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The Cost of Bearing a Sword: Locomotor Costs and Compensations in Relation to a Sexually Selected Trait in Xiphophorus

Abstract

Some of the most compelling traits to evolutionary biologists are sexually selected traits. The `un-natural' expression of these traits has puzzled biologists since the time of Charles Darwin. Why would a trait evolve that seems to decrease survival? Most of these traits evolve because they increase the reproductive fitness of the bearer, either through intra-sexual or inter-sexual competition. However, these sexually selected traits must meet the demands of both natural and sexual selection. It is therefore assumed that there are `costs' associated with the trait that limits their expression. In this dissertation I examined the performance costs of a sexually selected trait, expressed as an exaggerated morphological structure.

I first introduce a more integrative approach to the study of detecting costs of sexually selected traits. This approach incorporates additional aspects of the organism's phenotype that may have evolved to offset the costs of the sexually selected trait. I show that ignoring these `compensatory' traits may mask the cost of the sexually selected trait using synthesized data.

I then set out to examine the repeatability of various swimming performance measures. Using male guppies (Poecilia reticulata) I demonstrate that swimming performance measures are stable over various time scales. Both aerobic and anaerobic locomotor functions were examined over time scales varying from days to more than a year later. While most performance measures were not stable over a year, at least one was repeatable. Furthermore, although most performance measures decreased over a year, one significantly increased (maximum speed, Umax).

I then used phylogenetic comparative methods to determine the locomotor costs of the sexually selected `sword' among species of Xiphophorus and Priapella, taking into account compensatory traits. I showed that inter-specifically, the sword is not a cost to critical swimming speed among 19 species of Xiphophorus and Priapella. In fact, when compensatory traits and phylogenetic information are included, the sword had a significant positive effect on critical swimming speed among these species. This suggests that as this sexually selected trait evolved it was not a locomotor handicap.

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