Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Michael Goodrich


The relationship between social rank and sexual behavior of the male swordtail (Xiphophorus helleri) was studied. Initially three tanks were established with three males and two females in each. Social rank, aggressive and sexual levels were recorded by observation of the interactions which occurred in the tanks. A second series of observations were made with the fish regrouped according to their initial rank, that is, dominant males grouped together, middle ranking males together and the lowest ranking males together. Aggressive and sexual levels were the highest for the highest ranking males in all tanks and in both trials, although the levels of some subordinate fish were close to those of the dominant fish. When high ranking fish were placed together the levels of aggressive and sexual behavior increased strikingly for each one, including the male which became the lowest ranking male in the tank. In most cases aggressive and sexual behaviors increased or decreased both qualitatively and quantitatively in conjunction with a change in social rank. It was concluded that the amount of stress increased as the rank was lowered, and inhibited the sexual behavior accordingly. Thus, when a fish changed his rank, his expressed sexual behavior changed also.