Pick on someone your own size: ontogenetic shifts in mate choice by male garter snakes result in size-assortative mating

Data on over 950 natural matings of red-sided garter snakes, Thamnophis sir talis parietalis, in Manitoba revealed size-assortative pairing: large male s tended to mate with large females, and small males with small females. Un like previously reported cases of size-assortative mating, the causal mecha nism in these snakes involved a size-related shift in active mate selection by males. In the field, courtship as well as mating was size assortative ( albeit, with considerable scatter around the trend line). Staged trials in outdoor arenas showed that males of all sizes preferred to court large rath er than small females, but this preference was stronger in large males. Mal es adjusted their courtship intensity in response to the numbers and sizes of females and competing males, but did not change their preferences with r espect to female body size. Thus, size-assortative mating was not a direct consequence of large males excluding their smaller rivals from large female s. Males may be selective courters in this species because they have a limi ted supply of sperm and mating plugs, and hence can copulate effectively on ly a few times within the mating season. Given intense competition from lar ge males (which primarily court large females), small males may benefit fro m focusing on small females. Alternatively, small males may be less Capable of inducing sexual receptivity from large females. Mark-recapture data con firmed that males grow rapidly from one year to the next. Thus, the size-re lated shift in male mate choice was due to an ontogenetic change rather tha n the existence of multiple male morphs differing in both body size and cou rtship preference. (C) 2001 The Association for the Study of Animal Behavio ur.