Headstands: a sexually selected signal in the swordtail fish Xiphophorus nezahualcoyotl

" Behaviors associated with aggression in male–male competition may also be used in courtship. Headstands are performed by male Xiphophorus nezahualcoyotl during male–male interactions and by both males and females during courtship. This behavior consists of a descending vertical tilt of the head until the body is at a 45° to 90° angle with the substrate. We examined the function of headstands in male–male contests by investigating differences in their use by winners and losers, and correlations between headstands and other behaviors. We also investigated their function in mate choice by determining if females preferred videos of males performing headstands over videos of the same males not performing headstands. There was no correlation between performing the first headstand and biting first, or the total number of headstands and bites used. However, winners performed more headstands than losers in the second half of contests. We suggest that the headstand is an aggressive display providing information about Resource Holding Potential (RHP) in male contests. In addition, females had a significant preference for males performing headstands as compared to males that did not. We discuss the dual role of this behavior in sexual selection in relation to the information it may contain "