van Schaik’s socioecological model (van Schaik, 1989) hypothesized that predation risk and food distribution determine female gregariousness and their social relationships through the type of food competition females experience. Also, it predicts that the distribution of males is dependant upon female gregariousness. This model was later modified by van Schaik and colleagues (Sterck et al., 1997). The modified model incorporated ’habitat saturation’ and ’infanticide risk’ as selective forces to explain several phenomena that van Schaik’s classical model did not. At present, the main focus of the discussion in primate socioecology has shifted to ’infanticide risk’.
We review here the evidence for and against van Schaik’s classical and modified models, and point out the following problems to be solved. 1) The modified model cannot be a socioecological model until it is combined with the classical model mainly because it does not address the determining factors of each type of food competition (i.e., food distribution). 2) Out of four social categories addressed in the modified model (Dispersal-Egalitarian, Resident-Nepotistic, Resident-Egalitarian and Resident-Nepotistic-Tolerant), the ecological factors of RNT do not seem valid. In our opinion, RNT is expected where primates equally subsist on both types of food: a) high quality, scattered patches large enough to accommodate all group members, and b) high quality patches that are too small to accommodate all group members. 3) In RE and RNT categories, there is not yet enough evidence to prove either the relationship between food distribution and food competition or the relationship between food competition and social category. 4) The sooty mangabey (
Cercocebus torquatus atys) may be categorized into the fifth category (Female-Resident-Despotic) but this may not be explained by ’habitat saturation’ as incorporated into the modified model. If so, ecological factors of FRD should be investigated in more detail. 5) There is little evidence that dispersed females in DE species choose males with a greater ability to defend against infanticide, although high infanticide risk forces females to permanently associate with males as the modified model hypothesizes. 6) Female dispersal in most DE species seems to reduce infanticide risk since larger female groups are more attractive to incoming males. However, we should not jump to conclusions because convincing counter-evidence exists. That is, groups with a larger number of females are more successful than groups with fewer females in reducing infanticide through defensive female coalitions.
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