The Effects of Diet Quality on Xiphophorus Escape Response

Event Center A & B

A fundamental assumption of the theory of sexual selection is that sexually selected traits increase reproductive fitness. However, these traits may incur a survival cost. Among swordtail fishes, both the length of the sword (if present) and body size are sexually selected traits in males—a reduction in adult size would potentially incur both decreased mating success, and reduction in C-start performance. Here we examined the effects of diet (high protein, HQ, vs. low protein, LQ) on body size and C-start performances in three types of swordtail fish, Xiphophorus variatus (no swords) and two genetically distinct size classes within the genetically polymorphic X. multilineatus. To quantify escape performance we measured whole-body (relative to starting position) and relative (within-fish) maximum turn angles, turn rates, time to maximum body angles, and total displacement of the center of mass. LQ diet fish mass in all three groups was significantly less than HQ counterparts. We found effects of diet only in relative maximum turn angle and turn rate, with LQ fish achieving greater turn angles and rates. However, when controlling for the effects of length, only the relative maximum turn angle was significantly impacted by diet. Our results are somewhat surprising given the substantial differences in mass observed in these fish, and given that size seems to be a good predictor of escape performance in other fishes. For swordtails, smaller size due to a poor quality diet in the wild is likely to be detrimental to reproductive fitness, but perhaps less so to survivorship.