The Evolution of Mating Strategies in Bullfrogs, Rana catesbeiana

Variation in reproductive success within and between the sexes was investigated for 2 yr in bullfrogs, Rana catesbeiana. Number of successful matings per male ranged from zero to four per year; number of successful matings per female ranged from one to two per year The polygynous mating system of bullfrogs appeared to result from the differential abilities of older, larger males to control oviposition sites preferred by females. Thus, older, larger males controlled territories of higher quality as judged by lower percent embryo mortality prior to hatching. Factors affecting embryo mortality included developmental abnormalities resulting from high water temperature, and predation of embryos chiefly by the leech, Macrobdella decora. Females deposited their egg complement in the territory of their mate in 70 of 73 observed copulations. Male territories were used as oviposition sites at both low and high densities. No pattern was observed in the spatial positions of males in a chorus with respect to age, size, or mating success. Younger males employed two alternative mating strategies: (1) male parasitism-associating with an old, large territorial male and intercepting any females attracted by the territorial male; (2) opportunism-becoming temporally territorial in a recently vacated territory until threatened by a nearby large territorial male. Both strategies appeared to reduce competition from older, larger males and to bypass female choice. The ability of females to discriminate mate quality also appeared to be age dependent. Younger females were significantly more variable with respect to the quality of mate that they chose than were older females. Older females consistently selected the oldest, largest males in the population as mates. Two aspects concerning the influence of male age on male reproductive success were discussed. (1) Male age should influence the outcome of male-male competition for territories or for females directly (2) Female choice should include discrimination of males on the basis of male age not only because older males can control higher quality resources but also because older males display a phenotype that has successfully survived a wider range of environmental conditions than younger males. Such survival ability could provide information concerning the genetic quality of a male.