This dissertation examines components of male-male competition in order to determine factors that are important in contest dynamics and outcome, as well as the evolution of particular male traits. Specifically, I use northern swordtail fishes (Pisces: Poecilliidae, genus Xiphophorus ) to address the following questions: 1) What role does size play in conflict resolution; 2) How do males use the signal vertical bars in agonistic encounters; 3) What was the role of male - male communication in the evolution of the male trait vertical bars; 4) Are differences in fighting ability and aggression levels between barred and barless X. cortezi males the result of responses to the bars or inherent properties of being either barred or barless; and 5) Are there relationships between size and aggression both within and across species.
Results from dyadic contests indicate that competitive ability is largely the result of an individual’s size relative to that of his opponent. However, other factors contribute to the probability of winning a contest as well. For example, barless X. cortezi males appear to have higher resource holding power (RHP) than same sized barred males, and this advantage appears to result from overall higher aggression levels. Results from mirror image stimulation tests indicate that variable bar expression signals aggressive motivation in most barred species and additionally, signals aggressive intent in barred species that intensify the expression of the bars prior to biting.
Additionally, data indicate that other factors probably played a larger role in the evolution of the vertical bars than did male-male competition. However, once males gained the ability to vary the intensity of the bars, variable bar expression became an important component of communication in swordtail aggressive interactions. Finally, the relationship between absolute size and aggression varied within species, suggesting that the costs and benefits resulting from competition favor higher levels of aggression in larger males in some species and lower levels of aggression in others. However, there was a positive relationship between size and aggression across species. While many other factors affect male size in swordtails, these results suggest that male – male competition is important in the evolution of male size.