Evolution of Social Organization and Life-History Patterns Among Grouse

Abstract

Correlations of social structure, life-history patterns, and ecology among the seventeen species of grouse exemplify some general patterns in the evolution of mating systems among higher vertebrates. The species of grouse differ in the aggregation of displaying males, the permanence of heterosexual affiliations, the contributions of males to parental care, and the breeding sex ratio. Promiscous species (no durable heterosexual affiliation) are probably all polynous (the breeding sex ratio less than unity), but fall into two groups depending on whether the displaying males congregate at leks or disperse relatively evenly. In all of these promiscuous, polygymous species and in three monogramous species, the female cares for the young; dual parental care appears only in one monogamous species. During their first year, the males of polynous species do not mate or mate much less frequently, although females normally breed at one year of age, a situation termed sexual bimaturism. At least among birds and mammals, polygyny is normally associated with sexual bimaturism. Sexual bimaturism and polygyny among grouse correlated well both with large overall size, as indicated by female weight, and with greater sexual dimorphism in weight. These correlations also recur in other vertebrate families. Among the promiscuous grouse, the dispersion patterns of displaying males probably relate to the differences in predation pressures in open and forested habitats. In contrast with some other avian families, differences in social structure among grouse have little relationship to major differences in diet. Single parental care is not a sufficient condition for the evolution of polygyny among grouse. The association of sexual bimaturism with polygyny requires, in addition, an explanation of the adaptive advantages of deferred reproduction among males. Postponed reproduction by males will reduce the spread of their genes in a population unless compensated by a sufficient gain in early survival or increased fecundity later. Calculations of rates of reproductive increase for hypothetical lineages of males indicate that these compensating conditions can plausibly explain the evolution of delayed reproduction among male grouse. Theoretical considerations further suggest that larger size could favor the evolution of deferred reproduction, especially in males, and thus could contribute to the evolution of sexual bimaturism and polygyny in the larger species of grouse. Ecological circumstances, including the details of food dispersion, could thus influence the evolution of mating systems indirectly, through effects on the evolution of body sizes. As polygyny among higher vertebrates is normally associated with sexual bimaturism and is incompatible with full dual parental care, polygyny should evolve under ecological conditions in which the adaptive advantages of sexual bimaturism otuweigh those of dual parental care.