Intrapopulational variation of ejaculate traits and sperm depletion in red‐sided garter snakes

Abstract

AbstractFemale sexual promiscuity is a prevalent element of mating systems. One consequence of female sexual promiscuity is that male‐male competition often continues post‐copulation within the female's reproductive tract. According to theory, the number of sperm a male inseminates relative to his rivals strongly predicts his fertilization success. However, sperm quality is also important, especially when males are sperm limited and female sperm storage is prevalent. In this study, we examined intrapopulational variation in sperm numbers and ejaculate quality (sperm mobility) in male red‐sided garter snakes, Thamnophis sirtalis parietalis, and determined whether these traits varied with male body size and condition over successive matings. We obtained sperm by dissolving copulatory plugs collected from natural matings, which enabled us to also test whether males allocated more sperm to larger, more fecund females. We found significant variation in ejaculate quality among males and that small males transferred as many sperm as large males. Total sperm numbers declined significantly from a male's first to second ejaculate suggesting that males may become significantly sperm depleted across successive matings. The mass of the relatively sperm‐free posterior portion of the copulatory plug that remained after liberation of sperm was correlated with copulation duration. Males copulated longer with larger females; however, longer copulation durations did not correlate with total sperm. Thus, males may allocate more copulatory plug material to larger females to guard against her remating, instead of allocating more sperm.