Are snakes right-handed? Asymmetry in hemipenis size and usage in gartersnakes (Thamnophis sirtalis)

Male snakes possess paired reproductive systems (testes, efferent ducts, hemipenes and associated components of the kidneys), with an independent set on either side of the body. Our studies on gartersnakes (Thamnophis sirtalis parietalis) reveal significant morphological asymmetry in this system: testes, kidneys, and hemipenes on the right-hand-side of the body are larger than those on the left. Data from matings in the field, and in outdoor enclosures, suggest that this asymmetry has implications for reproductive behavior and, possibly, reproductive success. Copulations using the right hemipenis produced a larger gelatinous 'mating plug,' and may thus more effectively delay remating by the female. Although the overall usage of the two hemipenes in field matings averaged close to 50/50, hemipenis usage was not random. Males tended to alternate hemipenis use in successive matings, perhaps because of depletion of plug material. Also, male gartersnakes preferentially used their larger (right) hemipenis when mating at high body temperatures, perhaps because they are more able to make subtle postural adjustments (and thus, select the better system) under these conditions.