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Network optimality has been described in genes, proteins and human communicative networks. In the latter, optimality leads to the efficient transmission of information with a minimum number of connections. Whilst studies show that differences in centrality exist in animal networks with central individuals having higher fitness, network efficiency has never been studied in animal groups. Here we studied 78 groups of primates (24 species). We found that group size and neocortex ratio were correlated with network efficiency. Centralisation (whether several individuals are central in the group) and modularity (how a group is clustered) had opposing effects on network efficiency, showing that tolerant species have more efficient networks. Such network properties affecting individual fitness could be shaped by natural selection. Our results are in accordance with the social brain and cultural intelligence hypotheses, which suggest that the importance of network efficiency and information flow through social learning relates to cognitive abilities. [ABSTRACT FROM AUTHOR]

The clumped distribution of food resources promotes food defensibility and can lead to the monopolizing of resources by high-ranking individuals. However, the balance of power is set at different levels according to societies, meaning that resource partitioning should vary between them. This study investigates whether dominance asymmetry and resource partitioning are related in non-human primates by comparing two species with contrasting social styles, namely rhesus macaques (Macaca mulatta) which display strong social intolerance and a steep gradient of dominance, and Tonkean macaques (Macaca tonkeana), which exhibit higher levels of tolerance and more balanced dominance relationships. Study groups were kept in semi-free ranging conditions. Animals were provided with fruit in three different clumped conditions during 30-min trials. We found that higher-ranking rhesus macaques had priority for the access to fruit: these individuals spent longer in the feeding area in the first 10-min period of trials, while lower-ranking individuals had diminished access to fruit under the most clumped condition; this was associated with sustained agonistic interactions. Dominance effects were weaker in Tonkean macaques. They exhibited co-feeding between high- and low-ranking individuals in the first period; there was no significant effect of dominance even in the most clumped condition; and frequencies of agonistic interactions remained moderate relative to the number of individuals present in the feeding area. These results show that food resources were more equitably distributed among group members in tolerant macaques than in their intolerant counterparts. Dominance gradient and social tolerance may be considered as two aspects of the same phenomenon. [ABSTRACT FROM AUTHOR]

Maternal style can shape offspring behavioral profiles both in the early months and later in life. In nonhuman primates, mothering style can be described by three main dimensions: protectiveness, rejection, and warmth. Although many studies have been performed on this topic in Cercopithecines, we still know little about the more socially tolerant species of this subfamily. We aimed to determine the behavioral dimensions of maternal style in captive Macaca tonkeana, a species characterized by a particularly relaxed type of offspring rearing. We therefore investigated how mothering style influences offspring behavior at different developmental stages by observing the interactions of 30 offspring during the first 5 months of infant life and for 10 days when infants were 12 months old. We used principal component analysis to first extract the components from the variables describing the main interactions of mothers with infants and then the components from the variables describing the main interactions between offspring and their partners. We used linear and generalized mixed models to test the effects of maternal factors on offspring behavior. Our results showed that maternal style in Macaca tonkeana was described by two independent behavioral dimensions, which we labeled protectiveness and warmth. Maternal rejection was almost nonexistent in our captive groups. We did not find any relationship between maternal style components and infant behavior during the first 5 months of life. At 1 year of age, offspring reared by higher-warmth mothers showed greater sociability. Our findings are reminiscent of the dynamic properties of human parenting, corroborating the usefulness of comparative studies on parent–infant attachment. [ABSTRACT FROM AUTHOR]

Animals commonly face fluctuations in their environment and resources. To maximize their benefits, they need to integrate the risks attached to potential pay-offs. We do not know, however, to what extent individuals account for irregularity in the latter. We tested the sensitivity of monkeys (Cebus apella, Macaca tonkeana, M. fascicularis) to the irregularity of pay-offs in two different tasks. In a first experiment, the subjects were given an exchange task where the reward probability varied between different conditions, but yielded the same average pay-off. There was no evidence of subjects favoring either condition, meaning that they behaved in accordance with the predictions of the classical decision theory (Expected Utility Theory). In a second experiment, we offered to subjects a choice between two options involving different pay-off regularity. In this case, a wide range of inter-individual variation was found in the choices of individuals. Whereas monkeys accepted irregular pay-off in a rational way, there were individual biases in their preferences. These results indicate that the preferences of animals in a risky situation were not unequivocally shaped by the environment in which species have evolved. [ABSTRACT FROM AUTHOR]

Great apes appear to be the nonhuman primates most capable of recognizing trading opportunities and engaging in transfers of commodities with conspecifics. Spontaneous exchange of goods between them has not yet been reported. We tested gorillas (Gorilla gorilla), orangutans (Pongo pygmaeus), bonobos (Pan paniscus), and chimpanzees (Pan troglodytes) in a token-exchange task involving two conspecifics and a human experimenter. Tested in pairs, subjects had to exchange tokens with a partner to obtain food from the experimenter. We observed 4, 5, 264, and 328 transfers of tokens in gorillas, chimpanzees, orangutans, and bonobos, respectively. Most gifts were indirect in gorillas, chimpanzees, and bonobos, whereas most were direct in orangutans. The analysis showed no evidence of calculated reciprocity in interactions. A main finding of the study was the high rate of repeated gifts and begging gestures recorded in orangutans. This raises the question of the meaning of pointing in great apes and their ability to understand the communicative intent of others. [ABSTRACT FROM AUTHOR]

We investigated the initiation of group movements in white-faced capuchin monkeys, Cebus capucinus, with the aim of determining whether a single individual with high dominance status consistently leads movements or whether leadership is distributed between group members. The group studied was reared in semifree-ranging conditions. A multivariate analysis followed by univariate analyses demonstrated that leadership was not concentrated on a single individual in this species. All individuals could initiate a collective movement. Nearly half of group members regularly succeeded in recruiting at least three followers. Although both sexes had similar rates of start attempts, females succeeded more frequently than males. We found no significant effect of the dominance status on the percentage of successful attempts. The use of a slow speed, looking back towards the other group members, or trills by the initiator heightened the likelihood of success in group movement initiation. An initiator starting from a core position in a clumped group was more successful than one starting from an edge position in a clumped group or from a dispersed group. Furthermore, the probability of successful start attempts was higher when the group remained stationary for a long period. Leadership in white-faced capuchins appears to be distributed between group members rather than exclusively concentrated on high-ranking individuals. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. [Copyright &y& Elsevier]

We tested the social complexity hypothesis which posits that animals living in complex social environments should use complex communication systems. We focused on two components of vocal complexity: diversity (number of categories of calls) and flexibility (degree of gradation between categories of calls). We compared the acoustic structure of vocal signals in groups of macaques belonging to four species with varying levels of uncertainty (i.e. complexity) in social tolerance (the higher the degree of tolerance, the higher the degree of uncertainty): two intolerant species, Japanese and rhesus macaques, and two tolerant species, Tonkean and crested macaques. We recorded the vocalizations emitted by adult females in affiliative, agonistic and neutral contexts. We analysed several acoustic variables: call duration, entropy, time and frequency energy quantiles. The results showed that tolerant macaques displayed higher levels of vocal diversity and flexibility than intolerant macaques in situations with a greater number of options and consequences, i.e. in agonistic and affiliative contexts. We found no significant differences between tolerant and intolerant macaques in the neutral context where individuals are not directly involved in social interaction. This shows that species experiencing more uncertain social interactions displayed greater vocal diversity and flexibility, which supports the social complexity hypothesis. [ABSTRACT FROM AUTHOR]

The investigation of vocal similarity between individuals has provided evidence of the flexibility of communication signals. This study evaluates the impact of group membership, affiliative bonds, kinship and dominance on acoustic similarity in two primate species with different social styles, intolerant rhesus macaques and tolerant Tonkean macaques. We focused on the fundamental frequencies of the contact calls emitted by adult females. Close kinship promoted vocal similarity between individuals in both species, and also group membership in Tonkean macaques, indicating the involvement of experiential and/or genetic factors. In rhesus macaques more similarities were observed between partners with strong or weak dominance asymmetry than between those with medium asymmetry, which again points to the role of experience. No evidence was found that dominance influences vocal similarity in Tonkean macaques. Our results provide additional evidence to the flexibility of vocal signals produced by macaques, and reveal that it is influenced by social style. [ABSTRACT FROM AUTHOR]

Many studies investigate the decisions made by animals by focusing on their attitudes toward risk, that is, risk-seeking, risk neutrality, or risk aversion. However, little attention has been paid to the extent to which individuals understand the different odds of outcomes. In a previous gambling task involving 18 different lotteries (Pelé, Broihanne, Thierry, Call, & Dufour, 2014), nonhuman primates used probabilities of gains and losses to make their decision. Although the use of complex mathematical calculation for decision-making seemed unlikely, we applied a gradual decrease in the chances to win throughout the experiment. This probably facilitated the extraction of information about odds. Here, we investigated whether individuals would still make efficient decisions if this facilitating factor was removed. To do so, we randomized the order of presentation of the 18 lotteries. Individuals from 4 ape and 2 monkey species were tested. Only capuchin monkeys differed from others, gambling even when there was nothing to win. Randomizing the lottery presentation order leads all species to predominantly use a maximax heuristic. Individuals gamble as soon as there is at least one chance to win more than they already possess, whatever the risk. Most species also gambled more as the frequency of larger rewards increased. These results suggest optimistic behavior. The maximax heuristic is sometimes observed in human managerial and financial decision-making, where risk is ignored for potential gains, however low they may be. This suggests a shared and strong propensity in primates to rely on heuristics whenever complexity in evaluation of outcome odds arises. [ABSTRACT FROM AUTHOR]

Three mechanisms have been proposed to underlie reciprocation of social behaviors in gregarious animals: "calculated reciprocity," "emotional bookkeeping," and "symmetry-based reciprocity." Among these explanations, emotional book-keeping has received the broadest support from experimental and observational studies. On the other hand, 3 individual-based models have shown that reciprocation may emerge via "symmetry-based reciprocity," "emotional bookkeeping," or a combination of both mechanisms. Here, we use these 3 models to assess their relative fit with empirical data on reciprocation and social network structure across different groups and species of macaques. We collected grooming data from 14 groups and 8 macaque species and simulated each group in each model. We analyzed and quantitatively compared social network metrics of the empirical and the models' grooming networks. The 3 models captured fairly well the features of observed networks, and fitted data from wild groups better than captive ones. The emotional bookkeeping model seemed on average to fit slightly better the social networks metrics observed in empirical data, but failed to reproduce some grooming patterns. The symmetry-based models, on the other hand, fitted better other network parameters (e.g., modularity). No model generally fitted the data better than the others, and the fit with some metrics (e.g., modularity, centralization index) was low even after optimization. Thus, our analyses indicate that in the models social interactions may be simpler than in reality and models may miss social processes (e.g., third-party awareness). [ABSTRACT FROM AUTHOR]

Social bonds between group members affect individual fitness and wellbeing. While the impact of bond strength is well studied, the consequences of bond predictability and equitability are often overlooked. Similarly, whether bonds reflect short-term contingencies and/or long-term social strategies remains understudied. We investigated these questions in female crested macaques, Macaca nigra , which display a tolerant social style within a nepotistic hierarchical social structure. We analysed the structure of social bonds by testing whether similarity within dyads (in kinship, dominance and age) predicted the strength, predictability and equitability of bonds. We then tested the value of social bonds by analysing the effect of their characteristics on three fitness-related behaviours: coalitionary support, feeding-in-proximity and aggression. We found that the bond characteristics of females differed substantially from those of other species with comparable data: bonds were of average strength, of moderate endurance and relatively balanced. Stronger bonds were more equitable but less predictable than weaker bonds. Closely ranked females, but not kin or age peers, had stronger, more predictable and more equitable bonds than others. Coalitionary support was not related to any of the bond characteristics, feeding-in-proximity was positively associated with strength and predictability and aggression was positively linked to strength and negatively to equitability. These results highlight the complex picture of the benefits of social bonds in this species. They reflect the degrees of freedom tolerant macaque females can express in their social relationships within their stable social structure, a pattern that may not be given enough consideration in stable nepotistic hierarchical societies. Comparative research is necessary to establish whether these patterns are more general than previously thought or a specific feature of tolerant macaques. Investigating various characteristics of bonds together is paramount to appreciate the dynamics of social relationships and to better understand the social components of fitness. [ABSTRACT FROM AUTHOR]

In species that move in cohesive groups, animals generally reach decisions through socially distributed processes, and individual knowledge is expected to influence collective decision making. Pooling of information should not be considered a general rule, however, since conflicts of interest may occur between group members. When resources are limited or highly attractive, higher-ranking individuals can prevent others from accessing food, and subordinates may have an interest in withholding information about its location. We investigated the role individual knowledge may play in recruitment processes in four groups of horses ( Equus caballus). Animals were repeatedly released in a food search situation, in which one individual had been informed about the location of a preferred food, while another was a naïve control subject. Horses that were informed about the location of the feeding site were seen to approach the food source more steadily and were followed by a higher number of group members than their uninformed counterparts. Recruitment processes appeared mostly passive. Among the informed subjects, lower-ranking individuals were overall less followed than the higher-ranking ones. Most lower-ranking horses arrived alone at the feeding site. Non-followed informed subjects spent less time in active walk and used direct paths less frequently than followed animals, and they were joined by fewer partners at the attractive food source and spent more time feeding alone. This indicates that the influence of informed individuals on the behavior of other group members was a mixed process. Some horses brought nutritional benefits to their conspecifics by leading them to food supplies, whereas the behavior of others might be functionally deceptive. [ABSTRACT FROM AUTHOR]

Social thermoregulation is a cooperative strategy in which animals actively aggregate to benefit from the warmth of conspecifics in response to low ambient temperatures. Emperor penguins, Aptenodytes forsteri , use this behaviour to ensure their survival and reproduction during the Antarctic winter. An emperor penguin colony consists of a dynamic mosaic of compact zones, the so-called huddles, included in a looser network of individuals. To maximize energy savings, birds should adjust their huddling behaviour according to environmental conditions. Here, we examined the dynamics of emperor penguin aggregations, based on photo and video records, in relation to climatic factors. Environmental temperature, wind and solar radiation were the main factors contributing to huddle formation. The analysis of individual movements showed that birds originating from loose aggregations continually joined huddles. Sometimes, a small number of birds induced a movement that propagated to the entire huddle, causing its breakup within 2 min and releasing birds, which then integrated into looser aggregations. Different parts of the colony therefore appeared to continually exchange individuals in response to environmental conditions. A likely explanation is that individuals in need of warmth join huddles, whereas individuals seeking to dissipate heat break huddles apart. The regular growth and decay of huddles operates as pulses through which birds gain, conserve or lose heat. Originally proposed to account for reducing energy expenditure, the concept of social thermoregulation appears to cover a highly dynamic phenomenon that fulfils a genuine regulatory function in emperor penguins. [ABSTRACT FROM AUTHOR]

Why regularities in personality can be described with particular dimensions is a basic question in differential psychology. Nonhuman primates can also be characterized in terms of personality structure. Comparative approaches can help reveal phylogenetic constraints and social and ecological patterns associated with the presence or absence of specific personality dimensions. We sought to determine how different personality structures are related to interspecific variation in social style. Specifically, we examined this question in 6 different species of macaques, because macaque social style is well characterized and can be categorized on a spectrum of despotic (Grade 1) versus tolerant (Grade 4) social styles. We derived personality structures from adjectival ratings of Japanese (Macaca fuscata\ Grade 1), Assamese (M. assamensis\ Grade 2), Barbary (M. sylvanus; Grade 3), Tonkean (M. tonkeana; Grade 4), and crested (M. nigra\ Grade 4) macaques and compared these species with rhesus macaques (M. mulatta\ Grade 1) whose personality was previously characterized. Using a nonparametric method, fuzzy set analysis, to identify commonalities in personality dimensions across species, we found that all but 1 species exhibited consistently defined Friendliness and Openness dimensions, but that similarities in personality dimensions capturing aggression and social competence reflect similarities in social styles. These findings suggest that social and phylogenetic relationships contribute to the origin, maintenance, and diversification of personality. [ABSTRACT FROM AUTHOR]

Whereas most experiments indicate that monkeys have no theory of mind, a study carried out by Wood and collaborators (2007) claimed that they can make inferences about the intentions of another individual. We applied the experimental procedure devised by these authors to investigate whether monkeys can recognize goal-directed actions. We tested 16 Tonkean macaques (Macaca tonkeana) and 12 tufted capuchin monkeys (Sapajus apella). Each subject was submitted to 24 trials in randomized order. The experimenter presented 2 containers, 1 of which was potentially baited with a food reward. After the experimenter had either intentionally or accidentally made an action on 1 of the containers, the subject was asked to select 1 of them. We found that individuals in both species failed to distinguish between accidental and intentional actions. However, they displayed a significant preference for the container touched by the experimenter in the hand conditions, and not in the elbow conditions. These results do not support those reported by Wood and collaborators, but they are consistent with other studies concluding that monkeys are not capable of mind reading. [ABSTRACT FROM AUTHOR]

By distinguishing the attentional cues of their mates, animals can learn what part of their environment is of potential interest. However, recognizing the attentional states of others through auditory perception appears to be difficult, since these states are generally not accompanied by ostensive signals liable to reveal them. In this context, one study found that rhesus macaques withhold their action when unobserved, then concluding that they know what others can hear. We replicated this study by testing 18 Tonkean macaques in an experimental setting where subjects had to choose between two boxes containing a food reward. One box made a noise when opened, and the other opened silently. An experimenter was present and could either have her back to subjects or be facing them. If subjects aimed to avoid attracting the experimenter's attention, they were expected to select the silent box while the experimenter's back was turned. Results showed that subjects did not discriminate between boxes, whatever the experimenter's position. We thus found no evidence that Tonkean macaques are able to take the auditory attentional states of a human into account. It is therefore premature to conclude that monkeys can establish a link between hearing and knowing. [ABSTRACT FROM AUTHOR]

Species comparisons of personality structure (i.e., how many personality dimensions and the character-istics of those dimensions) can facilitate questions about the adaptive function of personality in nonhuman primates. Here we investigate personality structure in the brown capuchin monkey (Sapajus apella), a New World primate species, and compare this structure to those of chimpanzees (Pan troglodytes), orangutans (Pongo spp.), and rhesus macaques (Macaca mulatto). Brown capuchins evolved behavioral and cognitive traits that are qualitatively similar to those of great apes, and individual differences in behavior and cognition often reflect differences in personality. Thus, we hypothesized that brown capuchin personality structure would overlap more with great apes than with rhesus macaques. We obtained personality ratings from seven sites, including 127 brown capuchin monkeys. Principal-components analysis identified five personality dimensions (Assertiveness, Openness, Neuroticism, Sociability, and Attentiveness), which were reliable across raters and, in a subset of subjects, significantly correlated with relevant behaviors up to a year later. Comparisons between species revealed that brown capuchins and great apes overlapped in personality structure, particularly chimpanzees in the case of Neuroticism. However, in some respects (i.e., capuchin Sociability and Openness) the similarities between capuchins and great apes were not significantly greater than those between capuchins and rhesus macaques. We discuss the relevance of our results to brown capuchin behavior and the evolution of personality structure in primates. [ABSTRACT FROM AUTHOR]

Play signals are viewed as important means by which animals inform each other that bites, strikes, and throws that occur during play fighting are indeed playful rather than serious. One such signal is the open mouth play face that is common in primates and many other mammals. Unfortunately, as most play fighting involves biting, it can be ambiguous as to whether any instance of opening the mouth is performed to communicate playful intent or is simply a preparation for biting. In this study, open mouths co-occurring with the bared-teeth display (teeth-baring) in Tonkean macaques were used to assess the context in which facial gestures only relevant for signaling (i.e., teeth-baring is not necessary for biting) are used during play. Two predictions arising from the hypothesis that play signals are used to facilitate playful contact were tested: that the open mouth with teeth-baring should (1) be most frequent preceding contact, and (2) that it should be performed most often when bites are directed at orientations that is visible to the recipient. The data only partially support these predictions. The open mouth with teeth-baring is also frequently used when a monkey withdraws from playful contact. Moreover, it is associated with bites to body targets, such as the rump, that offer little prospect for detection by the recipient; this supports the possibility that play signals may sometimes be emitted not to communicate with the partner but with the performer itself. Thus, play signals serve multiple functions during play fighting. [ABSTRACT FROM AUTHOR]

Aggression is potentially disruptive for social groups. Although individuals witnessing a conflict are not directly threatened by aggressive interactions, the aftermath of aggression appears to be a period of social instability. We expected bystanders to respond to conflicts by affiliating with other group members and so reducing social tension. To test this hypothesis we collected data on two captive groups of Tonkean macaques, Macaca tonkeana. After an agonistic interaction, the behaviours of focal individuals uninvolved in the conflict were recorded over 5min postconflict periods, for comparison with baseline periods. The results showed that bystanders were more likely to show affiliation during postconflict periods than in baselines. We found that affiliation occurred more frequently between individuals linked by friendship, whereas no significant effect of kinship appeared, which may be related to the open social relationships reported in Tonkean macaques. Females initiated affiliation sooner than males and conflicts involving physical contact were more quickly followed by affiliation between bystanders. Rates of scratching tended to decrease after the first affiliative interaction. None the less, few signs of anxiety were observed in bystanders. Our results reflect the high propensity of Tonkean macaques to appease others and stop aggression. This study demonstrates that postconflict affiliation occurs between bystanders in a species characterized by tolerant social relationships. It could be a pervasive means of social cohesion among primates. [Copyright &y& Elsevier]

For coordinated movement, group-living individuals have to reach consensus decisions through recruitment processes. Success in recruitment can depend on the spatial distribution and behaviour of animals before and/or after departure, and on their affinitive relationships. We tested the effect of such factors on recruitment processes in a group of 19 ewe lambs, Ovis aries, at pasture. Two observers continuously videotaped the behaviours of animals from a platform located in the centre of the field. Results showed that group orientation, group vigilance and activities such as head movements, stillness and number of steps increased before departure. Using general linear modelling we found that changes in most of these variables predicted the number of participants in movements. Similarly, activity of the first mover was modified in the last 2min preceding departure, and at departure time the location and number of close neighbours of the first mover were especially influential in recruiting conspecifics. Animals first recruited were those that were close to the first mover and also its preferential partners. The behaviours of the second and third moving individuals could further influence the recruitment process. Moreover, there were clues that individuals were able to recruit others intentionally. Our findings emphasize that decision making in a group of domestic sheep was a continuous and distributed process. Recruitment depended both on group state and on the behaviour of individuals and their social relationships. [Copyright &y& Elsevier]

In this paper, I chronicle the Strasbourg population of Tonkean macaques (Macaca tonkeana) over a period of half a century. In 1972, Tonkean macaques were imported from Sulawesi, Indonesia, to eastern France, leading to the establishment of two social groups in the Strasbourg region several years later. Our research team studied the social behavior and cognitive abilities of these Tonkean macaques for four decades. The species is characterized by a high degree of social tolerance. This has proven to be very informative in comparative studies of macaque social behavior, opening a new perspective on the evolution of primate societies. Over the years, the population has grown, and more social groups have been formed. However, the fact that some of the Tonkean macaques were healthy carriers of the herpes B virus led to disagreements over their management and eventually to the elimination of the positive individuals. Many individuals from the Strasbourg population are now kept in sanctuaries, and the number of captive breeding groups is limited. We still have much to learn about Tonkean macaques and there is a need for studies carried out in their native habitat in Sulawesi., (© 2023. The Author(s), under exclusive licence to Japan Monkey Centre.)

Glucocorticoid and androgen hormones play a prominent role in male reproductive effort. Their production usually increases in non-human primates during mating competition, which may include rivalry for access to receptive females, struggles for high dominance rank, or social pressure on low-ranking individuals. It is generally assumed that glucocorticoids and androgens are associated with mating challenges rather than dominance status, but the involvement of multiple factors makes it difficult to disentangle the two. In this regard, Tonkean macaques provide a suitable model because they are characterized by relaxed dominance and year-round breeding, meaning that there is typically no more than one receptive female in a group, and thus first-ranking males can easily monopolize her. We studied two captive groups of Tonkean macaques over an 80-month period, recording the reproductive status of females, collecting urine from males and sampling behaviors in both sexes. Male urinary hormone concentrations could be affected by increased competition caused by the mating period, the number of males and the degree of female attractiveness. The highest increases in androgens were recorded in males performing female mate-guarding. Despite the importance of dominance status in determining which males can mate, we found no significant effect of male rank on glucocorticoids and only a marginal effect on androgens during mate-guarding. Both types of hormones were more directly involved in the mating effort of males than in their dominance status. Our results show that their function can be understood in light of the particular competitive needs generated by the species-specific social system., Competing Interests: Declaration of competing interest None., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Recruitment is a process by which animals can initiate collective movements: the action of an individual prompts conspecifics to follow. Although it has been hypothesized that animals may be able to intentionally recruit others, there is no experimental evidence of this to date. We tested this hypothesis in two pairs of Tonkean macaques in a situation requiring the subjects to find a food site in a 2800 m 2 area, and approach the site together to release rewards. Each subject was informed of the location of either highly or little-valued rewards. We recorded attention-action sequences in which an individual checked that his partner was attending to him before moving, and also simple departures (i.e., not preceded by eye contact). Analyses showed that sequences were more often followed by recruitment and leading the partner to a baited site than simple departures were. Moreover, subjects used attention-action sequences more frequently when informed of the location of the highly valued reward. This may be explained by the fact that the more motivated they were by the expected rewards, the more likely they were to actively recruit their partner. No such effect was found when subjects performed simple departures. We conclude that Tonkean macaques are capable of intentional recruitment because the subjects voluntarily behaved with the goal of influencing their partner's movement: they checked that the partner was paying attention to them and prompted him to follow by moving. Such performances can be accounted for either by associative learning or by intentional communication., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Comparative study of the social systems of macaques has revealed correlated variations between species in multiple social traits such as the asymmetry of dominance relationships, preferential treatment of kin, patterns of aggression and reconciliation, modes of socialization, and access to food resources. Macaques can be classified on a scale of four categories of social styles, ranging from the least to the most tolerant species. This led to the development of the covariation framework, which addresses the constraints responsible for the linkages between social traits, and their consequences on the evolution of primate social systems. Decades of research have provided a wealth of information that supports, complements, expands, or challenges the covariation framework. In this article, I review this body of knowledge and consider covariation in its two aspects, that is, as a pattern and as a hypothesis. I first consider the extent to which social styles can be invariant, the strength of correlations between traits, and the possible extension of the framework to nonhuman primates other than macaques. I then discuss how to formulate hypotheses, identify sources of linkage between traits, make predictions about the effects of social constraints, assess the tolerance dimension of social styles, and consider the breaking of linkages between traits. Whereas socioecological studies aim to understand how adaptation to the ecological environment determines the shape of social systems, the covariation framework is a complementary research program that seeks to unravel the internal processes that restrict or channel change in social behavior., (© 2021 American Association of Biological Anthropologists.)

Large-scale studies of individual differences in innovative behavior among nonhuman animals are rare because of logistical difficulties associated with obtaining observational data on a large number of innovative individuals across multiple locations. Here, we take a different approach, using observer ratings to study individual differences in innovative behavior in 127 brown capuchin monkeys (Sapajus [Cebus] sp.) from 15 social groups and 7 facilities. Capuchins were reliably rated by 1 to 7 raters (mean 3.2 ± 1.6 raters/monkey) on a 7-point Likert scale for levels of innovative behavior, task motivation, sociality, and dominance. In a subsample, we demonstrate these ratings are valid: Rated innovation predicted performance on a learning task, rated motivation predicted participation in the task, rated dominance predicted social rank based on win/loss aggressive outcomes, and rated sociality predicted the time that monkeys spent in proximity to others. Across all 127 capuchins, individuals that were rated as being more innovative were significantly younger, more social, and more motivated to engage in tasks. Age, sociality, and task motivation all had independent effects on innovativeness, whereas sex, dominance, and group size were nonsignificant. Our findings are consistent with long-term behavioral observations of innovation in wild white-faced capuchins. Observer ratings may, therefore, be a valid tool for studies of animal innovation. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

While there is no consensus about the definition of complexity, it is widely accepted that the ability to produce uncertainty is the most prominent characteristic of complex systems. We introduce new metrics that purport to quantify the complexity of living organisms and social organizations based on their levels of uncertainty. We consider three major dimensions regarding complexity: diversity based on the number of system elements and the number of categories of these elements; flexibility which bears upon variations in the elements; and combinability which refers to the patterns of connection between elements. These three dimensions are quantified using Shannon's uncertainty formula, and they can be integrated to provide a tripartite complexity index. We provide a calculation example that illustrates the use of these indices for comparing the complexity of different social systems. These indices distinguish themselves by a theoretical basis grounded on the amount of uncertainty, and the requirement that several aspects of the systems be accounted for to compare their degree of complexity. We expect that these new complexity indices will encourage research programmes aiming to compare the complexity levels of systems belonging to different realms., (© 2021 The Authors.)

The observation of specific reactions by animals to dead conspecifics raises questions about their capacity to understand death. We describe the behavior of a captive group of tufted capuchins in the hours following the death of an adult female. The behavior of 12 subjects was recorded over a 5-h period. Most group members displayed exploratory behaviors toward the corpse, peering at it, smelling, touching, lifting or pulling it at least once. Interactions with the corpse were particularly frequent in the first hour, then decreased in the following hours. The relatives of the deceased female were the most involved individuals, and her daughter spent more time near her body than the other members of the group. Rates of interaction with the body were especially high in subadult individuals. Most of the behaviors directed to the body of the deceased individual appeared to be investigative.

In anticipation of the current biodiversity crisis, it has become critical to rapidly and accurately assess biodiversity. DNA barcoding has proved efficient in facilitating the discovery and description of thousands of species and also provides insight into the dynamics of biodiversity. Here, we sequenced a portion of the mitochondrial cytochrome c oxidase subunit I (COI) gene from all morphospecies of reef brittle stars collected during a large-scale biodiversity survey in the southwestern Indian Ocean (SWIO). Three methods of species delineation (Automatic Barcode Gap Discovery, Generalized Mixed Yule Coalescent model, and Bayesian Poisson Tree Processes) showed concordant results and revealed 51 shallow reef species in the region. Mean intraspecific genetic distances (0.005-0.064) and mean interspecific genetic distances within genera (0.056-0.316) were concordant with previous echinoderm studies. This study revealed that brittle-star biodiversity is underestimated by 20% within SWIO and by >40% when including specimens from the Pacific Ocean. Results are discussed in terms of endemism, diversification processes, and conservation implications for the Indo-West Pacific marine biodiversity. We emphasize the need to further our knowledge on biodiversity of invertebrate groups in peripheral areas.

The level of glucocorticoids, especially if obtained from noninvasive sampling, can be used as an index of animal well-being, allowing evaluation of the animal's response to environmental modifications. Despite evidence that these hormones play a relevant role in energy metabolism regulation in perceived or real stress events, little is known regarding the factors that could modify the capability of animals to cope with relocation events. The aim of this research was to assess fecal cortisol metabolite concentrations before, during and after acute stress (transfer and relocation event) in two well-established social groups of Tonkean macaques (Macaca tonkeana). The results showed that the fecal levels of cortisol increased in individuals of both groups in response to the stress event, with a similar trend in males and females. Hormone levels were back to baseline values in both groups a few days after transfer and relocation. The presence of known social partners could be one of the factors that possibly facilitated the adaptation process.

Measuring variations in body mass is necessary to gain a deeper understanding of the evolution of life-history patterns, and it provides information on the timing of sexual maturity and the development of sexual dimorphism. In this study, we collected longitudinal data on body mass from infancy to adulthood in a captive population of Tonkean macaques (Macaca tonkeana). Tests to evaluate whether social group, maternal age, and dominance rank influenced growth rates showed that they had no significant effect. We investigated the timing and magnitude of breaking points in the growth paths of males and females, and checked whether these breaking points could correspond to specific reproductive and morphological developmental events. We found that male and female Tonkean macaques have roughly equivalent body masses until around the age of four, when males go through an adolescent growth spurt and females continue to grow at a constant rate. Males not only grow faster than females, but they also continue to grow for nearly one and a half years after females have attained their full body mass. Growth rate differences account for approximately two-thirds of the body mass sexual dimorphism; only the remaining third results from continued male growth beyond the age where full body mass is reached in females. We also discovered remarkable correspondences between the timing of testicular enlargement and the adolescent growth spurt in males, and between dental development and slowdown breaking points in both sexes.

The evolutionary study of social systems in non-human primates has long been focused on ecological determinants. The predictive value of socio-ecological models remains quite low, however, in particular because such equilibrium models cannot integrate the course of history. The use of phylogenetic methods indicates that many patterns of primate societies have been conserved throughout evolutionary history. For example, the study of social relations in macaques revealed that their social systems are made of sets of correlated behavioural traits. Some macaque species are portrayed by marked social intolerance, a steep dominance gradient and strong nepotism, whereas others display a higher level of social tolerance, relaxed dominance and a weaker influence of kinship. Linkages between behavioural traits occur at different levels of organization, and act as constraints that limit evolutionary responses to external pressures. Whereas these constraints can exert strong stabilizing selection that opposes the potential changes required by the ecological environment, selective mechanisms may have the potential to switch the whole social system from one state to another by acting primarily on some key behavioural traits that could work as pacemakers.

Whether or not non-human animals can plan for the future is a hotly debated issue. We investigate this question further and use a planning-to-exchange task to study future planning in the cooperative domain in two species of monkeys: the brown capuchin (Cebus apella) and the Tonkean macaque (Macaca tonkeana). The rationale required subjects to plan for a future opportunity to exchange tokens for food by collecting tokens several minutes in advance. Subjects who successfully planned for the exchange task were expected to select suitable tokens during a collection period (5/10 min), save them for a fixed period of time (20/30 min), then take them into an adjacent compartment and exchange them for food with an experimenter. Monkeys mostly failed to transport tokens when entering the testing compartment; hence, they do not seem able to plan for a future exchange with a human partner. Three subjects did however manage to solve the task several times, albeit at very low rates. They brought the correct version of three possible token types, but rarely transported more than one suitable token at a time. Given that the frequency of token manipulation predicted transport, success might have occurred by chance. This was not the case, however, since in most cases subjects were not already holding the token in their hands before they entered the testing compartment. Instead, these results may reflect subjects' strengths and weaknesses in their time-related comprehension of the task.

The ability to wait for a reward is a necessary capacity for economic transactions. This study is an age-related investigation of children's ability to delay gratification in an exchange task requiring them to wait for a significant reward. We gave 252 children aged 2-4 a small piece of cookie, then offered them an opportunity to wait for a predetermined delay period before exchanging it for a larger one. In a first experiment, the children had to exchange the initial food item for rewards two, four or eight times larger. Results showed that children aged 3-4 years old sustained longer time lags for larger rewards than for smaller rewards. This effect was not found in 2-year-old subjects. In a second experiment, a reward 40 times larger than the initial piece was offered to determine the maximum waiting time that children could sustain. All age groups increased their performances. Older children were more successful at waiting, but some children as young as 2 years old were able to tolerate delays of up to 16 min. Older children who chose to give up waiting earlier than their known capacity demonstrated anticipation skills which had not been seen in younger children, showing that they had anticipated an increase in the time lag, and that they had considered both time and reward value when making their decision. Despite the age effect, we did not establish any limits for delaying gratification in children. This study may have educational implications for dealing with behavioral misconduct, which is known to be related to impulsivity control in young children., (Copyright © 2011 Elsevier B.V. All rights reserved.)

It has been reported that wild capuchin monkeys exhibit several group-specific behavioural traditions. By contrast, experiments have found little evidence for the social learning assumed necessary to support such traditions. The present study used a diffusion chain paradigm to investigate whether a novel foraging task could be observationally learned by capuchins (Cebus apella) and then transmitted along a chain of individuals. We used a two-action paradigm to control for independent learning. Either of two methods (lift or slide) could be used to open the door of a foraging apparatus to retrieve food. Two chains were tested (N1=4; N2=5), each beginning with an experimenter-trained model who demonstrated to a partner its group-specific method for opening the foraging apparatus. After the demonstration, if the observer was able to open the apparatus 20 times by either method, then it became the demonstrator for a new subject, thus simulating the spread of a foraging tradition among 'generations' of group members. Each method was transmitted along these respective chains with high fidelity, echoing similar results presently available only for chimpanzees and children. These results provide the first clear evidence for faithful diffusion of alternative foraging methods in monkeys, consistent with claims for capuchin traditions in the wild.