The yellow-bellied marmot and the evolution of polygamy
Abstract
We have elaborated a model describing the development of polygynous mating systems. The bases of the model are measures of fertility and survival of adult females and their offspring, respectively, as functions of increasing harem size. Our data indicate that an adult female makes her greatest contribution to the next generation (is most fit) when she is monogamous. A second curve describing the fitness of the harem master with increasing harem size indicates that he is maximally fit when he maintains a harem of two or three females. Consideration of the optimal mating system for each sex leads to predictions concerning the types of behavior that would allow each sex to realize its optimal mating system. Females should react aggressively toward other females and solicit the attentions of the male. The aggressive behavior of the female should be linked with her reproductive state. Pregnant females will be more aggressive than nonpregnant females. The male, on the other hand, should actively recruit females and act to pacify interactions between females. These expectations are realized in marmots and a number of avian species. The reproductive success of females in the Gothic area is affected by the availability of food during gestation. When the growing season is early, more young are produced. In those years, however, the mean harem size is smaller, apparently due to increased aggression among adult females. The different reproductive strategies of males and females, and the influence of food availability on those strategies, lead to the prediction that bigamous matings should be the commonest mating type. This prediction is verified by our field data. It should be noted that bigamy is disadvantageous, relatively, to both males and females. A general formulation of optimal harem size based on the fitness of a male and the females in his harem is given. Large harems are expected to be variable and aggression among females should be reduced. Observations on pinnipeds support these predictions. Small harems would appear to be more stable. Consequently, behavior that would lead to pair formation and maintenance are expected. Pair-bonding in monogamous species of birds is in keeping with this prediction. When the optimal harem size is small, transient perturbations have effects opposite to those expected on a long-term basis. The effect of food abundance on marmots at Gothic is a decrease in mean harem size.
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