9 - Body size and sociobiology

Summary

Introduction

This chapter considers the effects of large body size on social patterns, in particular (i) group size and structure; (ii) male dominance systems; (iii) female mate choice. Except for group size, these features are not readily characterized in numerical terms, and so cannot be related allometrically to body mass. Instead I will employ a cost/benefit analysis, assessing likely gains and losses in the factors determining evolutionary fitness, i.e. survival chances, reproductive contributions, and offspring survival. An inherent shortcoming of such an approach is that it does not adequately allow for possible interactions between these components (Crook & Goss-Custard 1972; Wilson 1975).

Grouping patterns

Jarman (1974) pointed out that among African bovids group size tends to increase with increasing body size. He explained this pattern in terms of the trade-off between the feeding costs of group formation, and the resultant anti-predation benefits. Because of their high specific metabolic rates, small antelopes are selective feeders on high quality plant parts. These are thinly scattered and quickly depleted. Large ungulates in contrast are relatively fiber-tolerant. They experience a much higher density of acceptable food, which is more uniformly distributed and depleted less by other animals foraging in the same area than is the food of small antelope. Intermediate sized species exhibit a gradient between these extremes. Hence the feeding cost of having close companions decreases with increasing body size.

This explains why large ungulates should be more tolerant of nearby conspecifics while foraging than smaller ungulates, but not why they should actively remain in a group. Jarman thus considered predation risks.