Energetics of the Sword

In collaboration with Guillermina Alcaraz, I am investigating sword energetics in male swordtails. The sword consists of a caudal fin in which rays at the base of the fin are colored and lengthened. Using closed respirometry, we measured male O₂ consumption during courtship and routine swimming activities. The sword had a significant effect on both; males with longer swords expended more energy. And, although courtship was found to be energetically costly, sword length significantly increased this cost. Thus while mate choice favors longer swords, there is a corresponding cost. Second, we are investigating the effect of the sword on males swimming against a current. We first tested resting O₂ consumption rate as a baseline and found that it was not related to any sword measure. However, larger males at rest did consume more O₂. Male sword size showed the same relationship for O₂ consumption while swimming in a current; males with a greater sword area consumed more O₂. Both male size and sword area also had independent effects on aerobic scope, a measure of metabolic ability; larger males and males with greater sword area had a greater aerobic scope. In addition, males with less sword area required more time to recover to resting levels of O₂. These results suggest that while large swords favored by sexual selection may be costly in terms of changing direction, males with larger swords exhibit greater metabolic competence in a current. Our research will continue to investigate energetic costs of the sword and its hydrodynamic effects. Extensions of this research are currently providing undergraduates in Mexico with the opportunity to assist in both field and laboratory research.

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