Population density influences male-male competition in guppies.

This study tested the general prediction that population density affects male-male competition, female mate choice and the opportunity for sexual selection. By manipulating the density of guppies, Poecilia reticulata, while keeping the sex ratio constant, I found that male mating tactics were phenotypically plastic with respect to density. As density increased, males decreased their courtship displays. Male-male competition and mate searching were highest at intermediate densities. Population density had no significant effect on the total number of copulations, copulatory tactics or the percentage of postcopulatory guarding. Female preference for males with a higher percentage of orange coloration was similar at all density levels. The 'opportunity for sexual selection', which estimates the upper limit to which a selected trait can shift if directional selection is operating and was calculated as the variance in number of copulations per male divided by the square of the mean number of copulations, was negatively associated with population density. This may be due to the decrease in male-male competition at high density rather than female preference which was similar across density treatments. Copyright 1999 The Association for the Study of Animal Behaviour.