Head over heels: an examination of a possible mating signal in female swordtails, Xiphophorus cortezi

Females of many species display overt behavioural signs that provide information to males about their willingness to mate. We have observed female swordtails (Xiphophorus cortezi) performing ‘headstands’ and/or ‘pecks’ towards the substrate of their tank in the presence of males. To determine whether this behaviour is an attempt at foraging, a mating signal or a sign of aggression or stress, we exposed satiated individual female swordtails to either no fish, a female or a male fish and measured association time and frequency of headstand/pecking behaviours. Females did not perform these behaviours when there was no other fish in the tank. Although they spent equal amounts of time associating with either a male or female stimulus fish, they only performed headstands/pecks in the presence of males. Furthermore, in dichotomous choice tests with large and small males, females preferred to associate with large males and performed significantly more headstands/pecks in their presence. Finally, males were simultaneously exposed to videos of a free-swimming female and the same female intermittently pecking the bottom of the tank to examine their response to the signal. Larger males spent more time with and performed more courtship behaviours towards the pecking female, whereas smaller males spent more time associating with and courting the freely swimming female. These results suggest that headstands/pecks performed by female swordtails in the presence of males may be a signal of a female's willingness to mate and that the size of a male affects his response to this signal.