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Odd Couples, Extraordinary Differences Between the Sexes in the Animal Kingdom

Daphne J Fairbairn

......Similar harem-based, polygynous mating systems can be found in many large mammal species although not to the extreme seen in elephant seals. Notable examples include the various species of sea lions and fur seals, which are in a separate seal family (Otaridae) and represent a separate evolutionary origination of this type of mating. Many ungulate species (deer, sheep, bison, and antelope) have also evolved harem-based breeding systems, as have several primate species, and the parallels between these species and elephant seals are often striking. The key to these systems is that males are able to monopolize mating opportunities with females because fertile females are found in clusters or aggregations that can be defended by large, belligerent males; and the signature traits are marked sexual dimorphism in body weight, with males being the larger sex, and male-limited weaponry such as horns, antlers, tusks, and enlarged canine teeth. Intense selection on males to outcompete their rivals (sexual selec­tion) is certainly the main reason why males become so much larger than females, but we must be careful not to ignore the importance of selection on females as well. Females typically live within a dominance hierarchy of their own and gain advantages from increased size, but as in elephant seals, the benefits of large size for females are very modest when compared to the potential gains that large males can achieve. This is mainly because females give birth to only one or two offspring per breeding attempt and maximize their lifetime offspring production by producing one high quality offspring in each of many consecutive breeding seasons rather than by increasing the number of births per season. The net effect of this strategy is that females begin breeding at an earlier age and smaller size than males, and they divert energy into offspring production rather than growth, so that they remain much smaller than males throughout their adult lives. Elephant seals serve as the most extreme example of this pat­tern of sexual differences, but they are unique only in the magnitude of their sexual dimorphism and the scale of their ecological and geographic segregation. Similar patterns are repeated to a lesser degree in many large mammal species, and as we shall see in subsequent chapters, traces can be discerned in the patterns of sexual differences found in polygynous spe­cies in other taxa as well.