Effects of sex-ratio manipulation on territoriality and spawning success of male pupfish,Cyprinodon pecosensis

The sex ratio of Cyprinodon pecosensis was manipulated by varying the number of females but not of males. This resulted in some subordinate males changing their breeding tactics from satellites to territorials. The spawning success of breeding males was directly related, and the intensity of male-male competition was inversely related, to the number of available females. Time-budget data indicated a tradeoff between courtship and agonistic behaviour. In the male-biased treatment, males engaged in a greater number of agonistic interactions and fewer courtship bouts; conversely, in the female-biased treatment courtship behaviour was more frequent than aggression. A greater proportion of males adopted the satellite breeding tactic in the male-biased and even-sex-ratio treatments. Smaller, competitively inferior males were most sensitive to changes in the sex ratio. In the female-biased treatment, with an increase in mating opportunities and a decrease in the level of male-male competition for females, average mating success increased, more males were able to defend territories, and some satellite individuals were able to adopt the territorial breeding tactic. Temporal or spatial changes in the operational sex ratio of a population may have profound effects on the dynamics of the breeding system by affecting (1) the proportion of males adopting primary and secondary breeding tactics, (2) variance in reproductive success, and (3) intensity of sexual selection.