Friendships between males and lactating females in a free-ranging group of olive baboons (Papio hamadryas anubis): evidence from playback experiments

Peptides and primate personality: Central and peripheral oxytocin and vasopressin levels and social behavior in two baboon species (Papio hamadryas and Papio anubis)

The neurohormones oxytocin (OT) and arginine vasopressin (AVP) are involved in social behaviors and psychiatric conditions. However, more research on nonhuman primates with complex social behaviors is needed. We studied two closely-related primate species with divergent social and mating systems; hamadryas baboons (Papio hamadryas, n=38 individuals) and anubis baboons (Papio anubis, n=46). We measured OT in cerebrospinal fluid (CSF, n=75), plasma (n=81) and urine (n=77), and AVP in CSF (n=45), and we collected over 250 hours of focal behavioral observations. Using Bayesian multivariate models, we found no clear species difference in hormone levels; the strongest support was for hamadryas having higher CSF OT levels than anubis (posterior probability [PP] for females = 0.75, males = 0.84). Looking at nine specific behaviors, OT was associated with affiliative behaviors (approach, proximity, grooming, PP ∼ 0.85 - 1.00), albeit inconsistently across sources of measurement (CSF, plasma, and urine, which were uncorrelated with each other). Most behaviors had low repeatability (R ∼ 0 - 0.2), i.e. they did not exhibit stable between-individual differences (or "personality"), and different behaviors did not neatly coalesce into higher-order factors (or "behavioral syndromes"), which cautions against the use of aggregate behavioral measures and highlights the need to establish stable behavioral profiles when testing associations with baseline hormone levels. In sum, we found some associations between peptides and social behavior, but also many null results, OT levels from different sources were uncorrelated, and our behavioral measures did not indicate clear individual differences in sociability.

Infant spatial relationships with adult males in a wild primate: males as mitigators or magnifiers of intergenerational effects of early adversity?

Adult male mammals can provide infants with protection and enhance their access to resources. They can also pose a risk to infants, either directly through infanticide or other aggression, or indirectly by placing infants at increased risk of conspecific or heterospecific conflict. Both benefits and costs may be especially important for offspring born to mothers in poor condition. Here we present the most detailed analysis to date of the influence of adult non-human primate males on a wide range of infant behaviors, and a description of the predictors of individual infants' proximity to adult males. We show that the number of adult males near an infant predicts many infant behavioral traits, including aspects of the mother-infant relationship, infant activity budgets, and the frequency of social interactions with non-mothers. Infant exposure to adult males is statistically significantly repeatable over time (R = 0.16). This repeatability is partially explained by whether the infant's mother experienced early life adversity: offspring of high-adversity mothers spent time in close proximity to more males during the first months of life. Our results are consistent with the possibility that the effects of maternal early life adversity can be mitigated or magnified by relationships with adult males.

Socially bonded females face more sexual coercion in a female-philopatric primate

Sexual coercion is a manifestation of sexual conflict increasing male mating success while inflicting costs to females. Although previous work has examined inter-individual variation in male sexually coercive tactics, little is known about female counter-strategies. We investigated whether social bonding mitigates the extent of sexual coercion faced by female mandrills (Mandrillus sphinx), as a putative mechanism linking sociality to fitness. Surprisingly, females faced the most coercion from those males with whom they formed the strongest bonds, while the strength of a female-male bond was also positively correlated with coercion from all other males. Finally, greater social integration in the female network was positively correlated with coercion, through a direct 'public exposure' mechanism and not mediated by female reproductive success or retaliation potential. Altogether, this study shows that neither between- nor within-sex bonds are protective against sexual coercion and identifies, instead, a hidden cost of social bonding.

Tradeoffs between mating effort and parenting effort in a polygynandrous mammal

Reproductive strategies are defined by expenditures of time and energy devoted to mating effort, which increases mating opportunities, and parenting effort, which enhances the survival of offspring. We examine tradeoffs between mating effort and parenting effort in male olive baboons, Papio anubis, a species in which males compete for mating opportunities, but also form ties to lactating females (primary associations) that represent a form of parenting effort. Males that are involved in more primary associations invest less in mating effort than males who are involved in fewer primary associations. Males that are involved in more primary associations play a smaller role in establishing proximity to their primary associates than other males, suggesting that males operate under temporal constraints. There is also some evidence that involvement in primary associations negatively affects paternity success. Taken together, the data suggest that males face tradeoffs between mating effort and parenting effort.

Primate Sex and Its Role in Pleasure, Dominance and Communication

Sexual intercourse in the animal kingdom functions to enable reproduction. However, we now know that several species of non-human primates regularly engage in sex outside of the times when conception is possible. In addition, homosexual and immature sex are not as uncommon as were once believed. This suggests that sex also has important functions outside of reproduction, yet these are rarely discussed in sex-related teaching and research activities concerning primate behaviour. Is the human sexual experience, which includes pleasure, dominance, and communication (among others) unique, or do other primates also share these experiences to any extent? If so, is there any way to measure them, or are they beyond the rigour of scientific objectivity? What would be the evolutionary implications if human-like sexual experiences were found amongst other animals too? We comment on the evidence provided by our close relatives, non-human primates, discuss the affective and social functions of sex, and suggest potential methods for measuring some of these experiences empirically. We hope that this piece may foster the discussion among academics and change the way we think about, teach and research primate sex.

Infant adoptions in wild bonnet macaques (Macaca radiata)

Though uncommon, adoption of orphaned infants has been observed in both wild and captive non-human primates. In two groups of wild bonnet macaques (Macaca radiata), we observed five instances of infants being cared for after they lost their mothers at a pre-weaning age (< 6 months). Orphaned infants had one or more caregivers (juvenile, subadult, and adult female or male) involved in carrying, grooming, hugging, and protecting them. Adoption did not appear to be related to the age/sex class of the infant, or directly to the mother's rank. Although the dominance rank of the mother of an orphaned infant did not have a direct effect on orphan survivorship, it determined the number of caregivers available to the orphaned infant, and infant survivorship was positively related to the number of caregivers of the orphaned infant. Thus, survivorship was likely a function of the mother's sociality. Two other infants born to high-ranking mothers were also adopted by more individuals and survived longer than the infants of low-ranking mothers.

Yet Another Non-Unique Human Behaviour: Leave-Taking in Wild Chacma Baboons (Papio ursinus)

Simple Summary

Although greeting is well-studied across animal species, its counterpart, leave taking, is little studied in nonhumans. Here, we review the previous limitations of leave-taking research and use this to develop a new method for studying leave taking in nonhumans. Using videos of chacma baboons in Gorongosa National Park, Mozambique, we compared behaviours at the end of social departures to nonsocial departures. We found that shifting orientation towards the direction of parting was significantly more likely in social departures compared to nonsocial departures. As the first evidence of leave taking in a wild nonhuman species, we suggest that leave taking is not uniquely human as previously argued, and that our method could be used to further explore the presence of leave taking in other nonhuman species.

Abstract

Leave taking is a common, possibly universal, feature of human social behaviour that has undergone very little empirical research. Although the importance remains unknown, it has been suggested to play an important role in managing separations, mitigating the risk, and increasing social bonding beyond the interaction itself. In nonhuman species, the literature is virtually absent, but identifying leave taking beyond humans may provide unique insights into the evolutionary history of this behaviour and shed light onto its proximate and ultimate function(s). Methods to study leave taking are not well-established, and the variation in definitions, measures, and control variables presented in past studies poses additional challenges. Baboons are a valuable model for investigating human behavioural evolution: as a flexible, highly adaptable, and social primate whose radiation is, similarly to humans, associated with the emergence of the African savannah biome. Using the framework and definition proposed by Baehren, we investigated the presence of leave taking in a wild, generalist primate and tested a range of candidate behaviours on prerecorded video footage: (1) self-scratching, (2) eye gaze, and (3) orientation in the direction of parting. Using multivariate analysis, controlling for interaction duration and individual variation, our results show that orientation in the direction of parting occurs predominantly before social separation events. These results indicate evidence of leave taking in a wild nonhuman population and contrast with previous ideas that this is a uniquely human behaviour. The presence of leave taking in baboons suggests a deep evolutionary history of this behaviour, warranting further investigation into its function and presence across other nonhuman primate species.

No evidence that grooming is exchanged for coalitionary support in the short- or long-term via direct or generalized reciprocity in unrelated rhesus macaques

Abstract

Reciprocity is a prominent explanation for cooperation between non-kin. Studies designed to demonstrate reciprocity often focus on direct reciprocity in the timescale of minutes to hours, whereas alternative mechanisms like generalized reciprocity and the possibility of reciprocation over longer timescales of months and years are less often explored. Using a playback experiment, we tested for evidence of direct and generalized reciprocity across short and longer timescales. We examined the exchange of grooming for coalitionary support between unrelated female rhesus macaques in a population with a complete genetic pedigree. Females that received grooming were not more responsive to calls for coalitionary support from unrelated female group mates compared to control females that received agonism or no interaction — even when the call belonged to a females’ most recent grooming partner. Similarly, females were not more responsive to calls for support from their most frequent unrelated grooming partner of the last two years, nor if they received large amounts of grooming from all other females in their group. We interpret these results as an absence of evidence for direct or generalized reciprocity on any timescale in the exchange of grooming for coalitionary support in rhesus macaques. If grooming is exchanged for support in this population, it is with an intensity below our ability to detect it or over a longer timescale than we examined. We propose by-product explanations may also be at play and highlight the importance of investigating multiple mechanisms when testing apparently cooperative behaviors.

Significance statement

The receipt of help can make some animals more likely to provide help in return, whether it be a singular act, or many acts accumulated over months. Similarly, the receipt of help, be it one act of aid, or a group’s worth of help over time, can make some animals more likely to pay help forward to others. Studies on Old World monkeys suggest females may give grooming and in return receive aid in future physical conflicts. Using a playback experiment, we found female rhesus macaques were not more responsive to calls for intervention in a simulated conflict after being groomed by unrelated females, even if the calling combatant was her most recent, or a long-time grooming partner. Our results suggest females in our study population may be receiving benefits other than support in conflicts for the grooming they provide.

Pathways to paternal care in primates

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

Who is there? Captive western gorillas distinguish human voices based on familiarity and nature of previous interactions

The ability to recognize conspecifics by their acoustic signals is of crucial importance to social animals, especially where visibility is limited, because it allows for discrimination between familiar and unfamiliar individuals and facilitates associations with and the avoidance of particular conspecifics. Animals may also benefit from an ability to recognize and use the information coded into the auditory signals of other species. Companion species such as dogs, cats, and horses are able to discriminate between familiar and unfamiliar human voices; however, whether this ability is widespread across vertebrates is still unknown. Using playback experiments, we tested whether western gorillas living at Zoo Atlanta were able to discriminate between the voices of subgroups of people: i.e., unfamiliar individuals, familiar individuals with whom the gorillas had positive interactions, and familiar individuals with whom they had negative interactions. Gorillas responded significantly more often (longer gazing duration, higher gazing frequency, shorter latency, and larger number of distress behaviors) to the voices of unfamiliar and familiar-negative individuals than to those of familiar-positive individuals, indicating that they recognized the voices of subgroup of people based on familiarity and possibly the nature of the relationship with them. Future studies should determine whether this is also the case in the wild, where interspecific associations with humans are less intense than they are in captive settings.

Genetic ancestry predicts male-female affiliation in a natural baboon hybrid zone

Opposite-sex social relationships are important predictors of fitness in many animals, including several group-living mammals. Consequently, understanding sources of variance in the tendency to form opposite-sex relationships is important for understanding social evolution. Genetic contributions are of particular interest due to their importance in long-term evolutionary change, but little is known about genetic effects on male-female relationships in social mammals, especially outside of the mating context. Here, we investigate the effects of genetic ancestry on male-female affiliative behaviour in a hybrid zone between the yellow baboon, Papio cynocephalus, and the anubis baboon, Papio anubis, in a population in which male-female social bonds are known predictors of life span. We place our analysis within the context of other social and demographic predictors of affiliative behaviour in baboons. Genetic ancestry was the most consistent predictor of opposite-sex affiliative behaviour we observed, with the exception of strong effects of dominance rank. Our results show that increased anubis genetic ancestry is associated with a subtle, but significantly higher, probability of opposite-sex affiliative behaviour, in both males and females. Additionally, pairs of anubis-like males and anubis-like females were the most likely to socially affiliate, resulting in moderate assortativity in grooming and proximity behaviour as a function of genetic ancestry. Our findings indicate that opposite-sex affiliative behaviour partially diverged during baboon evolution to differentiate yellow and anubis baboons, despite overall similarities in their social structures and mating systems. Furthermore, they suggest that affiliative behaviour may simultaneously promote and constrain baboon admixture, through additive and assortative effects of ancestry, respectively.

Who is there? Captive western gorillas distinguish human voices based on familiarity and nature of previous interactions

The ability to recognize conspecifics by their acoustic signals is of crucial importance to social animals, especially where visibility is limited, because it allows for discrimination between familiar and unfamiliar individuals and facilitates associations with and the avoidance of particular conspecifics. Animals may also benefit from an ability to recognize and use the information coded into the auditory signals of other species. Companion species such as dogs, cats, and horses are able to discriminate between familiar and unfamiliar human voices; however, whether this ability is widespread across vertebrates is still unknown. Using playback experiments, we tested whether western gorillas living at Zoo Atlanta were able to discriminate between the voices of subgroups of people: i.e., unfamiliar individuals, familiar individuals with whom the gorillas had positive interactions, and familiar individuals with whom they had negative interactions. Gorillas responded significantly more often (longer gazing duration, higher gazing frequency, shorter latency, and larger number of distress behaviors) to the voices of unfamiliar and familiar-negative individuals than to those of familiar-positive individuals, indicating that they recognized the voices of subgroup of people based on familiarity and possibly the nature of the relationship with them. Future studies should determine whether this is also the case in the wild, where interspecific associations with humans are less intense than they are in captive settings.

Balancing costs and benefits in primates: ecological and palaeoanthropological views

Maintaining the balance between costs and benefits is challenging for species living in complex and dynamic socio-ecological environments, such as primates, but also crucial for shaping life history, reproductive and feeding strategies. Indeed, individuals must decide to invest time and energy to obtain food, services and partners, with little direct feedback on the success of their investments. Whereas decision-making relies heavily upon cognition in humans, the extent to which it also involves cognition in other species, based on their environmental constraints, has remained a challenging question. Building mental representations relating behaviours and their long-term outcome could be critical for other primates, but there are actually very little data relating cognition to real socio-ecological challenges in extant and extinct primates. Here, we review available data illustrating how specific cognitive processes enable(d) modern primates and extinct hominins to manage multiple resources (e.g. food, partners) and to organize their behaviour in space and time, both at the individual and at the group level. We particularly focus on how they overcome fluctuating and competing demands, and select courses of action corresponding to the best possible packages of potential costs and benefits in reproductive and foraging contexts. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

MHC-Based Mate Choice in Wild Golden Snub-Nosed Monkeys

The genes of the major histocompatibility complex (MHC) are an important component of the vertebrate immune system and play a significant role in mate choice in many species. However, it remains unclear whether female mate choice in non-human primates is based on specific functional genes and/or genome-wide genes. The golden snub-nosed monkey (Rhinopithecus roxellana) lives in a multilevel society, which consists of several polygynous one-male-several-female units. Although adult females tend to mainly socialize with one adult male, females often initiate extra-pair copulations with other males resulting in a high proportion of offspring being fathered by extra-pair males. We investigated the effects of adaptive MHC genes and neutral microsatellites on female mate choice in a wild R. roxellana population. We sequenced 54 parent-offspring triads using two MHC class II loci (Rhro-DQA1 and Rhro-DQB1) and 20 microsatellites from 3 years of data. We found that the paternities of offspring were non-randomly associated with male MHC compositions not microsatellite genotypes. Our study showed that the fathers of all infants had significantly less variance for several estimates of genetic similarity to the mothers compared with random males at both MHC loci. Additionally, the MHC diversity of these fathers was significantly higher than random males. We also found support for choice based on specific alleles; compared with random males, Rhro-DQA1^∗ 05 and Rhro-DQB1^∗ 08 were more common in both the OMU (one-male unit) males and the genetic fathers of offspring. This study provides new evidence for female mate choice for MHC-intermediate dissimilarity (rather than maximal MHC dissimilarity) and highlights the importance of incorporating multiple MHC loci and social structure into studies of MHC-based mate choice in non-human primates.

Philopatry at the frontier: A demographically driven scenario for the evolution of multilevel societies in baboons (Papio)

The baboons (Papio sp.) exhibit marked interspecies variation in social behavior. The thesis presented here argues, first, that male philopatry is a crucial factor, arguably the crucial factor, underlying the other distinctive features (one-male units, multilevel society) shared by hamadryas and Guinea baboons, but not other species of Papio. The second suggestion is that male philopatry as a population norm was not an adaptation to a particular habitat or set of ecological circumstances but evolved in the common ancestor of hamadryas and Guinea baboons as a response to natural selection in the demographic context peculiar to the frontier of a rapidly expanding population. Other derived features of social structure (male-male tolerance, some facultative female dispersal) subsequently evolved to accommodate male philopatry. The mitochondrial genetic population structure of extant baboons preserves a footprint of the initial expansion of 'modern' Papio. Immediately after the expansion, male-philopatric, multilevel populations with a general physical and behavioral resemblance to Guinea baboons occupied the whole northern hemisphere range of the genus. Behavioral and physical autapomorphies of hamadryas baboons evolved in a subpopulation of this ancestral northern base, in response to a less productive habitat of the Horn of Africa. Subsequently, ancestral olive baboons 'reinvented' male dispersal. They and yellow baboons, another male-dispersing species, then replaced most of the male-philopatric northern populations, by male-driven introgression and nuclear swamping.

Breaking conversational rules matters to captive gorillas: A playback experiment

Across human cultures, conversations are regulated by temporal and social rules. The universality of conversational rules suggests possible biological bases and encourages comparisons with the communicative interactions of nonhuman animals. Unexpectedly, few studies have focused on other great apes despite evidence of proto-conversational rules in monkeys, thus preventing researchers from drawing conclusions on potential evolutionary origins of this behaviour. A previous study showed however that western lowland gorillas engage in soft call interactions that seem temporally- and socially-ruled. Indeed, interactions occurred mainly between individuals close in age who followed a preset response delay, thus preventing call overlap. Here, we experimentally investigated the presence of these rules in a captive gorilla group, using a violation-of-expectation paradigm. Head orientation responses suggest that the respect of response delay matters to subjects, but the importance of the interlocutors' age proximity appeared less clear. The intensity of the response varied with subjects' age in a context-dependent way, supporting a possible role of learning. Our findings support the growing number of studies highlighting the importance of vocal turn-taking in animals and a possible sociogenesis of this ability. The capacity to "converse" might have been a key step in the co-evolution of communication and complex sociality.

Shifts in Male Reproductive Tactics over the Life Course in a Polygynandrous Mammal

In polygynous and polygynandrous species, there is often intense male-male competition over access to females, high male reproductive skew, and more male investment in mating effort than parenting effort [1]. However, the benefits derived from mating effort and parenting effort may change over the course of males' lives. In many mammalian species, there is a ∩-shaped relationship between age, condition, and resource holding power as middle-aged males that are in prime physical condition outcompete older males [2-8] and sire more infants [9-12]. Thus, males might derive more benefits from parenting effort than mating effort as they age and their competitive abilities decline [13]. Alternatively, older males may invest more effort in making themselves attractive to females as mates [14]. One way that older males might do so is by developing relationships with females and providing care for their offspring [14, 15]. Savannah baboons provide an excellent opportunity to test these hypotheses. They form stable multi-male, multi-female groups, and males compete for high ranking positions. In yellow and chacma baboons (Papio cynocephalus and P. ursinus), there is a ∩-shaped relationship between male age and dominance rank [12], and high rank enhances paternity success [12, 16]. Lactating female baboons form close ties ("primary associations" hereafter) with particular males [15-20], who support them and their infants in conflicts [15, 19] and buffer their infants from rough handling [20]. Females' primary associates are often, but not always, the sires of their current infants [16, 20-22].

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.

Insights into the evolution of social systems and species from baboon studies

Baboons, members of the genus Papio, comprise six closely related species distributed throughout sub-Saharan Africa and southwest Arabia. The species exhibit more ecological flexibility and a wider range of social systems than many other primates. This article summarizes our current knowledge of the natural history of baboons and highlights directions for future research. We suggest that baboons can serve as a valuable model for complex evolutionary processes, such as speciation and hybridization. The evolution of baboons has been heavily shaped by climatic changes and population expansion and fragmentation in the African savanna environment, similar to the processes that acted during human evolution. With accumulating long-term data, and new data from previously understudied species, baboons are ideally suited for investigating the links between sociality, health, longevity and reproductive success. To achieve these aims, we propose a closer integration of studies at the proximate level, including functional genomics, with behavioral and ecological studies.

Male-female relationships in olive baboons (Papio anubis): Parenting or mating effort?

Long-term male-female bonds and bi-parental investment in offspring are hallmarks of human society. A key question is how these traits evolved from the polygynandrously mating multimale multifemale society that likely characterized the Pan-Homo ancestor. In all three species of savanna baboons, lactating females form strong ties (sometimes called "friendships") with one or more adult males. For yellow baboons (Papio cynocephalus) and chacma baboons (Papio ursinus), several lines of evidence suggest that these relationships are a form of male parenting effort. In olive baboons (Papio anubis), females are thought to preferentially mate with their "friends", and male-female bonds may thus function as a form of mating effort. Here, we draw on behavioral and genetic data to evaluate the factors that shape male-female relationships in a well-studied population of olive baboons. We find support for the parenting effort hypothesis in that sires have stronger bonds with their infants' mothers than do other males. These bonds sometimes persist past weaning age and, in many cases, the sire of the previous infant is still a close partner of the female when she nurses her subsequent offspring. We find that males who have the strongest bonds with females that have resumed cycling, but are not currently sexually receptive, are more likely to sire the female's next offspring but the estimate is associated with large statistical uncertainty. We also find that in over one third of the cases, a female's successive infants were sired by the same male. Thus, in olive baboons, the development of stable breeding bonds and paternal investment seem to be grounded in the formation of close ties between males and anestrous females. However, other factors such as male dominance rank also influence paternity success and may preclude stability of these bonds to the extent found in human societies.

Oestrous females avoid mating in front of adult male bystanders in wild chacma baboons

In social species, female mating strategies can be constrained by both male and female groupmates through sexual conflict and reproductive competition, respectively. This study tests if females adjust their sexual behaviour according to the presence of male and female bystanders in wild chacma baboons (Papio ursinus) and assesses their relative importance. Our results show that oestrous females initiate fewer copulations in the presence of adult male bystanders, irrespective of whether they are mate-guarded or not. This inhibitory effect probably reflects a response to indirect sexual coercion by males, whose close proximity may dissuade females to initiate copulations with rival males to avoid punishment and/or aggressive mating interference. By contrast, females initiate more matings with their mate-guard in the presence of higher-ranking female bystanders, which may reflect an attempt to secure bodyguard services from their mate when they feel threatened. These results emphasize the importance of intra- and intersexual conflicts in shaping female sexual behaviour in this promiscuous society.

Baboon vocal repertoires and the evolution of primate vocal diversity

A remarkable and derived trait of humans is the faculty for language, and considerable research effort has been devoted to understanding the evolution of speech. In contrast to spoken language, which constitutes a (learned) symbolic communication system, the acoustic structure of nonhuman primate vocalizations is largely genetically fixed. Yet, appreciable differences between different genera and species may exist. Environmental conditions, sexual selection, and characteristics of the social system have been invoked to explain these differences. Here, we studied the acoustic variation of call types and vocal repertoires in the genus Papio. Because the genus comprises both stable groups as well as multi-level societies, and reveals striking variation in the degree of aggressiveness from south to north, it constitutes a promising model to assess the link between social system characteristics and vocal communication. We found that, the vocal repertoires of the different species were composed of the same general call types. A quantitative analysis of the acoustic features of the grunts and loud calls of chacma (Papio ursinus), olive (P. anubis), and Guinea (P. papio) baboons showed subtle acoustic differences within call types, however. Social system characteristics did not map onto acoustic variation. We found no correlation between the structure of grunts and geographic distance; the same was true for female loud calls. Only for male loud calls from three populations, call structure varied with geographic distance. Our findings corroborate the view that the structure of nonhuman primate vocalizations is highly conserved, despite the differences in social systems. Apparently, variation in rate and intensity of occurrence of signals, probably due to different behavioral dispositions in species, are sufficient to allow for plasticity at the level of the social relationships, mating patterns, and social organization.

Numerical assessment in the wild: insights from social carnivores

Playback experiments have proved to be a useful tool to investigate the extent to which wild animals understand numerical concepts and the factors that play into their decisions to respond to different numbers of vocalizing conspecifics. In particular, playback experiments have broadened our understanding of the cognitive abilities of historically understudied species that are challenging to test in the traditional laboratory, such as members of the Order Carnivora. Additionally, playback experiments allow us to assess the importance of numerical information versus other ecologically important variables when animals are making adaptive decisions in their natural habitats. Here, we begin by reviewing what we know about quantity discrimination in carnivores from studies conducted in captivity. We then review a series of playback experiments conducted with wild social carnivores, including African lions, spotted hyenas and wolves, which demonstrate that these animals can assess the number of conspecifics calling and respond based on numerical advantage. We discuss how the wild studies complement those conducted in captivity and allow us to gain insights into why wild animals may not always respond based solely on differences in quantity. We then consider the key roles that individual discrimination and cross-modal recognition play in the ability of animals to assess the number of conspecifics vocalizing nearby. Finally, we explore new directions for future research in this area, highlighting in particular the need for further work on the cognitive basis of numerical assessment skills and experimental paradigms that can be effective in both captive and wild settings.This article is part of a discussion meeting issue 'The origins of numerical abilities'.

Long-term-stability of relationship structure in family groups of common marmosets, and its link to proactive prosociality

Cooperatively breeding, group-living common marmosets show differentiated relationships, where more strongly bonded dyads within a group engage more in affiliative interactions than less strongly bonded ones. Intriguingly, recent results suggest that strong bonds do not only occur between breeding partners but between individuals from any sex or status, and that strong-bond partners exhibit correlated oxytocin fluctuations (dyadic oxytocin synchrony, OTS) over a period of six weeks. To date, it is unclear whether such relationships are stable over time and whether they are also reflected in higher partner-specific proactive prosociality. To assess the long-term stability of the relationship structure of common marmoset family groups, we investigated whether hormonal and behavioral markers of group structure (dyadic OTS, dyadic affiliation, and individual group integration) in common marmoset families remained stable over a period of six months. We collected baseline urinary OT and social behavior of 36 dyads from three family groups in a non-reproductive period (period A), and again six months later, around the birth of new infants (period B). Patterns of dyadic OTS, dyadic affiliation, and individual group integration were consistent between the two study periods. Oxytocin data from a fourth group (10 dyads), collected in two non-reproductive periods separated by a period of more than five years, could replicate this finding. Furthermore, OTS was also correlated with proactive prosociality that was assessed experimentally for 38 dyads during an earlier study. These results suggest that differentiated relationships are stable over time, even between group members other than the breeding pair, and that more strongly bonded partners also show higher levels of proactive prosociality. Future studies are necessary to identify whether these relationships have an adaptive function, perhaps with regard to positive consequences on cooperativeness.

Charting the neglected West: The social system of Guinea baboons

OBJECTIVES

Primate social systems are remarkably diverse, and thus play a central role in understanding social evolution, including the biological origin of human societies. Although baboons have been prominently featured in this context, historically little was known about the westernmost member of the genus, the Guinea baboon (Papio papio).

MATERIAL AND METHODS

Here, we summarize the findings from the first years of observations at the field site CRP Simenti in the Niokolo Koba National Park in Senegal.

RESULTS

Guinea baboons reveal a nested multi-level social organization, with reproductive units comprising one "primary" male, one to several females, young, and occasionally "secondary" males at the base of the society. Three to five units form "parties," which team up with other parties to form a "gang." Different gangs have largely overlapping home ranges and agonistic interactions between different parties or gangs are rare. Some but not all strongly socially bonded males are highly related, and population genetic and behavioral evidence indicate female-biased dispersal. Females play an important role in intersexual bond formation and maintenance, and female tenure length varies between a few weeks to several years.

DISCUSSION

While the social organization resembles that of hamadryas baboons (P. hamadryas), the social structure differs considerably, specifically in terms of low male aggressiveness and female freedom. Despite substantial differences in social organization and social structure, the acoustic structure of Guinea baboon vocalizations does not differ substantially from that of other baboon taxa. With its multi-level organization, stable bonds between males and females, as well as a high-degree of male-male cooperation and tolerance, Guinea baboons constitute an intriguing model for reconstructing human social evolution.

Patterns and Consequences of Male-Infant Relationships in Wild Assamese Macaques (Macaca assamensis)

Male care for offspring is unexpected in polygynandrous mammals. Evidence from nonhuman primates, however, indicates not only the existence of stable male-immature associations in multimale-multifemale groups, but also male care in the form of protection from infanticidal attacks and conspecific harassment. Here, we investigate the relationship characteristics, dynamics, and consequences of male-immature associations in wild Assamese macaques, Macaca assamensis, at Phu Khieo Wildlife Sanctuary, Thailand, to inform hypotheses of their evolutionary origins. Female Assamese macaques reproduce seasonally and do not signal ovulation, resulting in low mating and paternity skew. However, male-immature associations are predicted by paternity, and male behavior potentially reflects paternal effort. We present focal animal data on 12 immatures followed from birth beyond weaning into their juvenile life (1188 focal hours). The distribution of composite sociality indices suggests that male-immature relationships were highly differentiated. Association patterns and the degree of differentiation remained stable from 6 mo well into the juvenile phase, suggesting that male protection extends beyond the phase of high infanticide risk. Based on Hinde indices, immatures were responsible for maintaining the relationships. The likelihood that an infant was associated with its preferred male increased if the mother was absent and if other males were present in proximity, suggesting that immatures sought protection. The presence of the preferred male did not decrease the rate of mild aggression immatures received from group members, but the stronger the relationship between an immature and a male, the more often it received agonistic support from him. Future studies will have to assess whether this agonistic support translates into improved fitness and represents true paternal care.

Sex and friendship in a multilevel society: behavioural patterns and associations between female and male Guinea baboons

One key question in social evolution is the identification of factors that promote the formation and maintenance of stable bonds between females and males beyond the mating context. Baboons lend themselves to examine this question, as they vary in social organisation and male-female association patterns. We report the results from the first systematic observations of individually identified wild female Guinea baboons. Guinea baboons live in a multilevel society with female-biased dispersal. Although several males could be found within 5 m of females, each female chiefly associated with one "primary" male at the 2 m distance. Social interactions occurred predominantly with the primary male, and female reproductive state had little influence on interaction patterns. The number of females per primary male varied from 1 to 4. During the 17-month study period, half of the females transferred between different males one or multiple times. A subset of females maintained weaker affiliative nonsexual relationships with other "secondary" males. Units composed of primary males with females, and occasional secondary males, apparently form the core of the Guinea baboon society. The social organisation and mating patterns of Guinea and hamadryas baboons may have a common evolutionary origin, despite notable differences in relationship quality. Specifically, Guinea baboon females appear to have greater leverage in their association patterns than hamadryas baboon females. Although we cannot yet explain the lack of overt male control over females, results generally support the notion that phylogenetic descent may play an important role in shaping social systems.

Distress Calls of a Fast-Flying Bat (Molossus molossus) Provoke Inspection Flights but Not Cooperative Mobbing

Many birds and mammals produce distress calls when captured. Bats often approach speakers playing conspecific distress calls, which has led to the hypothesis that bat distress calls promote cooperative mobbing. An alternative explanation is that approaching bats are selfishly assessing predation risk. Previous playback studies on bat distress calls involved species with highly maneuverable flight, capable of making close passes and tight circles around speakers, which can look like mobbing. We broadcast distress calls recorded from the velvety free-tailed bat, Molossus molossus, a fast-flying aerial-hawker with relatively poor maneuverability. Based on their flight behavior, we predicted that, in response to distress call playbacks, M. molossus would make individual passing inspection flights but would not approach in groups or approach within a meter of the distress call source. By recording responses via ultrasonic recording and infrared video, we found that M. molossus, and to a lesser extent Saccopteryx bilineata, made more flight passes during distress call playbacks compared to noise. However, only the more maneuverable S. bilineata made close approaches to the speaker, and we found no evidence of mobbing in groups. Instead, our findings are consistent with the hypothesis that single bats approached distress calls simply to investigate the situation. These results suggest that approaches by bats to distress calls should not suffice as clear evidence for mobbing.

The adaptive value of secondary males in the polygynous multi-level society of hamadryas baboons

OBJECTIVES

One-male social systems are usually characterized by polygyny and reproductive exclusion by a single resident male. Sometimes, however, secondary males join these groups, and this may carry fitness costs and/or benefits to the resident male. In hamadryas baboons (Papio hamadryas hamadryas), which live in one-male units (OMUs) with female defense polygyny within a multi-level social system, secondary "follower" males often reside in OMUs. Our aim here is to examine possible benefits of these secondary males to hamadryas resident males.

MATERIALS AND METHODS

Using 6 years of data from 65 OMUs in a band of wild hamadryas baboons in Ethiopia, we compared demographic and reproductive parameters of OMUs with and without secondary "follower" males to assess whether their presence conferred any reproductive benefits to resident "leader" males.

RESULTS

Leaders with followers had tenure lengths almost twice as long, acquired more than twice as many females, retained females longer, and had three times as many infants during their tenure compared to leaders without followers.

DISCUSSION

Hamadryas follower males enabled leaders to retain females for longer periods of time-likely through unit defense, social relationships with OMU members, and/or infant protection. Hamadryas leaders appear to be able to monopolize access to females despite the presence of followers, and as such any enhanced reproduction derived from the presence of followers likely increases the fitness of the leader rather than the follower. Thus the relationship between leaders and followers in hamadryas society appears to be a mutually beneficial one and tolerance of secondary males may be an adaptive reproductive strategy characterizing hamadryas leader males.

Mares prefer the voices of highly fertile stallions

We investigated the possibility that stallion whinnies, known to encode caller size, also encoded information about caller arousal and fertility, and the reactions of mares in relation to type of voice. Voice acoustic features are correlated with arousal and reproduction success, the lower-pitched the stallion's voice, the slower his heart beat and the higher his fertility. Females from three study groups preferred playbacks of low-pitched voices. Hence, females are attracted by frequencies encoding for large male size, calmness and high fertility. More work is needed to explore the relative importance of morpho-physiological features. Assortative mating may be involved as large females preferred voices of larger stallions. Our study contributes to basic and applied ongoing research on mammal reproduction, and questions the mechanisms used by females to detect males' fertility.