Abstract: The neocortex is widely held to have been the focus of mammalian brain evolution, but what selection pressures explain the observed diversity in its size and structure? Among primates, comparative studies suggest that neocortical evolution is related to the cognitive demands of sociality, and here I confirm that neocortex size and social group size are positively correlated once phylogenetic associations and overall brain size are taken into account. This association holds within haplorhine but not strepsirhine primates. In addition, the neocortex is larger in diurnal than in nocturnal primates, and among diurnal haplorhines its size is positively correlated with the degree of frugivory. These ecological correlates reflect the diverse sensory-cognitive functions of the neocortex.

Abstract: If culture is defined as variation acquired and maintained by social learning, then culture is common in nature. However, cumulative cultural evolution resulting in behaviors that no individual could invent on their own is limited to humans, song birds, and perhaps chimpanzees. Circumstantial evidence suggests that cumulative cultural evolution requires the capacity for observational learning. Here, we analyze two models the evolution of psychological capacities that allow cumulative cultural evolution. Both models suggest that the conditions which allow the evolution of such capacities when rare are much more stringent than the conditions which allow the maintenance of the capacities when common. This result follows from the fact that the assumed benefit of the capacities, cumulative cultural adaptation, cannot occur when the capacities are rare. These results suggest why such capacities may be rare in nature.

Abstract: Although many animal species can represent numerical values, little is known about how salient number is relative to other object properties for nonhuman animals. In one hypothesis, researchers propose that animals represent number only as a last resort, when no other properties differentiate stimuli. An alternative hypothesis is that animals automatically, spontaneously, and routinely represent the numerical attributes of their environments. The authors compared the influence of number versus that of shape, color, and surface area on rhesus monkeys' (Macaca mulatta) decisions by testing them on a matching task with more than one correct answer: a numerical match and a nonnumerical (color, surface area, or shape) match. The authors also tested whether previous laboratory experience with numerical discrimination influenced a monkey's propensity to represent number. Contrary to the last-resort hypothesis, all monkeys based their decisions on numerical value when the numerical ratio was favorable.

Abstract: Complex social relationships among nonhuman primates appear to contribute to individual reproductive success. Experiments with and behavioral observations of natural populations suggest that sophisticated cognitive mechanisms may underlie primate social relationships. Similar capacities are usually less apparent in the nonsocial realm, supporting the view that at least some aspects of primate intelligence evolved to solve the challenges of interacting with conspecifics.

Abstract: Social dominance is usually viewed from the perspective of intragroup competition over access to limited resources. The present paper, while not denying the importance of such competition, discusses the dominance concept among monkeys and apes in the context of affiliative bonding, social tolerance, and the reconciliation of aggressive conflicts. Two basic proximate mechanisms are supposed to provide a link between dominance and interindividual affiliation, namely, formalization of the dominance relationship (i.e., unequivocal communication of status), and conditional reassurance (i.e., the linkage of friendly coexistence to formalization of the relationship). Ritualized submission is imposed upon losers of dominance struggles by winners; losers are offered a “choice” between continued hostility or a tolerant relationship with a clearly signalled difference in status. If these two social mechanisms are lacking, aggression is bound to have dispersive effects. In their presence, aggression becomes a well-integrated, even constructive component of social life. In some higher primates this process of integration has reached the stage where status differences are strongly attenuated. In these species, sharing and trading can take the place of overt competition. The views underlying this “reconciled hierarchy” model are only partly new, as is evident from a review of the ethological literature. Many points are illustrated with data on a large semi-captive colony of chimpanzees (Pan troglodytes), particularly data related to striving for status, reconciliation behavior, and general association patterns. These observations demonstrate that relationships among adult male chimpanzees cannot be described in terms of a dichotomy between affiliative and antagonistic tendencies. Male bonding in this species has not been achieved by an elimination of aggression, but by a set of powerful buffering mechanisms that mitigate its effects. Although female chimpanzees do exhibit a potential for bonding under noncompetitive conditions, they appear to lack the buffering mechanisms of the males.