Between-group variation in female dispersal, kin composition of groups, and proximity patterns in a black-and-white colobus monkey (Colobus vellerosus)

Social network dynamics, infant loss, and gut microbiota composition in female Colobus vellerosus during time periods with alpha male challenges

The gut microbiota of group-living animals is strongly influenced by their social interactions, but it is unclear how it responds to social instability. We investigated whether social instability associated with the arrival of new males and challenges to the alpha male position could explain differences in the gut microbiota in adult female Colobus vellerosus at Boabeng-Fiema, Ghana. First, we used a data set collected during May-August 2007 and May 2008-2009 that consisted of (i) 50 fecal samples from adult females in eight social groups for V4 16S rRNA sequencing to determine gut microbiota composition, and (ii) demographic and behavioral data ad libitum to determine male immigration, challenges to the alpha male position, and infant births and deaths. Sørensen and Bray-Curtis beta diversity indices (i.e., between-sample microbiota variation) were predicted by year, alpha male stability, group identity, and age. Next, we used a more detailed behavioral data set collected during focal observations of adult females in one group with a prolonged alpha male takeover and three cases of infant loss, to create 12-month versus 3-month 1-m proximity networks that preceded and overlapped the gut microbiome sampling period in that group. The long versus short-term networks were not correlated, suggesting temporal variation in proximity networks. In this group, beta diversity among the five adult females was predicted by similarity in infant loss status and short-term (rather than yearly) 1-m proximity ties. Although the mechanism driving this association needs to be further investigated in future studies, our findings indicate that alpha male takeovers are associated with gut microbiota variation and highlight the importance of taking demographic and social network dynamics into account.

All together now: Assessing variation in maternal and nonmaternal handling of wild Colobus vellerosus infants

Primatologists have a long-standing interest in the study of maternal care and nonmaternal handling (NH) of infants stemming from recognition that early social relationships can have enduring consequences. Though maternal care and NH often include expression of similar behaviors, they are regularly studied in isolation from each other with nonoverlapping terminology, thereby overlooking possible interplay between them and obfuscating potential developmental ramifications that ensue from trade-offs made between maternal (MH) and NH during infancy. To that end, identifying how MH and NH patterns interact and contribute to the total handling (TH) infants receive is a critical first step. We present durational handling data collected from 25 wild Colobus vellerosus infants from 2016 to 2017 and assess the relationships between TH, MH, and NH. Patterns of social affiliation are shaped in part by surrounding context, and therefore, we also assess whether NH and TH differ in their responsivity to various infant and social group characteristics. Ninety-four percent of observed handling was MH, while just 5.5% was NH. Young infants who received more MH (excluding nursing) also received more NH; there was no relationship between the two in older infants. Infants in larger groups participated in more handling of all types. Additionally, NH time was associated with infant sex and group stability. Non-nursing TH time was associated with group stability and infant cohort size. Though NH variation likely confers social-networking advantage, in this population NH is not a major contributor to TH and would not effectively replace reduced MH. The positive association between MH and NH during early infancy suggests that colobus mothers may play a mediating role in shaping infant socialization. This is a first step in elucidating how different forms of handling relate to one another in wild primates and in identifying the impact of handling on infant socialization.

Weak, but not strong, ties support coalition formation among wild female chimpanzees

In social species, individuals may be able to overcome competitive constraints on cooperation by leveraging relationships with familiar, tolerant partners. While strong social ties have been linked to cooperation in several social mammals, it is unclear the extent to which weak social ties can support cooperation, particularly among non-kin. We tested the hypothesis that weakly affiliative social relationships support cooperative coalition formation using 10 years of behavioural data on wild female chimpanzees. Female chimpanzees typically disperse and reside with non-kin as adults. Their social relationships are differentiated but often relatively weak, with few dyads sharing strong bonds. Females occasionally form aggressive coalitions together. Three measures of relationship quality-party association, five-metre proximity and whether a dyad groomed-positively predicted coalitions, indicating that relationship quality influenced coalition partnerships. However, dyads that groomed frequently did not form more coalitions than dyads that groomed occasionally, and kin did not cooperate more than expected given their relationship quality. Thus, strong bonds and kinship did not bolster cooperation. We conclude that cooperative coalitions among female chimpanzees depend on social tolerance but do not require strong bonds. Our findings highlight social tolerance as a distinct pathway through which females can cultivate cooperative relationships. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

Joint intergroup aggression in female colobus monkeys (Colobus vellerosus) is associated with grooming bonds, male participation, and group size

Cooperative home range defense is common in primates, despite a collective action problem that arises when group members benefit from winning the intergroup encounter regardless of whether they participate. The costs associated with this collective action problem may be mitigated by residing in small groups, residing with kin, or by forming strong bonds with group members. The potential to decouple the effects of these variables provided an opportunity to investigate which of these three variables best explains coparticipation in intergroup encounters among adult and subadult female colobus at Boabeng-Fiema, Ghana. Because males are often the main participants, we also investigated the relationship between female-female coparticipation and adult and subadult male participation. We collected intergroup behaviors from 94 adult and subadult individuals in eight groups during 1 year. We quantified female grooming bond strength and approach rates using focal samples. We classified female dyads as close kin (i.e., halfsiblings or more closely related) or nonkin based on partial pedigrees and genotypes generated from 17 STR loci. Female-female coparticipation was higher in dyads with stronger grooming bonds but was not associated with dyadic kinship, approach rate, or age class. Female coparticipation decreased with increasing female group size as expected if there is a collective action problem. Females coparticipated less in groups with more males and male intergroup aggression, possibly because there is less need for female-female cooperation if males are participating in the intergroup encounter. Females in smaller groups may not only benefit from increased female-female cooperation during intergroup encounters, they are also likely to reside with a higher-quality alpha male, both of which may increase the likelihood of winning intergroup encounters. There may be strong selection for facultative female dispersal in populations like the Boabeng-Fiema colobus in which small groups are associated with multiple benefits and cooperation is not affected by kinship.

High levels of infant handling by adult males in Rwenzori Angolan colobus (Colobus angolensis ruwenzorii) compared to two closely related species, C. guereza and C. vellerosus

Infant handling (holding or carrying) by adult males is rare in mammals; however, high levels have been reported in some primates. Though infant handling is a costly behaviour, there are many benefits that male handlers can accrue. Infant handling by males is most conspicuous in platyrrhines and tends to be uncommon in catarrhines. In the latter species, research on male-infant interactions has focused on low-cost behaviours, such as proximity and grooming. However, to better understand the evolution of infant handling by males, more data on its occurrence across the Primate order are essential, even in species where it is relatively uncommon. We compare the occurrence of infant handling by males in three closely related species of catarrhine: Colobus vellerosus, C. guereza, and C. angolensis ruwenzorii. We collected focal animal samples on infants to quantify infant handling rates and durations, and found that adult male C. a. ruwenzorii handled infants much more frequently and for much longer than males in the other two species. We discuss how C. a. ruwenzorii's unique social organization may explain high levels of infant handling by adult males in this species. More long-term and detailed comparisons of infant handling across species and populations will shed light on how sociality has shaped the evolution of this behaviour in the Primate order.

Using strontium isotopes to determine philopatry and dispersal in primates: a case study from Kibale National Park

Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) allow researchers to track changes in mobility throughout an animal's life and could theoretically be used to reconstruct sex-biases in philopatry and dispersal patterns in primates. Dispersal patterns are a life-history variable that correlate with numerous aspects of behaviour and socio-ecology that are elusive in the fossil record. The present study demonstrates that the standard archaeological method used to differentiate between 'local' and 'non-local' individuals, which involves comparing faunal isotopic ratios with environmental isotopic minima and maxima, is not always reliable; aspects of primate behaviour, local environments, geologic heterogeneity and the availability of detailed geologic maps may compromise its utility in certain situations. This study instead introduces a different methodological approach: calculating offset values to compare ⁸⁷Sr/⁸⁶Sr of teeth with that of bone or local environments. We demonstrate this method's effectiveness using data from five species of primates, including chimpanzees, from Kibale National Park, Uganda. Tooth-to-bone offsets reliably indicate sex-biases in dispersal for primates with small home ranges while tooth-to-environment offset comparisons are more reliable for primates with larger home ranges. Overall, tooth-to-environment offsets yield the most reliable predictions of species' sex-biases in dispersal.

Abundance, density, and social structure of African forest elephants (Loxodonta cyclotis) in a human-modified landscape in southwestern Gabon

Population monitoring is critical to effective conservation, but forest living taxa can be difficult to directly observe. This has been true of African forest elephants (Loxodonta cyclotis), for which we have limited information regarding population size and social behavior despite their threatened conservation status. In this study, we estimated demographic parameters using genetic capture-recapture of forest elephants in the southern Industrial Corridor of the Gamba Complex of Protected Areas in southwestern Gabon, which is considered a global stronghold for forest elephants. Additionally, we examined social networks, predicting that we would find matrilineal structure seen in both savanna and forest elephants. Given 95% confidence intervals, we estimate population size in the sampled area to be between 754 and 1,502 individuals and our best density estimate ranges from 0.47 to 0.80 elephants per km². When extrapolated across the entire Industrial Corridor, this estimate suggests an elephant population size of 3,033 to 6,043 based on abundance or 1,684 to 2,832 based on density, approximately 40–80% smaller than previously suggested. Our social network analysis revealed approximately half of network components included females with different mitochondrial haplotypes suggesting a wider range of variation in forest elephant sociality than previously thought. This study emphasizes the threatened status of forest elephants and demonstrates the need to further refine baseline estimates of population size and knowledge on social behavior in this taxon, both of which will aid in determining how population dynamics in this keystone species may be changing through time in relation to increasing conservation threats.

Female Assamese macaques bias their affiliation to paternal and maternal kin

Forming strong social bonds can lead to higher reproductive success, increased longevity, and/or increased infant survival in several mammal species. Given these adaptive benefits, understanding what determines partner preferences in social bonding is important. Maternal relatedness strongly predicts partner preference across many mammalian taxa. The role of paternal relatedness, however, has received relatively little attention, even though paternal and maternal kin share the same number of genes, and theoretically similar preferences would therefore be expected for paternal kin. Here, we investigate the role of maternal and paternal relatedness in female affiliation in Assamese macaques (Macaca assamensis), a species characterized by a relatively low male reproductive skew. We studied a wild population under natural conditions using extensive behavioral data and relatedness analyses based on pedigree reconstruction. We found stronger affiliative relationships and more time spent grooming between maternal kin and paternal half-sisters compared with nonkin, with no preference of maternal over paternal kin. Paternally related and nonrelated dyads did not form stronger relationships when they had less close maternal kin available, but we would need a bigger sample size to confirm this. As expected given the low reproductive skew, affiliative relationships between paternal half-sisters closer in age were not stronger than between paternal half-sisters with larger age differences, suggesting that the kin bias toward paternal kin was not mediated by age similarity. An alternative way through which paternal kin could get familiarized is mother- and/or father-mediated familiarity.

Divergence in gut microbial communities mirrors a social group fission event in a black-and-white colobus monkey (Colobus vellerosus)

Host behavior and social factors have increasingly been implicated in structuring the composition of gut microbial communities. In social animals, distinct microbial communities characterize different social groups across a variety of taxa, although little longitudinal research has been conducted that demonstrates how this divergence occurs. Our study addresses this question by characterizing the gut microbial composition of an African Old World monkey, the black-and-white colobus (Colobus vellerosus), before and after a social group fission event. Gut microbial taxonomic composition of these monkeys was profiled using the V-4 hypervariable region of the bacterial 16S ribosomal RNA gene, and pairwise-relatedness values were calculated for all individuals using 17 short tandem repeat loci and partial pedigree information. The two social groups in this study were found to harbor distinct microbial signatures after the fission event from which they emerged, while these communities were not divergent in the same individuals before this event. Three genera were found to differ in abundance between the two new social groups: Parabacteroides, Coprococcus, and Porphyromonadaceae. Additionally, although this fission happened partially along lines of relatedness, relatedness did not structure the differences that we found. Taken together, this study suggests that distinct gut microbial profiles can emerge in social groups in <1 year and recommends further work into more finely mapping the timescales, causes, and potentially adaptive effects of this recurring trend toward distinct group microbial signatures.

Individual dispersal decisions affect fitness via maternal rank effects in male rhesus macaques

Natal dispersal may have considerable social, ecological and evolutionary consequences. While species-specific dispersal strategies have received much attention, individual variation in dispersal decisions and its fitness consequences remain poorly understood. We investigated causes and consequences of natal dispersal age in rhesus macaques (Macaca mulatta), a species with male dispersal. Using long-term demographic and genetic data from a semi-free ranging population on Cayo Santiago, Puerto Rico, we analysed how the social environment such as maternal family, group and population characteristics affected the age at which males leave their natal group. While natal dispersal age was unrelated to most measures of group or population structure, our study confirmed earlier findings that sons of high-ranking mothers dispersed later than sons of low-ranking ones. Natal dispersal age did not affect males’ subsequent survival, but males dispersing later were more likely to reproduce. Late dispersers were likely to start reproducing while still residing in their natal group, frequently produced extra-group offspring before natal dispersal and subsequently dispersed to the group in which they had fathered offspring more likely than expected. Hence, the timing of natal dispersal was affected by maternal rank and influenced male reproduction, which, in turn affected which group males dispersed to.

A unique case of extra-group infant adoption in free-ranging Angola black and white colobus monkeys (Colobus angolensis palliatus)

Infant adoption has been reported in a variety of primate taxa both in captive and natural settings. Adoption by females may be adaptive by increasing inclusive fitness via shared genes between adoptive mother and adoptee or by providing valuable maternal practice which, in turn, may increase the female's future reproductive success. Others have argued that adoption may be non-adaptive and the result of a general attraction toward infants. Our study examines a unique case of adoption by an adult female Angola black and white colobus monkey (Colobus angolensis palliatus) who adopted an extra-group infant alongside her own biological infant. We compare infant behaviors and mother-infant interactions between biological infant and adoptee and then compare both biological infant and adoptee behavioral profiles to those of infants under normal circumstances. Data were collected from July 2014 to June 2015 on three habituated groups in the Diani Forest of Kenya. Scan sampling and pooled data were used to create daily and monthly behavioral profiles for the biological infant and adoptee, as well as a mean monthly profile of four infants under normal circumstances. Data include time spent (1) clinging to mother/adoptive mother, (2) clinging to another individual, (3) behaving independently, and (4) behaving in close proximity to mother/adoptive mother. Initially, the adoptee struggled to achieve behavioral profiles consistent with those of the biological infant and normal colobus infants of the same age as he spent significantly more time moving independently and significantly less time clinging to the adoptive mother. After the mysterious death of the biological infant in mid-January 2015, the adoptee assumed a behavioral profile similar to that of infants under normal conditions. This case does not support adaptive hypotheses for adoption (i.e., inclusive fitness or learning to mother). Instead, because the biological infant died, possibly due to the presence of the adoptee, we argue that this case of infant adoption was non-adaptive. Ultimately, this adoption appears to have been an outcome of the adoptee's persistent desire to be cared for and the female's strong propensity to engage in allomaternal behavior.

Population genetic insights into the social organization of Guinea baboons (Papio papio): Evidence for female-biased dispersal

Sex differences in philopatry and dispersal have important consequences on the genetic structure of populations, social groups, and social relationships within groups. Among mammals, male dispersal and female philopatry are most common and closely related taxa typically exhibit similar dispersal patterns. However, among four well-studied species of baboons, only hamadryas baboons exhibit female dispersal, thus differing from their congenerics, which show female philopatry and close-knit female social relationships. Until recently, knowledge of the Guinea baboon social system and dispersal pattern remained sparse. Previous observations suggested that the high degree of tolerance observed among male Guinea baboons could be due to kinship. This led us to hypothesize that this species exhibits male philopatry and female dispersal, conforming to the hamadryas pattern. We genotyped 165 individuals from five localities in the Niokolo-Koba National Park, Senegal, at 14 autosomal microsatellite loci and sequenced a fragment of the mitochondrial hypervariable region I (HVRI) of 55 individuals. We found evidence for higher population structuring in males than in females, as expected if males are the more philopatric sex. A comparison of relatedness between male-male and female-female dyads within and among communities did not yield conclusive results. HVRI diversity within communities was high and did not differ between the sexes, also suggesting female gene flow. Our study is the first comprehensive analysis of the genetic population structure in Guinea baboons and provides evidence for female-biased dispersal in this species. In conjunction with their multilevel social organization, this finding parallels the observations for human hunter-gatherers and strengthens baboons as an intriguing model to elucidate the processes that shaped the highly cooperative societies of Homo.

Social bonds in the dispersing sex: partner preferences among adult female chimpanzees

In most primate societies, strong and enduring social bonds form preferentially among kin, who benefit from cooperation through direct and indirect fitness gains. Chimpanzees, Pan troglodytes, differ from most species by showing consistent female-biased dispersal and strict male philopatry. In most East African populations, females tend to forage alone in small core areas and were long thought to have weak social bonds of little biological significance. Recent work in some populations is challenging this view. However, challenges remain in quantifying the influence of shared space use on association patterns, and in identifying the drivers of partner preferences and social bonds. Here, we use the largest data set on wild chimpanzee behaviour currently available to assess potential determinants of female association patterns. We quantify pairwise similarities in ranging, dyadic association and grooming for 624 unique dyads over 38 years, including 17 adult female kin dyads. To search for social preferences that could not be explained by spatial overlap alone, we controlled for expected association based on pairwise kernel volume intersections of core areas. We found that association frequencies among females with above-average overlap correlated positively with grooming rates, suggesting that associations reflected social preferences in these dyads. Furthermore, when available, females preferred kin over nonkin partners for association and grooming, and variability was high among nonkin dyads. While variability in association above and below expected values was high, on average, nonkin associated more frequently if they had immature male offspring, while having female offspring had the opposite effect. Dominance rank, an important determinant of reproductive success at Gombe, influenced associations primarily for low-ranking females, who associated preferentially with each other. Our findings support the hypothesis that female chimpanzees form well-differentiated social relationships that are of potential adaptive value to females and their offspring.

The evolution of group dispersal with leaders and followers

In many species, individuals disperse in groups. While there are empirical studies that explore the proximate incentives for group dispersal, theoretical research has primarily examined the consequences rather than the evolution of this phenomenon. We design a simple model to study the origin and evolution of group dispersal. We assume that like many other group activities associated with collective movement, group dispersal in our model is initiated by leaders. We use the theory of inclusive fitness to examine the incentives for leading and following in this context. High relatedness, significant reductions in the cost of dispersal due to dispersing in groups, and reproductive skew in favour of followers facilitates the emergence of group dispersal. In contrast to some previous theoretical work, which has either concluded that leadership is uniformly altruistic or that it is uniformly selfish, we find that at evolutionary equilibrium the incentives for leading can be either selfish or altruistic. The nature of result (selfish or altruistic) depends on ecological and social conditions such as the cost of dispersal and the relatedness between leaders and followers. Our model demonstrates that kin selection is sufficient and that individual differences in condition and ability are not necessary to promote the emergence and maintenance of leader-follower relationships.

Familiarity is more important than phenotypic similarity in shaping social relationships in a facultative female dispersed primate, Colobus vellerosus

Animals often bias affiliative behaviors toward kin, but it is unclear what mechanism most species use to discriminate kin. We investigated if facultative dispersed female primates use phenotype matching and/or familiarity to discriminate female kin. We studied 38 adult female Colobus vellerosus at Boabeng-Fiema, Ghana. We determined dyadic co-residency status and age proximity using long-term demographic data, R-values from 17 short tandem repeat loci, and interaction rates using focal samples collected during one year. Approach rates were not strongly affected by how long females had resided together, which contrasts to the familiarity hypothesis. Females approached and groomed maternal kin more than other females, which supports the mother-mediated familiarity hypothesis. Females did not discriminate paternal half siblings from non-kin, and they did not prefer to interact with females of similar age. Short-term co-resident kin did not bias affiliation toward each other, indicating that female colobus cannot consistently recognize less familiar kin via phenotype matching or that biasing behaviors toward less familiar kin is not beneficial. Despite showing facultative dispersal that may reduce the accuracy of using familiarity as a kin recognition mechanism, female choice of social partners was based on familiarity, which conforms to the pattern observed in many female philopatric primates.

Factors influencing male affiliation and coalitions in a species with male dispersal and intense male–male competition, Colobus vellerosus

Male Colobus vellerosus compete intensely for access to females, which sometimes leads to mortal wounding. Yet, males often form cooperative relationships to overtake prime-aged males and immigrate into bisexual groups. We investigated the factors that predicted the presence of coalitions and affiliative relationships among males in this species. Interactions among males in 292 dyads from six groups were examined from 2004 to 2010 at Boabeng-Fiema, Ghana. Affiliation rates among males were higher and aggression rates lower when one or both males in the dyad were subadult, compared to adult male dyads. Affiliation rates tended to be higher among males that were kin but no other aspect of male relationships predicted affiliation. Coalitions among males were rarely observed and primarily occurred in the context of joint defense against extra-group males (93.5% of events). Adult males were more likely to provide coalitionary support than subadults and coalitions occurred significantly more often when both males were high ranking, since these males probably benefited most in terms of reproductive success from excluding extra-group males. Rank-changing and leveling coalitions among low-ranking males appear to be quite rare or absent in C. vellerosus. The costs of these types of coalitions may be too high or male group size too small on average for these types of coalitions to have been selected for. The overall low rates of affiliation and coalitions among male C. vellerosus are likely influenced by male-biased dispersal and the high level of male–male competition.