INTRODUCTION

The study of seasonal variation in the environment and the role this plays in influencing behavior and physiology of chimpanzees has been the focus of numerous studies in recent years. Chimpanzee habitat shows a highly seasonal pattern in food abundance and distribution, which has long been known to influence many aspects of social behavior and ranging patterns.

Several social behaviors of chimpanzees are seasonal. For example, Stanford et al. (1994) found that hunting activity at Gombe peaked during the dry season. Chimpanzee party size varies seasonally (Wrangham 1977; Wallis & Bettinger, unpublished data; Chapman et al. 1995; Matsumota-Oda et al. 1998). At Gombe, there are significantly more adult males and estrous females present in a party during the dry season than during the wet season (Wallis & Bettinger, unpublished data).

It was previously assumed that within-group feeding competition drives the correlation between party size and the availability of food in chimpanzees (Wrangham 1977; Chapman et al. 1995) and may thus explain the seasonal trend in party size. However, the presence of sexually receptive females influences party size much more than do the local conditions of food abundance or distribution (Anderson 2001; Anderson et al., Chapter 6). Several investigators have found a correlation between party size and the presence of estrous females (Matsumoda-Oda 1999; Wallis & Reynolds 1999; Anderson 2001; Anderson et al., Chapter 6). Thus, the key to understanding some seasonal social patterns in chimpanzees may lie in further investigation of female reproductive physiology.