Female-female aggression functions in mate defence in an Asian agamid lizard

Female-female aggression and its functions are poorly understood compared with male-male aggression. Here, we examined the role of female-female aggression in mate defence in an Asian agamid lizard, Phrynocephalus vlangalii, in which male neighbours are valuable to females in both mating success and resource defence. We provided three social contexts by pairing a resident female with either a neighbour male, unfamiliar male or unfamiliar female, then introduced a tethered unknown female (intruder). We carried out our experiments during and outside the mating season to test the links between female-female aggression and mate defence. The aggressive responses of resident females in these different social contexts were compared by quantifying variation in their territorial displays. Resident females were faster to display to the intruder in the presence of a neighbour male compared with both the unfamiliar male and female. We also found that female lizards signalled faster in the presence of the neighbour male than the other contexts, but only during the mating season. To separate the effects of familiarity from those of sex, we carried out a second experiment in which we paired a resident female with a neighbour male or neighbour female, before introducing a tethered unfamiliar female. In this experiment, resident females responded sooner to intruder females when paired with the neighbour male, although signalling speeds were equivalent. Taken together, our results suggest female-female aggression is used for both resource and mate defence, and so might be under direct selection. Our study highlights that female aggression more broadly requires further work.