Sexual selection on body size, territory and plumage variables in a population of Darwin's finches.

Abstract

The population of medium ground finches Geospiza fortis on I. Daphne Major, Galapagos was studied between 1979 and 1981. Adult males outnumbered females by 2 or 3 to 1 as a result of differential mortality during a drought in 1977. The skewed sex ratio resulted in a high potential for sexual selection in this monogamous species: about half the variance in male reproductive success was attributable to variation in mating success. Females preferred to mate with larger males in 2 out of 3 years. Territory size and extent of adult plumage were also positively correlated with mating success, and all 3 characters contributed significantly when combined into a regression model using probit analysis. Plumage and territory size characters are tentatively interpreted as being indicators to females of male quality. High quality males probably contribute increased parental care and possibly genetic quality to their offspring. There is no evidence that sub-adult plumage has adaptive value with respect to mating success. Territory size may be a sexually selected trait more commonly than is generally assumed and sexual selection may prove to be a general explanation for 'superterritoriality'. Body size was under selection in part because it positively correlated with male quality; other types of sexual selection (active female preferences and male-male competition) may also be operating.-from Author