Visual phenotype matching: cues to paternity are present in rhesus macaque faces

Multiple mechanisms for inbreeding avoidance used simultaneously in a wild ape

Mating with close kin can have considerable negative fitness consequences, which are expected to result in selective pressure for inbreeding avoidance mechanisms, such as dispersal, mate choice and post-copulatory biases. Captive studies have suggested that inbreeding avoidance through mate choice is far less widespread than expected and may be absent where other mechanisms already limit inbreeding. However, few studies have examined multiple mechanisms of inbreeding avoidance simultaneously, particularly in the wild. We use 13 years of detailed dispersal, copulation and paternity data from mountain gorillas to examine inbreeding avoidance. We find that partial dispersal of both sexes results in high kinship in multimale groups, but that copulations between close kin occur 40% less than expected. We find strong kin discrimination in mate choice, with significant avoidance of maternal kin but more limited avoidance of paternal kin. We find no evidence for post-copulatory inbreeding avoidance. Our analyses support familiarity-based mechanisms of kin identification and age-based avoidance that limits mating between fathers and daughters in their natal group. Our findings demonstrate that multiple complementary mechanisms for inbreeding avoidance can evolve in a single species and suggest that inbreeding avoidance through mate choice may enable more flexible dispersal systems to evolve.

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.

Comparison of Scalp ERP to Faces in Macaques and Humans

Face recognition is an essential activity of social living, common to many primate species. Underlying processes in the brain have been investigated using various techniques and compared between species. Functional imaging studies have shown face-selective cortical regions and their degree of correspondence across species. However, the temporal dynamics of face processing, particularly processing speed, are likely different between them. Across sensory modalities activation of primary sensory cortices in macaque monkeys occurs at about 3/5 the latency of corresponding activation in humans, though this human simian difference may diminish or disappear in higher cortical regions. We recorded scalp event-related potentials (ERPs) to presentation of faces in macaques and estimated the peak latency of ERP components. Comparisons of latencies between macaques (112 ms) and humans (192 ms) suggested that the 3:5 ratio could be preserved in higher cognitive regions of face processing between those species.

Bonds of bros and brothers: Kinship and social bonding in postdispersal male macaques

Group-living animals often maintain a few very close affiliative relationships-social bonds-that can buffer them against many of the inevitable costs of gregariousness. Kinship plays a central role in the development of such social bonds. The bulk of research on kin biases in sociality has focused on philopatric females, who typically live in deeply kin-structured systems, with matrilineal dominance rank inheritance and life-long familiarity between kin. Closely related males, in contrast, are usually not close in rank or familiar, which offers the opportunity to test the importance of kinship per se in the formation of social bonds. So far, however, kin biases in male social bonding have only been tested in philopatric males, where familiarity remains a confounding factor. Here, we studied bonds between male Assamese macaques, a species in which males disperse from their natal groups and in which male bonds are known to affect fitness. Combining extensive behavioural data on 43 adult males over a 10-year period with DNA microsatellite relatedness analyses, we find that postdispersal males form stronger relationships with the few close kin available in the group than with the average nonkin. However, males form the majority of their bonds with nonkin and may choose nonkin over available close kin to bond with. Our results show that kinship facilitates bond formation, but is not a prerequisite for it, which suggests that strong bonds are not restricted to kin in male mammals and that animals cooperate for both direct and indirect fitness benefits.

Same father, same face: Deep learning reveals selection for signaling kinship in a wild primate

Many animals rely on facial traits to recognize their kin; however, whether these traits have been selected specifically for this function remains unknown. Using deep learning for face recognition, we present the first evidence that interindividual facial resemblance has been selected to signal paternal kinship. Mandrills (Mandrillus sphinx) live in matrilineal societies, in which females spend their entire lives not only with maternal half-sisters (MHS) but also with paternal half-sisters (PHS). We show that PHS have more differentiated social relationships compared to nonkin, suggesting the existence of kin recognition mechanisms. We further demonstrate that facial resemblance increases with genetic relatedness. However, PHS resemble each other visually more than MHS do, despite both kin categories sharing similar degrees of genetic relatedness. This paternally derived facial resemblance among PHS indicates selection to facilitate kin recognition. This study also highlights the potential of artificial intelligence to study phenotypic evolution.

Presence of kin-biased social associations in a lizard with no parental care: the eastern water dragon (Intellagama lesueurii)

Numerous studies have observed kin-biased social associations in a variety of species. Many of these studies have focused on species exhibiting parental care, which may facilitate the transmission of the social environment from parents to offspring. This becomes problematic when disentangling whether kin-biased associations are driven by kin recognition, or are a product of transmission of the social environment during ontogeny, or a combination of both. Studying kin-biased associations in systems that lack parental care may aid in addressing this issue. Furthermore, when studying kin-biased social associations, it is important to differentiate whether these originate from preferential choice or occur randomly as a result of habitat use or limited dispersal. Here, we combined high-resolution single-nucleotide polymorphism data with a long-term behavioral data set of a reptile with no parental care to demonstrate that eastern water dragons (Intellagama lesueurii) bias their nonrandom social associations toward their kin. In particular, we found that although the overall social network was not linked to genetic relatedness, individuals associated with kin more than expected given availability in space and also biased social preferences toward kin. This result opens important opportunities for the study of kinship-driven associations without the confounding effect of vertical transmission of social environments. Furthermore, we present a robust multiple-step approach for determining whether kin-biased social associations are a result of active social decisions or random encounters resulting from habitat use and dispersal patterns.

Parent-offspring facial resemblance increases with age in rhesus macaques

Kin recognition is a key ability which facilitates the acquisition of inclusive fitness benefits and enables optimal outbreeding. In primates, phenotype matching is considered particularly important for the recognition of patrilineal relatives, as information on paternity is unlikely to be available via social familiarity. Phenotypic cues to both paternal and maternal relatedness exist in the facial features of humans and other primates. However, theoretical models suggest that in systems with uncertainty parentage it may be adaptive for offspring to conceal such cues when young, in order to avoid potential costs of being discriminated against by unrelated adults. Using experienced human raters, we demonstrate in a computer-based task that detection of parent-offspring resemblances in the faces of rhesus macaques (Macaca mulatta) increases significantly with offspring age. Moreover, this effect is specific to information about kinship, as raters were extremely successful at discriminating individuals even among the youngest animals. To our knowledge, this is the first evidence in non-humans for the age-dependent expression of visual cues used in kin recognition.

Why rate when you could compare? Using the "EloChoice" package to assess pairwise comparisons of perceived physical strength

We introduce "EloChoice", a package for R which uses Elo rating to assess pairwise comparisons between stimuli in order to measure perceived stimulus characteristics. To demonstrate the package and compare results from forced choice pairwise comparisons to those from more standard single stimulus rating tasks using Likert (or Likert-type) items, we investigated perceptions of physical strength from images of male bodies. The stimulus set comprised images of 82 men standing on a raised platform with minimal clothing. Strength-related anthropometrics and grip strength measurements were available for each man in the set. UK laboratory participants (Study 1) and US online participants (Study 2) viewed all images in both a Likert rating task, to collect mean Likert scores, and a pairwise comparison task, to calculate Elo, mean Elo (mElo), and Bradley-Terry scores. Within both studies, Likert, Elo and Bradley-Terry scores were closely correlated to mElo scores (all rs > 0.95), and all measures were correlated with stimulus grip strength (all rs > 0.38) and body size (all rs > 0.59). However, mElo scores were less variable than Elo scores and were hundreds of times quicker to compute than Bradley-Terry scores. Responses in pairwise comparison trials were 2/3 quicker than in Likert tasks, indicating that participants found pairwise comparisons to be easier. In addition, mElo scores generated from a data set with half the participants randomly excluded produced very comparable results to those produced with Likert scores from the full participant set, indicating that researchers require fewer participants when using pairwise comparisons.

Genetic studies on the Cayo Santiago rhesus macaques: A review of 40 years of research

Genetic studies not only contribute substantially to our current understanding of the natural variation in behavior and health in many species, they also provide the basis of numerous in vivo models of human traits. Despite the many challenges posed by the high level of biological and social complexity, a long lifespan and difficult access in the field, genetic studies of primates are particularly rewarding because of the close evolutionary relatedness of these species to humans. The free-ranging rhesus macaque (Macaca mulatta) population on Cayo Santiago (CS), Puerto Rico, provides a unique resource in this respect because several of the abovementioned caveats are of either minor importance there, or lacking altogether, thereby allowing long-term genetic research in a primate population under constant surveillance since 1956. This review summarizes more than 40 years of genetic research carried out on CS, from early blood group typing and the genetic characterization of skeletal material via population-wide paternity testing with DNA fingerprints and short tandem repeats (STRs) to the analysis of the highly polymorphic DQB1 locus within the major histocompatibility complex (MHC). The results of the paternity studies also facilitated subsequent studies of male dominance and other factors influencing male reproductive success, of male reproductive skew, paternal kin bias, and mechanisms of paternal kin recognition. More recently, the CS macaques have been the subjects of functional genetic and gene expression analyses and have played an important role in behavioral and quantitative genetic studies. In addition, the CS colony has been used as a natural model for human adult-onset macular degeneration, glaucoma, and circadian rhythm disorder. Our review finishes off with a discussion of potential future directions of research on CS, including the transition from STRs to single nucleotide polymorphism (SNP) typing and whole genome sequencing.

Neonatal representation of odour objects: distinct memories of the whole and its parts

Extraction of relevant information from highly complex environments is a prerequisite to survival. Within odour mixtures, such information is contained in the odours of specific elements or in the mixture configuration perceived as a whole unique odour. For instance, an AB mixture of the element A (ethyl isobutyrate) and the element B (ethyl maltol) generates a configural AB percept in humans and apparently in another species, the rabbit. Here, we examined whether the memory of such a configuration is distinct from the memory of the individual odorants. Taking advantage of the newborn rabbit's ability to learn odour mixtures, we combined behavioural and pharmacological tools to specifically eliminate elemental memory of A and B after conditioning to the AB mixture and evaluate consequences on configural memory of AB. The amnesic treatment suppressed responsiveness to A and B but not to AB. Two other experiments confirmed the specific perception and particular memory of the AB mixture. These data demonstrate the existence of configurations in certain odour mixtures and their representation as unique objects: after learning, animals form a configural memory of these mixtures, which coexists with, but is relatively dissociated from, memory of their elements. This capability emerges very early in life.

Primate Kinship: Contributions from Cayo Santiago

Research on Cayo Santiago and Japan deserves credit for launching the study of primate kinship and for continuing to help shape it in important ways. This review describes the origins of kinship research on Cayo Santiago, beginning with Donald Sade's pioneering work establishing the concepts of kin preferences, matrilineal dominance systems and incest avoidance. It then reviews subsequent research by later Cayo Santiago researchers and alumni, focusing primarily on maternal kinship. Together these researchers have greatly expanded our knowledge of kin preferences in rhesus in terms of (i) what age-sex classes, behaviors and types of kin show them, (ii) the ways in which kinship interfaces with rank, sex, age, and dispersal patterns, and (iii) the graded and variably limited nature of kin preferences in terms of degree of relatedness. Second, the argument for kin selection at least for some types of behavior has survived challenges posed by several alternative explanations, and has been both strengthened by recent findings of paternal kin preferences and narrowed by studies showing that unilateral altruism may extend only to very close kin. Third, work on Cayo Santiago has contributed to an appreciation that both current conditions and inherent social characteristics may influence the strength of kin preferences, and fourth, it has contributed to an understanding of the possible origins of our own species' family systems. Cayo Santiago became a leader in kinship research in large part because of management practices that produce known extended lineages. These lineages have promoted and accelerated research on kinship, prompting other researchers to investigate its importance in other groups and species, where its effects only then became clear. The extended lineages remain valuable tools for research on a species that lives in a broad range of environments in the wild, including those with key parallels to Cayo Santiago.

The self prefers itself? Self-referential versus parental standards in face attractiveness

Preference for phenotypic similarity in assortative mating may be influenced by either a preference for self-similarity or parent-similarity. The aim of the current study was to assess whether people's preference in face attractiveness is influenced by resemblance to the opposite sex parent's face (parental phenotype) or their own face (self-based phenotype or "self-imprinting"). We used computerized image manipulations of facial photographs of participants, their mothers and fathers. The original photographs were combined with 78% of the participants' opposite sex prototype face (i.e., male and female prototypes made from equal contributions of a hundred faces), creating morphs where the contribution from the familiar faces went unnoticed. Female and male participants ranked these images together with the opposite-sex prototype different familiar morphs. Each participant did the same for the morphs generated with other same-sex participants' faces and of their parents. We found that the female participants preferred the self-based morphs to the prototype faces. Male participants showed a general tendency towards self-referential standard. Parental face morphs were ranked low in attractiveness, which may be accounted for by the age difference of the faces blended into the self-based versus parental face morphs, since we used present-time photographs of both the participants and their parents.

Monkeys spontaneously discriminate their unfamiliar paternal kin under natural conditions using facial cues

Kin recognition can enhance inclusive fitness via nepotism and optimal outbreeding. Mechanisms allowing recognition of patrilineal relatives are of particular interest in species in which females mate promiscuously, leading to paternity uncertainty. Humans are known to detect facial similarities between kin in the faces of third parties, and there is some evidence for continuity of this ability in nonhuman primates . However, no study has yet shown that this propensity translates into an ability to detect one's own relatives, one of the key prerequisites for gaining fitness benefits. Here we report a field experiment demonstrating that free-ranging rhesus macaques (Macaca mulatta) spontaneously discriminate between facial images of their paternal half-siblings and unrelated individuals, when both animals are unfamiliar to the tested individual. Specifically, subjects systematically biased their inspection time toward nonkin when the animals pictured were of their own sex (potential threats), relative to when they were of the opposite sex (potential mates). Our results provide strong evidence for visual phenotype matching and the first demonstration in any primate that individuals can spontaneously detect their own paternal relatives on the basis of facial cues under natural conditions.

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.

Triadic social interactions operate across time: a field experiment with wild chimpanzees

Social animals cooperate with bonding partners to outcompete others. Predicting a competitor's supporter is likely to be beneficial, regardless of whether the supporting relationship is stable or transient, or whether the support happens immediately or later. Although humans make such predictions frequently, it is unclear to what extent animals have the cognitive abilities to recognize others' transient bond partners and to predict others' coalitions that extend beyond the immediate present. We conducted playback experiments with wild chimpanzees to test this. About 2 h after fighting, subjects heard recordings of aggressive barks of a bystander, who was or was not a bond partner of the former opponent. Subjects looked longer and moved away more often from barks of the former opponents' bond partners than non-bond partners. In an additional experiment, subjects moved away more from barks than socially benign calls of the same bond partner. These effects were present despite differences in genetic relatedness and considerable time delays between the two events. Chimpanzees, it appears, integrate memories of social interactions from different sources to make inferences about current interactions. This ability is crucial for connecting triadic social interactions across time, a requirement for predicting aggressive support even after a time delay.

Female rhesus macaques discriminate unfamiliar paternal sisters in playback experiments: support for acoustic phenotype matching

Widespread evidence exists that when relatives live together, kinship plays a central role in shaping the evolution of social behaviour. Previous studies showed that female rhesus macaques (Macaca mulatta) recognize familiar maternal kin using vocal cues. Recognizing paternal kin might, however, be more difficult as rhesus females mate promiscuously during the possible conception period, most probably concealing paternity. Behavioural observations indicate that semi free-ranging female rhesus macaques prefer to associate with their paternal half-sisters in comparison to unrelated females within the same group, particularly when born within the same age cohort. However, the cues and mechanism/s used in paternal kin discrimination remain under debate. Here, we investigated whether female rhesus macaques use the acoustic modality to discriminate between paternal half-sisters and non-kin, and tested familiarity and phenotype matching as the underlying mechanisms. We found that test females responded more often to calls of paternal half-sisters compared with calls of unrelated females, and that this discrimination ability was independent of the level of familiarity between callers and test females, which provides, to our knowledge, the first evidence for acoustic phenotype matching. Our study strengthens the evidence that female rhesus macaques can recognize their paternal kin, and that vocalizations are used as a cue.

The impact of paternity on male-infant association in a primate with low paternity certainty

In multimale groups where females mate promiscuously, male-infant associations have rarely been studied. However, recent studies have shown that males selectively support their offspring during agonistic conflicts with other juveniles and that father's presence accelerates offspring maturation. Furthermore, it was shown that males invest in unrelated infants to enhance future mating success with the infant's mother. Hence, infant care might provide fitness gain for males. Here, we investigate male-infant associations in rhesus macaques (Macaca mulatta), a primate with low paternity certainty as females mate with multiple partners and males ensure paternity less efficiently through mate-guarding. We combined behavioural data with genetic paternity analyses of one cohort of the semi-free-ranging population of Cayo Santiago (Puerto Rico) and recorded affiliative and aggressive interactions between focal subjects and adult males from birth to sexual maturation (0-4 years) of focal subjects. Our results revealed that 9.6% of all interactions of focal subjects involved an adult male and 94% of all male-infant interactions were affiliative, indicating the rareness of male-infant aggression. Second and most interestingly, sires were more likely to affiliate with their offspring than nonsires with unrelated infants. This preference was independent of mother's proximity and emphasized during early infancy. Male-infant affiliation rose with infant age and was pronounced between adult males and male rather than female focal subjects. Overall, our results suggest that male-infant affiliation is also an important component in structuring primate societies and affiliation directed towards own offspring presumably represent low-cost paternal care.