Social organization and foraging in emballonurid bats

Summary

• 1. A general model of mating system evolution in mammals is developed, which takes into account the different male strategies of resource defense, female group defense, and male mating aggregations. The critical environmental variables determining differential defensibility of females and resources are identified by generalizing the resource defense model of Orians (1969). The model is then applied to available data on African antelopes (Jarman, 1974) to establish a set of hypothetical relations between certain patterns of habitat use and mating structures. The resulting relations are only likely to apply to species in which food determines female dispersion and in which any resource defense exhibited by males is directed towards food supplies.

• 2. The relations developed for antelopes are then compared to recently published data on mating systems in five neotropical emballonurid bats (Bradbury and Vehrencamp, 1976a).

• 3. Antelopes and the bats are found to share the following features. Species living in wet and stable forests tend to be fine-grained socially and to have groups consisting of monogamous pairs or nested male-female territories. Species in more seasonal habitats show an inverse relation between the size stability of groups and the duration of use of a given foraging site. As the model predicts, in both groups resource defense occurs where groups are least stable and female defense where groups are most stable. Also as the model predicts, the numbers of females accessible to each male and the number of reproductive males per group can be anticipated in each of the two taxa wherever sufficient data for the critical variables are available.

• 4. Antelopes and bats differ in the following ways. Whereas body size is a good predictor of antelope habitat use and social dispersions, it is a poor predictor for emballonurid patterns. Similarly, although the numbers of females per male generally increase with group size in antelopes, this correlation does not hold for the bats in this study. These differences lead to the conclusion that application of the general model cannot be simplified by measurement of a few variables such as body size or group size, but instead will generally require actual measurements of the critical resource dispersion parameters in the field.