Neurobiology of Social Behavior

The Primate Order comprises over 300 species and a wide range of different social systems (Smuts et al., 1987). Very few species have a fully solitary lifestyle. Most of the others are characterized by permanent associations between two or more adults and their young. Pair living is relatively rare, being characteristic of only 3% to 4% of primate species. Some of these socially monogamous species (e.g., tamarins and marmosets) have a flexible social organization, in which one additional adult male or female may be temporarily associated with the breeding pair. Stable groups with one adult male and several reproductively active females (i.e., harems) are shown by approximately 35% of primate species. The most common type of social organization in nonhuman primates consists of social groups with multiple adult males and females and their young. These multimale/ multifemale social groups represent the stable form of organization of about 45% primate species, with an additional 15% of species showing fluctuations between such groups and groups with a harem structure. Variation in social organization among primate species has been explained on the basis of variation in ecological variables such as diet, food-related competition and cooperation (e.g., Wrangham, 1980 AQ1 ), and need for protection from predators (e.g., van Schaik AQ2 ) or infanticide (e.g. Dunbar, 1988). Phylogenetic history, however, also accounts for variation in social organization, as groups of closely related species and genera tend to have similar social systems (Rendall & Di Fiore, 1995). Finally, within-species variation in social organization may result from variation in local ecological and demographic conditions. Regardless of the variation in social systems, it is clear that nonhuman primates are generally highly social organisms, in which successful survival and reproduction depend on complex social interactions with other conspecifics. Accordingly, most primate species exhibit complex behavioral adaptations for communication, affiliation, aggression, mating, and parenting. Group-living monkeys and apes often use vocalizations to alert others of the presence of food and predators, and also to keep in contact with other group members during travel. Facial expressions and body postures play an important role in close-range affiliative, agonistic, and sexual interactions, particularly among Old World monkeys and the great apes. Olfactory and tactile signals are also used in these and other contexts, although olfactory communication is relatively underdeveloped in most primate species relative to other mammals. Social bonds between family and group members are established and maintained through contact, proximity, and grooming. Grooming is an altruistic behavior that can be exchanged for tolerance, sex, or coalitionary support during fights. Aggression and submission, often expressed with facial expressions and body postures, result in the establishment of dominance relationships and hierarchies. In many species, conflict outcomes and dominance ranks are determined not by the individuals’ body size and strength but by coalitionary support received from other individuals. Fights between