Primates are noted f o r their mental abilities but the selective basis f o r such traits has Aleles, Alouatta] MOST PRIMATES SHOW A REMARKABLE CAPACITY for learning and retention. As noted by Eisenberg (1973), what appear to set primate societies apart from those of other relatively long-lived and large-brained animals are particular sets of attributes related to the storage and retrieval of a great deal of independently acquired information about the environment. The complex brain of higher primates forms the basis for a plasticity of behavior that permits a wide variety of responses to a given situation (Napier 1970). Hominids are at the zenith of this trend and many hypotheses have been advanced as to why increasing mental complexity might have been favored in their evolution. Hunting is frequently singled out as a critical factor (Waahburn and Lancaster 1968). According to this hypothesis, proto-humans living in savanna-mosaic areas during the Pliocene may FOODS A N D M E N T A L DEVELOPMENT 535have become increasingly dependent on animal protein in the diet. This dependency, in time, could have resulted in strong selective pressure toward increased hunting efficiency through cooperative hunting, food sharing, manufacture of tools and a symbolic system of communication (Hockett and Ascher 1964; Holloway 1967). It is indeed possible that selective pressures related to increased hunting efficiency may have served as an important stimulus in the evolution of the hominid brain. What is omitted from this and similar explanations, however, is why increasing mental complexity might have been favored in hominid evolution in the first place. The key to understanding any adaptive trait comes not only from a knowledge of current selective pressures but also from the past evolutionary history of the species in question. New adaptations are not creations sui generis but rather are modifications of past adaptations. Some critical pressures in the evolutionary history of certain higher primates resulted in an increasing dependence on memory and learning-the hominid line represents an intensification of what must have been a preexisting trend in primate evolution.Early primate field studies were largely concerned with the behavior of savanna-living primates since these were thought to offer possible analogues for hominid evolution (Washburn and DeVore 1961). It must be remembered, however, that proto-humans did not evolve in the savannas, but rather came gradually to them from tropical forest>. bringing with them many millions of years of adaptations to forest conditions. To u n d w stand the origins of mental complexity in hominids, we must look not only at life in the savannas but also at life in tropical forests, for it is here that the first steps toward an increasing dependence on mental abilities must have been taken. What pressures on forestliving primates might have stimulated evolution along these lines?Larger, forest-living primates today are all either partially or entirely primary consumers-that is to say, ...

In lorisines (Loris, Nycticebus, Perodicticus, Arctocebus), the tip of the ulna is reduced to the dimensions of a styloid process, a new and more proximal ulnar head is developed, and the pisiform is displaced distally away from its primitive contact with the ulna. In some Nycticebus, intra-articular tissues separate the ulna from the triquetrum. These traits are not seen in other quadrupedal primates, but they are characteristic of extant hominoids. Among hominoids, these features have been interpreted as adaptations to arm-swinging locomotion. Since hominoid-like features of the wrist joint are found in lorisines, but not in New World monkeys that practice arm-swinging locomotion, these features may have been evolved in both lorisines and large hominoids to enhance wrist mobility for cautious arboreal locomotion involving little or no leaping. Most of the other morphological traits characteristic of modern hominoids can be explained as adaptations to cautious quadrupedalism as well as to brachiation, and may have developed for different reasons in different lineages descended from an unspecialized cautious quadruped resembling Alouatta.

Primates show a strong positive relationship between body weight and home range area. Dietary habits also influence home range area. Folivorous primates occupy smaller home range areas for their body weight than do frugivores and omnivores. Primates generally require smaller home range area per individual than solitary terrestrial mammals, but primates living in social groups have much larger total home range than individual solitary mammals. This trend may necessitate higher expenditures of energy in food-gathering or modifications in movement patterns.