ABSTRACT

Khao Yai gibbons can be described as fruit-pulp specialists that maintain a fruit-dominated diet year-round by switching to lower-quality fallback foods-namely figs-when the overall abundance of ripe fruit is low. During the fruit-rich hot season the study animals traveled long distances, visiting multiple fruiting trees in succession and foraging more heavily on insects than during other times of the year. At the same time gibbons exhibited high rates of nonsubsistence behavior, including grooming, play, and friendly interactions with neighboring groups. When ripe fruit was scarce, gibbons coped by exhibiting an economical foraging strategy that optimized energy use by increasing feeding time while limiting time devoted to nonsubsistence behaviors and the distance traveled per day. In contrast to many other populations Khao Yai gibbons recurrently visited large feeding patches (including both figs and nonfig fruit trees) throughout the year. These findings indicate that from the standpoint of primate socioecology it is important to decouple group size and patch size. In Khao Yai the importance of patch size to gibbons appears to lie not in their ability to jointly exploit small food patches, but in their ability to recurrently exploit and defend large patches, thereby minimizing search effort

while maximizing energy gain. Because such benefits can be realized only if animals have reliable access to known food sources, territoriality has evolved in concert with the gibbon’s dietary preference for ripe fruit. Ultimately social monogamy in gibbons likely emerged in response to male parental care, at first indirectly in the form of resource defense territoriality and second, in the form of direct parental care as exhibited by predator protection and grooming.