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Understanding the ecological and social factors that influence group size is a major focus of primate behavioural ecology. Studies of species with fission–fusion social organizations have offered an insightful tool for understanding ecological drivers of group size as associations change over short temporal and spatial scales. Here we investigated how the fission–fusion dynamics of spider monkeys (Ateles geoffroyi) at Runaway Creek, Belize were affected by fruit availability. When males and females were analyzed together, we found no association between fruit availability and subgroup size. However, when females were analyzed separately, we found that when fruit availability increased, so did subgroup size. In all analyses, higher fruit availability did not influence subgroup spatial cohesion. Our results point to the complexity of understanding grouping patterns, in that while ecological factors make groups of specific sizes advantageous, social factors also play an important determining role.

Among mammals, the order Primates is exceptional in having a high taxonomic richness in which the taxa are arboreal, semiterrestrial, or terrestrial. Although habitual terrestriality is pervasive among the apes and African and Asian monkeys (catarrhines), it is largely absent among monkeys of the Americas (platyrrhines), as well as galagos, lemurs, and lorises (strepsirrhines), which are mostly arboreal. Numerous ecological drivers and species-specific factors are suggested to set the conditions for an evolutionary shift from arboreality to terrestriality, and current environmental conditions may provide analogous scenarios to those transitional periods. Therefore, we investigated predominantly arboreal, diurnal primate genera from the Americas and Madagascar that lack fully terrestrial taxa, to determine whether ecological drivers (habitat canopy cover, predation risk, maximum temperature, precipitation, primate species richness, human population density, and distance to roads) or species-specific traits (body mass, group size, and degree of frugivory) associate with increased terrestriality. We collated 150,961 observation hours across 2,227 months from 47 species at 20 sites in Madagascar and 48 sites in the Americas. Multiple factors were associated with ground use in these otherwise arboreal species, including increased temperature, a decrease in canopy cover, a dietary shift away from frugivory, and larger group size. These factors mostly explain intraspecific differences in terrestriality. As humanity modifies habitats and causes climate change, our results suggest that species already inhabiting hot, sparsely canopied sites, and exhibiting more generalized diets, are more likely to shift toward greater ground use.

Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships. Publisher PDF

Spider monkeys (Ateles sp.) are characterized by high fission-fusion dynamics, meaning their social grouping pattern is fluid and consists of subgroups that vary in size, composition, and spatial cohesion over time. In this study, we quantify the fission-fusion dynamics of a group of spider monkeys at Runaway Creek Nature Reserve in Belize by measuring subgroup size, spatial cohesion, and stability using data spanning 5 years. We then test whether variation in these three subgroup measures differ according to season, subgroup sex composition, and the reproductive status of female subgroup members. Our results show that subgroups were larger in size and less stable in membership during the wet season compared to the dry season. All-female subgroups were less spatially cohesive but more stable in membership than all-male subgroups. Finally, we report that subgroups with one or more non-lactating females (i.e., without nursing young) were smaller on average than subgroups containing lactating females with nursing young. These data contribute to a growing body of research documenting the ecological and social dimensions along which grouping patterns might vary.

Understanding the ecological and social factors that influence group size is a major focus of primate behavioural ecology. Studies of species with fission-fusion social organizations have offered an insightful tool for understanding ecological drivers of group size as associations change over short temporal and spatial scales. Here we investigated how the fission-fusion dynamics of spider monkeys (Ateles geoffroyi) at Runaway Creek, Belize were affected by fruit availability. When males and females were analyzed together, we found no association between fruit availability and subgroup size. However, when females were analyzed separately, we found that when fruit availability increased, so did subgroup size. In all analyses, higher fruit availability did not influence subgroup spatial cohesion. Our results point to the complexity of understanding grouping patterns, in that while ecological factors make groups of specific sizes advantageous, social factors also play an important determining role.

In species in which males and females exhibit different association patterns, the use of vocalizations that regulate interindividual distance may differ between the sexes. Spider monkey social groups are characterized by high fission–fusion dynamics and sex differences in association patterns; female–female associations have been described as more passive than those between philopatric males. Individuals of both sexes produce whinny vocalizations, which may allow callers and receivers to mediate interindividual spacing based on existing social relationships. As such, we hypothesized individuals of each sex would use whinny vocalizations at different rates and in different contexts. To investigate sex differences in the rate of whinnying across behavioral contexts, we collected focal animal samples on Yucatan spider monkeys (Ateles geoffroyi yucatanensis) over 8 mo at Runaway Creek Nature Reserve, Belize. In addition, we recorded all changes in subgroup composition to investigate whether a female’s likelihood of calling was influenced by the number of conspecifics joining, or leaving their subgroup. We found that females called at higher rates than males in most behavioral contexts, especially foraging. The probability that females would call increased during subgroup fissions and fusions, and correlated positively with the number of individuals joining or leaving their subgroup. Male calling rates did not differ across contexts, and males generally called less than females. Our results suggest that whinnying by females may allow callers to mediate interindividual spacing in contexts where proximity risks increasing feeding competition. In species in which the sexes associate in qualitatively different ways, vocalizations may play a role in maintaining these differences.

Sexual segregation is a recognized dimension of the socioecology of many vertebrates, but it has not been systematically examined in primates. We investigated temporal patterns of sexual segregation in spider monkeys (Ateles geoffroyi yucatanensis) using a test that distinguishes sexual segregation from aggregation and random association between the sexes. We further investigated how sexual segregation varies over time as a function of food availability, and then tested other possible factors that might be causally linked to sexual segregation in spider monkeys. We predicted that male philopatry and cooperative territorial defence leads to sexual dimorphism in behavior, which in turn creates different optimal energetic requirements for males and females as reflected in differing activity budgets and diet. We investigated sexual segregation in a group of 33–35 spider monkeys at Runaway Creek Nature Reserve in Belize over 23 mo in 2008–2009. We used the sex compositions of subgroups recorded in scan samples to test the occurrence of sexual segregation at monthly and biweekly time scales.We found that males and females were significantly segregated in 15 out of the 23 mo of the study, and that periods of nonsegregation coincided with months of low food availability. The sexes differed significantly in activity and diet; males spent more time traveling, and less time resting and feeding than females, and they had a higher proportion of ripe fruits in their diets than did females. We propose that sexual segregation in spider monkeys is primarily a form of social segregation that results from males and females pursuing optimal dietary and behavioral strategies to satisfy sex-specific energetic demands. We further suggest that sexual segregation represents an important constraint on fission–fusion dynamics that should be considered when assessing the potential for variability in subgroup composition.

Infant tolerance by adult males has been observed in many primate species with multimale–multifemale mating systems, but males do not usually initiate interactions with infants. In male philopatric species, such as spider monkeys, adult males within a community exhibit high levels of cooperation and affiliation, and they might therefore be motivated to create bonds with potential future allies. Based on this hypothesis we predicted that adult male spider monkeys would participate in infant handling more than adult females and they would preferentially direct handling toward male infants. Between January 2008 and July 2010, we collected 884 h of observation on a community of wild spider monkeys at Runaway Creek Nature Reserve in Belize. During this period we observed 120 incidences of affiliative interactions between infants and adults other than their mother. The adult initiated the majority of nonmother adult–infant interactions (78 %). All available infants (5 males, 7 females) were handled during the study. All 9 of the community adult males handled infants but only 7 of 14 adult females did so. Adult males handled infants significantly more often than did adult females and males also handled young infants more often than older infants. Significant infant sex differences in handling appeared in infants >6 mo when adult males handled males significantly more than females. The patterns of infant handling among age–sex class dyads reflect the affiliative social patterns that we see in adult spider monkeys. These results provide support for the hypothesis that adult males preferentially handle male infants as a strategy for fostering social bonds.

There have been several previous studies of the loud, long-distance call of chimpanzees, termed a ‘pant hoot’. Some have explored the possibility that there are acoustically distinct subtypes of pant hoots that communicate to distant listeners different information about the caller's behaviour, or ecological and social circumstances. However, results to date have been either inconclusive or conflicting. To help resolve these issues, we undertook research on pant hoots produced by wild chimpanzees, Pan troglodytes schwienfurthii, living in the Budongo Forest, Uganda. In this paper, we report the results of acoustic analysis of 201 pant hoot series produced by seven adult males. Principal components analysis (PCA) was used to identify key dimensions of structural variation in the calls, which were then related to a large set of behavioural, social and ecological variables associated with call production. Although there was little evidence for distinctive pant hoot subtypes according to many of the social and ecological dimensions traditionally recognized, a number of significant patterns were identified. Specifically, pant hoots were produced with higher probability at abundant food sources, and they were more likely to contain a ‘let-down’ phase when produced in specific behavioural contexts, such as travelling and upon arrival at a food source. In addition, pant hoots produced while travelling along the ground in small parties prior to joining-up with other community members were consistently different from all other pant hoots, varying reliably in the fundamental frequency of their build-up elements, the tonal quality of climax elements and in the presence of a let-down phase. We discuss the possible mechanistic bases for these differences in the pattern of calling and detailed spectral structure of the calls and consider the implications for referential communication.

Evolution plays a central role in the design of all living things, producing a diversity of extraordinary adaptations. The details of evolutionary design can only be fully understood in a historical context, within the broader perspective it offers on how design solutions to recent specific environmental pressures are critically shaped by species' history — that is, by the requirement that they be integrated with the myriad pre-existing physical, physiological, and solutions of behaviour to other adaptive challenges that species' history entails. In short, organisms are a blend of adaptive potential and historical inertia; the degrees of freedom are not infinite and so what is evolutionarily possible over most timeframes depends critically on the starting conditions. This article outlines these frameworks and highlights broad conceptual weaknesses in them.

A key component of nonhuman primate vocal communication is the production and recognition of clear cues to social identity that function in the management of these species’ individualistic social relationships. However, it remains unclear how ubiquitous such identity cues are across call types and age-sex classes and what the underlying vocal production mechanisms responsible might be. This study focused on two structurally distinct call types produced by infant baboons in contexts that place a similar functional premium on communicating clear cues to caller identity: (1) contact calls produced when physically separated from, and attempting to relocate, mothers and (2) distress screams produced when aggressively attacked by other group members. Acoustic analyses and field experiments were conducted to examine individual differentiation in single vocalizations of each type and to test mothers’ ability to recognize infant calls. Both call types showed statistically significant individual differentiation, but the magnitude of the differentiation was substantially higher in contact calls. Mothers readily discriminated own-offspring contact calls from those of familiar but unrelated infants, but did not do so when it came to distress screams. Several possible explanations for these asymmetries in call differentiation and recognition are considered.

Taxonomy.- Taxonomy and Biogeography of the Primates of Western Uganda.- Ecology.- Factors Influencing Variation in the Population Densities of Colobus guereza, Within Selectively Logged Forest at the Budongo Forest Reserve.- How Does the Golden Monkey of the Virungas Cope in a Fruit-Scarce Environment?.- The Diet of Olive Baboons (Papio anubis) in the Budongo Forest Reserve, Uganda.- Aspects of Diet, Foraging, and Seed Predation in Ugandan Forest Baboons.- The Interaction of Hormones with Ecological Factors in Male Budongo Forest Chimpanzees.- The Role of Diet in Self-Medication Among Chimpanzees in the Sonso and Kanyawara Communities, Uganda.- Geophagy in Chimpanzees (Pan troglodytes schweinfurthii) of the Budongo Forest Reserve, Uganda.- Nutritional Aspects of the Diet of Wild Gorillas.- The Parasites of the Gorillas in Bwindi Impenetrable National Park, Uganda.- Preliminary GIS Analysis of Range Use by Sympatric Mountain Gorillas and Chimpanzees in Bwindi Impenetrable National Park, Uganda.- Behavior and Physiology.- Comparison of Sex Differences in Gregariousness in Fission-Fusion Species.- Urinary Estrone Conjugates and Reproductive Parameters in Kibale (Kanyawara) and Budongo (Sonso) Chimpanzees.- Frequent Copulations by Females and High Promiscuity in Chimpanzees in the Kalinzu Forest, Uganda.- Effects of Injury on the Locomotion of Free-Living Chimpanzees in the Budongo Forest Reserve, Uganda.- Effect of Snare Injuries on the Fig-Feeding Behavior of Chimpanzees of the Budongo Forest, Uganda.- Deciphering Junglespeak.- Instrumental Leaf Use by Chimpanzees of the Budongo Forest (Sonso Community).- Conservation.- A Survey of Prosimians in the National Parks and Forest Reserves of Uganda.- The Diets, Preferences, and Overlap of the Primate Community in the Budongo Forest Reserve, Uganda.- Behavioral Patterns of Colobus in Logged and Unlogged Forests.- Threats to, and Protection of, the Chimpanzees of the Budongo Forest Reserve.- Gorillas Living on the Edge.- Local Ecological Perceptions of Chimpanzees and Forest Resources.

Chimpanzee (Pan troglodytes) predation has been found in communities across Africa. Little evidence of such behaviour has been previously reported from the Budongo Forest, western Uganda. Here we present preliminary observations of predatory behaviour for the Sonso community, collected between August 1994 and February 2002. We find that the Sonso chimpanzees do not appear to hunt frequently, despite a relatively high density of potential prey, particularly Colobus guereza and Cercopithecus mitis. They seem similar to other eastern chimpanzees in their focus on immature colobus monkeys and in targeting ungulates, and the absence of red colobus (Procolobus badius) - the most common prey species elsewhere for chimpanzees - may be a significant factor.

We make three points: (1) Overlooked studies of nonhuman communication originally inspired, but no longer support, the blinkered view of mental continuity that Penn et al. critique. (2) Communicative discontinuities between animals and humans might be rooted in social-cognitive discontinuities, reflecting a common lacuna in Penn et al.'s relational reinterpretation mechanism. (3) However, relational reinterpretation need not be a qualitatively new representational process.