. We obtained a detailed reconstruction of population stratification and fine-scale patterns of isolation, migration, and connectivity, including a comprehensive picture of admixture with bonobos (Pan paniscus). Unlike humans, chimpanzees did not experience extended episodes of long-distance migrations, which might have limited cultural transmission. Finally, based on local rare variation, we implement a fine-grained geolocalization approach demonstrating improved precision in determining the origin of confiscated chimpanzees.

Evolutionary theories predict that sibling relationships will reflect a complex balance of cooperative and competitive dynamics. In most mammals, dispersal and death patterns mean that sibling relationships occur in a relatively narrow window during development, and/or only with same-sex individuals. Besides humans, one notable exception are mountain gorillas, in which non-sex biased dispersal, relatively stable group composition, and the long reproductive tenures of alpha males mean that animals routinely reside with both same and opposite-sex, and full and half siblings, throughout their lives. Using nearly 40,000 hours of observation data collected over 14 years on 699 sibling and 1258 non-sibling pairs of wild mountain gorillas, we demonstrate that individuals have strong affiliative preferences for full and maternal siblings over paternal siblings or unrelated animals, consistent with an inability to discriminate paternal kin. Intriguingly, however, aggression data imply the opposite. Aggression rates were statistically indistinguishable among all types of dyads except one: in mixed-sex dyads, non-siblings engaged in substantially more aggression than siblings of any type. This pattern suggests mountain gorillas may be capable of distinguishing paternal kin, but nonetheless choose not to affiliate with them over non-kin. A preference for maternal kin occurs despite gorillas not possessing low male reproductive skew, the key characteristic believed to underlie such biases. These results call into question reasons for strong maternal kin biases when paternal kin are identifiable, familiar, and similarly likely to be long-term social partners, and suggest behavioral mismatches at play during a transitional period in mountain gorilla society.

Evolutionary theories predict that sibling relationships will reflect a complex balance of cooperative and competitive dynamics. In most mammals, dispersal and death patterns mean that sibling relationships occur in a relatively narrow window during development and/or only with same-sex individuals. Besides humans, one notable exception is mountain gorillas, in which non-sex-biased dispersal, relatively stable group composition, and the long reproductive tenures of alpha males mean that animals routinely reside with both maternally and paternally related siblings, of the same and opposite sex, throughout their lives. Using nearly 40,000 hr of behavioral data collected over 14 years on 699 sibling and 1235 non-sibling pairs of wild mountain gorillas, we demonstrate that individuals have strong affiliative preferences for full and maternal siblings over paternal siblings or unrelated animals, consistent with an inability to discriminate paternal kin. Intriguingly, however, aggression data imply the opposite. Aggression rates were statistically indistinguishable among all types of dyads except one: in mixed-sex dyads, non-siblings engaged in substantially more aggression than siblings of any type. This pattern suggests mountain gorillas may be capable of distinguishing paternal kin but nonetheless choose not to affiliate with them over non-kin. We observe a preference for maternal kin in a species with a high reproductive skew (i.e. high relatedness certainty), even though low reproductive skew (i.e. low relatedness certainty) is believed to underlie such biases in other non-human primates. Our results call into question reasons for strong maternal kin biases when paternal kin are identifiable, familiar, and similarly likely to be long-term groupmates, and they may also suggest behavioral mismatches at play during a transitional period in mountain gorilla society.

Associating with kin provides individual benefits

Associating with kin provides individual benefits but requires that these relationships be detectable. In humans, facial phenotype matching might help assess paternity; however, evidence for it is mixed. In chimpanzees, concealing visual cues of paternity may be beneficial due to their promiscuous mating system and the considerable risk of infanticide by males. On the other hand, detecting kin can also aid chimpanzees in avoiding inbreeding and in forming alliances that improve kin-mediated fitness. Although previous studies assessing relatedness based on facial resemblance in chimpanzees exist, they used images of captive populations in whom selection pressures and reproductive opportunities are controlled and only assessed maternity or paternity of adult offspring. In natural populations, the chances of infanticide are highest during early infancy, suggesting that young infants would benefit most from paternity concealment, whereas adults and subadults would benefit from the detection of all types of kin, including half-siblings. In our experiment, we conducted an online study with human participants, in which they had to assess the relatedness of chimpanzees based on facial similarity. To address previous methodological constraints, we used chimpanzee images across all ages, as well as maternal and paternal half-siblings. We found that kin status was detected above chance across all relatedness categories, with easier kin detection of father-offspring pairs, females, and older chimpanzees. Together, these findings support the existence of paternity confusion in infant chimpanzees and provide a possible mechanism for incest avoidance and kin-based social alliances in older individuals. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

The ultimate payoff of behaviours depends not only on their direct impact on an individual, but also on the impact on their relatives. Local relatedness—the average relatedness of an individual to their social environment—therefore has profound effects on social and life history evolution. Recent work has begun to show that local relatedness has the potential to change systematically over an individual’s lifetime, a process called kinship dynamics. However, it is unclear how general these kinship dynamics are, whether they are predictable in real systems and their effects on behaviour and life history evolution. In this study, we combine modelling with data from real systems to explore the extent and impact of kinship dynamics. We use data from seven group-living mammals with diverse social and mating systems to demonstrate not only that kinship dynamics occur in animal systems, but also that the direction and magnitude of kinship dynamics can be accurately predicted using a simple model. We use a theoretical model to demonstrate that kinship dynamics can profoundly affect lifetime patterns of behaviour and can drive sex differences in helping and harming behaviour across the lifespan in social species. Taken together, this work demonstrates that kinship dynamics are likely to be a fundamental dimension of social evolution, especially when considering age-linked changes and sex differences in behaviour and life history. A model of how local relatedness changes with age fits empirical patterns from seven group-living mammal species and reveals that patterns differ between the sexes and the potential behavioural consequences of these changes.

Like many animals, adult male chimpanzees often compete for a limited number of mates. They fight other males as they strive for status that confers reproductive benefits and use aggression to coerce females to mate with them. Nevertheless, small-bodied, socially immature adolescent male chimpanzees, who cannot compete with older males for status nor intimidate females, father offspring. We investigated how they do so through a study of adolescent and young adult males at Ngogo in Kibale National Park, Uganda. Adolescent males mated with nulliparous females and reproduced primarily with these first-time mothers, who are not preferred as mating partners by older males. Two other factors, affiliation and aggression, also influenced mating success. Specifically, the strength of affiliative bonds that males formed with females and the amount of aggression males directed toward females predicted male mating success. The effect of male aggression toward females on mating success increased as males aged, especially when they directed it toward females with whom they shared affiliative bonds. These results mirror sexual coercion in humans, which occurs most often between males and females involved in close, affiliative relationships.

Extended breeding bonds and direct paternal care are rare in group-living species with high levels of male-male competition. In polygynandrously mating savannah baboons, many males form close ties (primary associations) to certain females outside the mating context. These relationships have likely evolved as a form of male parenting effort because roughly 50-75% of primary associates are the sires of the females? current infants. Mismatches between the formation of primary associations and paternity have been interpreted as kin recognition errors, which arise because males rely on imperfect proxies of paternity. Alternatively, mismatches may reflect prior paternity history if males form enduring relationships with the mothers of their offspring. We tested these hypotheses in a wild population of olive baboons, Papio anubis. The behaviour of sires near the time of conception was considerably different from the behaviour of nonsire primary associates, suggesting that most males were not misled by ambiguous behavioural cues of paternity. Instead, previous paternity history was an important predictor of the strength of ties between males and lactating females. Both paternity of the current infant and paternity of the previous infant influenced the probability that males would establish close ties to females, and these effects were largely additive. These findings indicate that male-female bonds in olive baboons extend long past lactation and suggest that selection may have favoured prolonged paternal investment in offspring. ? 2021 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Paleoclimate reconstructions have enhanced our understanding of how past climates have shaped present-day biodiversity. We hypothesize that the geographic extent of Pleistocene forest refugia and suitable habitat fluctuated significantly in time during the late Quaternary for chimpanzees (Pan troglodytes). Using bioclimatic variables representing monthly temperature and precipitation estimates, past human population density data, and an extensive database of georeferenced presence points, we built a model of changing habitat suitability for chimpanzees at fine spatio-temporal scales dating back to the Last Interglacial (120,000 BP). Our models cover a spatial resolution of 0.0467° (approximately 5.19 km2 grid cells) and a temporal resolution of between 1000 and 4000 years. Using our model, we mapped habitat stability over time using three approaches, comparing our modeled stability estimates to existing knowledge of Afrotropical refugia, as well as contemporary patterns of major keystone tropical food resources used by chimpanzees, figs (Moraceae), and palms (Arecacae). Results show habitat stability congruent with known glacial refugia across Africa, suggesting their extents may have been underestimated for chimpanzees, with potentially up to approximately 60,000 km2 of previously unrecognized glacial refugia. The refugia we highlight coincide with higher species richness for figs and palms. Our results provide spatio-temporally explicit insights into the role of refugia across the chimpanzee range, forming the empirical foundation for developing and testing hypotheses about behavioral, ecological, and genetic diversity with additional data. This methodology can be applied to other species and geographic areas when sufficient data are available.

This paper deals with psychiatric comorbidity among 60 women problem drinkers treated in a specialized women-only treatment programme (EWA) at Karolinska Hospital, Stockholm, Sweden. The programme attracts women who have not been previously treated for alcohol problems. The methods used were structured interviews (SCID-I and SCID-II) applied at least 10 days after the start of treatment. All but two of the women had a definite alcohol dependence according to the DSM-III-R, and a majority (60%) also fulfilled the criteria for at least one psychiatric disorder during their lifetime. However, only 23% had a personality disorder (PD), and all subjects with a PD also had at least one Axis I disorder. The most common disorders were mood disorders (48%) and anxiety disorders (38%). However, alcohol dependence developed without definite pre-existing psychiatric disorders among a substantial proportion of the women (40%). It remains to be seen whether and how psychiatric disturbances among female problem drinkers affect treatment compliance and long-term outcome.