Introduction

During the three decades since the above statement was made a great deal of theoretical and empirical work has established a number of plausible functions for animal grouping. These functions typically relate to feeding efficiency or predator avoidance. Such generalizations may allow us to see overall patterns and generate hypotheses which can be tested experimentally. However, by considering the selection pressures on groups in general, there is the danger of missing some of the details. Instead, the relative advantages and disadvantages of group membership may vary in different parts of the group (Bertram 1978). For example, although individuals at the edge of a group may be more likely to be preyed upon than those at the center, they may also be more likely to obtain food. The costs and benefits to an organism as a function of location within the group has rarely been assessed. I hope to convince the reader that groups are not homogeneous units, and that different selective pressures operating in different locations within a group lead to position preferences by its members.