The Evolution of Mating Systems in Grouse

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Abstract
Promiscuity in grouse has been explained by the theory that delayed breeding by males causes an unbalanced breeding sex ratio, which forces females to accept polygamy. Subadult males are said to delay breeding because the risks are too high, not because they cannot effectively compete for mates. Evidence contradicts several predictions of this hypothesis. Another hypothesis, derived from sexual selection theory and published information, is presented. Sexual selection theory predicts that selection affecting females should determine mating system characteristics. Comparative evidence shows that the principal differences between monogamous and promiscuous grouse occur in spring. These are that females of monogamous species use male territories to acquire food and build a nest, while females of promiscuous species do not; and females of monogamous species rely on male vigilance to detect predators during the prelaying and laying periods, unlike females of promiscuous species. There is no evidence for consistent behavioral differences during other stages of the breeding cycle. The observed behavioral differences can be explained by the hypothesis that females of monogamous grouse are limited by the availability of food in spring, while females of promiscuous grouse are limited by digestive rate. In monogamous species females should be able to increase their rate of food intake by foraging on male territories to reduce competition and by relying on male vigilance for predators to increase the amount of time available for finding food. Females of promiscuous species would not be able to increase digestive rate by these behaviors, and would be able to avoid increased conspicuousness by minimizing their association with males. Scarce food resources should favor monogamy, while abundant food resources should favor promiscuity. The available evidence is consistent with this hypothesis, but is insufficient to prove it.