What determines lek size? Cognitive constraints and per capita attraction of females limit male aggregation in an acoustic moth

It has been proposed that leks arise because of increased mating benefits in aggregations of displaying males, and some evidence supports this hypothesis. But observations also indicate that lekking aggregations include only a small percentage of the males in a population, implying that certain factors limit lek size. Potential factors include increasing travel costs to find and form large but distant aggregations, greater attraction of predators and higher levels of aggression. Any one of these constraints may cause the number of females arriving at larger leks to decelerate such that per capita male attractiveness, and hence mating success, declines above an optimum lek size. None the less, relatively little empirical work has examined what determines lek size. In particular the possibility that cognitive aspects might constrain lek size has rarely been considered. We studied this question in Achroia grisella, an acoustic moth in which singing males form small aggregations that attract females. We created artificial leks in the laboratory and tested their relative attractiveness to females; we also tested male preferences to form and join such aggregations. Females preferred male aggregations over solitary singers, but the marginal per capita attractiveness of an aggregation of n+2 males versus n males waned for n≥5. Similarly we found that males were attracted to other males singing in the vicinity, but this effect disappeared for n≥4. We infer that lek size is limited because the marginal per capita attractiveness of larger leks only occurs for small groups. This constraint probably arises because females distinguish leks by overall song rate but are neuroethologically incapable of discriminating rates above a threshold value corresponding to groups of four to six males. These findings emphasize the critical role that neural constraints may play in determining fundamental parameters of complex behaviours such as lekking. •We used binary choices to examine female preference for lek size in an acoustic moth.•Female preferential attraction for larger leks declined with lek size.•Saturation of neuroreceptors can explain the pattern of female preferences.•Female preference for lek size and male tendency to aggregate roughly matched.•Neuroethological constraints can ultimately limit lek size.