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

Abstract

Female 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.

Size dependence in non-sperm ejaculate production is reflected in daily energy expenditure and resting metabolic rate

“…Indeed, in redsided garter snakes, total plug mass is positively correlated with male and female size (Friesen et al, 2014c,d;Shine et al, 2000a). Males allocate more plug material, but not more sperm, to larger females, and males tend to copulate longer with larger females (Friesen et al, 2014c). We found further support in the present study for larger females receiving larger copulatory plugs, but water content has the strongest relationship with female size.…”

“…Female mass did not strongly affect energy density (in kJ g −1 : R 2 =0.154, F 1,18 =3.091, P=0.097), dry plug mass (in g: R 2 =0.152, F 1,18 =3.039, P=0.099), or total energy per plug (in kJ: female mass, F 1,18 =3.704, P=0.072), but plugs from larger females contained significantly more water (in g: female mass, F 1,18 =9.897, P=0.006). We note that this study has smaller sample sizes than those that have found significant effects of male and female size on plug mass in this species (Friesen et al, 2013(Friesen et al, , 2014cShine et al, 2000a).…”

“…On average, larger males deposit slightly larger plugs and longer copulations tend to produce larger plugs as well. Nevertheless, there is no relationship between male size and copulation duration, indicating that larger males deposit plug material more quickly, perhaps because they have greater storage capacity and associated delivery ducts than smaller males (Friesen et al, 2013(Friesen et al, , 2014cShine et al, 2000a). Secondly, copulating males have increased blood lactate compared with courting males, which suggests that mating incurs an additional energetic cost over courtship alone (Shine et al, 2004b).…”

“…1). Although males are less likely to produce plugs and also become sperm deleted after successive matings (Friesen et al, 2013(Friesen et al, , 2014c, it is not known whether smaller males become deficient in plug material faster than larger males. The shift in RQ, seen only in small mating males (Fig.…”

“…Twenty males were allowed access to females while placed in small cylindrical arenas (Friesen et al, 2014c). In one arena we placed 20 large (>35 g) males and in the other arena we placed 20 small (<30 g) males.…”