Copulatory plugs, restricted mating opportunities and reproductive competition among male garter snakes.

Abstract

REPRODUCTIVE competition for females among males of a given species can take many forms, some of which (like mating plugs) may place constraints on the mating opportunities of males and intensify competition for available females. I have studied the reproductive behaviour of two garter snake species (Thamnophis sirtalis and T. butleri) in the field for 5 years. They do not exhibit pair-bonding, paternal care of offspring, territoriality, or intrasexual combat. Sexual selection in this mating system should favour males which compete to locate, court and mate with the maximum number of females1. Yet I have observed numerous solitary female Thamnophis within the same time periods and areas in which other females were being vigorously courted, often by several males. I have previously observed that the anterior cloaca of any recently mated female Thamnophis contains a copulatory plug which occludes the oviductal orifices2. The plug evidently is formed by the copulating male after sperm transfer. I interpreted this as a form of intrasexual competition in which the successfully copulating male makes the female temporarily unavailable to other males and reduces the likelihood of multiple inseminations. Some reports3,4 indicate that multiple inseminations are possible. I present here evidence from the field that male garter snakes recognise females with a copulatory plug and behave as if these females were unavailable.