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3 <br />evolution of mating systems, which is currently the subject of <br />considerable theorizing. But before proceeding to do that, I <br />must make clear what is meant by a mating system. <br />Mating systems encompass the interactions of males and <br />females. The emphasis is on the dynamics of getting males and <br />females together. For example, does one male mate with several <br />females (polygyny), or one female with several males (polyandry)? <br />Or does a given male remain paired with a single female <br />(monogamy)? Mode of reproduction, e.g., external or internal <br />fertilization, is a separate issue and is usually more <br />conservative in the course of evolution. <br />A central feature in theories about mating systems is the <br />search for first causes. A simple scheme goes as follows: In a <br />given species, the distribution of its food in space and time <br />determines the pattern of distribution of the animals. If the <br />food is distributed in discrete patches, the animals tend to <br />bunch up. Given other conditions, such bunching permits one large <br />male to exclude other males and to hold a harem at the patch of <br />food. On the other hand, living in open areas may result in the <br />animals forming groups for protection from predators; the <br />resulting mating system will depend on the sex ratio of the <br />group. One can make similar arguments for such factors as <br />population density and suitable places to leave eggs or to rear <br />young. <br />The bottom line is that features of the environment impose <br />spatial patteans on the animals living there. Depending on the <br />interaction of a number of variables, and the constraints imposed <br />by the nature of the species and its evolutionary history,