Operational sex ratio influences female preference and male-male competition in guppies.

Manipulation of the operational sex ratio (OSR) in guppies, Poecilia reticulata, causes changes in male-male competition and female mate choice. In this study OSR is defined as the number of sexually active males divided by the total number of sexually active adults of both sexes. The rate of male courtship displays decreased, and interference behaviours between males increased, at male-biased OSRs. The OSR influenced both copulatory tactics and postcopulatory guarding. All copulations followed sigmoid displays, except at an OSR of five males to one female where 60% of copulations occurred during sneak attempts. Compared with copulations that followed sigmoid displays, successful sneak copulations were followed by a shorter period of postcopulatory mate guarding and a shorter refractory period before males resumed courtship activities. Females preferred males with more orange colour whenever they had a choice, and the preference for orange colour was stronger with more male-biased OSRs. The OSR thus influences the presence, absence or relative importance of both female mate choice and male-male competition which, in turn, should affect the evolution of secondary sexual traits. Copyright 1999 The Association for the Study of Animal Behaviour.