Breeding behavior and mating system in relation to body size in Rana chensinensis, a temperate frog endemic to northern China

Abstract

Knowledge of how mating success is related to body size may provide insight into the evolution of social systems. This study investigated the mating system and relevant social behavior of a temperate anuran (Rana chensinensis) at three localities in northern China. During chorusing, males aggregated and persisted in the communal spawning ponds with a density of 10–26 frogs per m2 water area and operational sex ratio of 15–28 males to 1 female. The males frequently grabbed any conspecifics they encountered, releasing the grip if the individual was a male, but holding the grip when it was a female. A significant positive relationship between male and female body lengths of pairs in amplexus was detected from all the sites during the five breeding seasons, but the average correlation coefficients of determination of 18% indicated that the size-assortative mating constituted a minority of the species’ mating system. Pairing probability decreased with increased size differences between sexes, which could have prevented a few adult animals in a population from forming pairs. The strong aggregation of males could limit the opportunities for large males to exhibit contest advantages in mating and for both sexes to choose a large mate. Infrequent occurrence of scramble competition (averaging 8% of the recorded amplectant pairs) and the lack of observed takeovers, probably because of reclusion of amplectant pairs at the bottom of spawning ponds, suggested a weak role of amplexus displacement in generating non-random mating. Our results suggest that although non-random mating with respect to body size may be expected in explosive breeders, the social environment characterized by male aggregation may also impose weak selective pressure for the evolution of non-random mating.