Swimming with a sword: tail beat kinematics in relation to sword length in Xiphophorus

1. Exaggerated morphological structures that evolve under sexual selection have the potential to alter functional relationships and hence affect aspects of movement. These effects may be more pronounced when the exaggerated morphological trait is coupled to the propulsive system. 2. Many studies have examined the effects of sexually selected traits on whole-organism performance, but few have documented their effects on the kinematics of locomotion. 3. Using four swordtail (Xiphophorus) species that vary naturally in their expression of the sexually selected sword, and an experimental manipulation for the species in our sample with the longest sword (X. alvarezi), we examined how variation in sword length affects the kinematics of swimming. 4. Among the four species, we found few differences in tail beat kinematics, despite the large variation in sword length among species. In particular, the two species with long swords did not differ from the species lacking a sword, suggesting no locomotor ‘cost’ of having long swords. 5. Using experimental manipulation, sword removal significantly increased tail beat amplitude, but not frequency, suggesting a potential increase in thrust production. 6. Our comparative results suggest that swimming kinematics do not vary much with sword length, despite the variation in this sexually selected trait among the four species. This result suggests that other physiological mechanisms may be compensating for sword length, or as has been suggested recently, the sword may not impose a significant swimming cost.