Intraspecific variation in ejaculate traits of the northern watersnake (Nerodia sipedon)

Abstract

Sperm competition occurs when the sperm of more than one male compete to fertilize the eggs of a female. In reptiles, sperm competition is particularly prevalent and is an important agent of sexual selection in males. Spermatogenesis in reptiles can be energetically expensive, suggesting that there may be costs to producing high-quality ejaculates. The northern watersnake Nerodia sipedon has a mating system characterized by aggregations in which a single female mates with multiple males, resulting in high levels of multiple paternities. Under these circumstances sperm competition is likely important, and selection should favour sperm and ejaculate traits that enhance a male's reproductive success. In this study, we examined intraspecific variation in ejaculate quality (sperm length, motility, sperm density, spermactocrit) in male watersnakes and determined whether ejaculate traits varied with body size and condition, using both size-corrected mass and haematocrit as indices of condition. We found large variation among males in all these traits, except for sperm length. Although there was significant variation in sperm length among males, the majority of variation in sperm length occurred within rather than among individuals. Males with high haematocrit had sperm that were less variable with respect to length, and large males produced ejaculates that were less concentrated with respect to sperm than small males. The lack of condition dependence of most ejaculate traits is consistent with previous studies that indicate that male reproductive effort in this species is generally not energy limited, perhaps because of opportunistic foraging during the mating season.

Between‐male variation in sperm size, velocity and longevity in sand martins Riparia riparia

“…A second hypothesis is that sperm characteristics are condition‐dependent sexually selected traits (Rowe and Houle 1996). Condition‐dependence has indeed been found in sperm swimming speed and sperm length (Simmons and Kotiaho 2002, Malo et al 2005, Schulte‐Hostedde and Montgomerie 2006, García‐Gonzáles and Simmons 2007). However, we found no significant correlations between body condition and sperm characteristics, and even found that males with shorter tarsi produced sperm with higher overall velocity (Fig.…”

“…To test whether male phenotype may signal sperm characteristics in the sand martin, we examined correlations between both sperm length and velocity and body measures and condition. As intra‐ejaculate variation in sperm morphology may be indicative of more or less stable spermatogenesis (Schulte‐Hostedde and Montgomerie 2006) we also tested the correlation between intra‐ejaculate variance (SD) in sperm length and body measures and condition.…”

Multiple paternity in reptiles: patterns and processes

“…This assumption is untested in reptiles and its verification would require studies documenting that fertilization success in sperm competition is a heritable trait or that it is correlated with genetic quality more generally. No studies of the heritability of fertilization success exist in reptiles and there is also precious little evidence for intraspecific variation in sperm traits and their heritability (Schulte‐Hostedde & Montgomerie 2006; Uller et al . forthcoming).…”

Intraspecific Variation in Sperm Length Is Negatively Related to Sperm Competition in Passerine Birds

“…For example, strong stabilizing or directional selection would be expected to reduce additive genetic variance in traits under selection. A significant intraspecific variation in sperm morphometry has been documented in a wide range of taxa (e.g., Ward 1998; Morrow and Gage 2001; Schulte‐Hostedde and Millar 2004; Hettyey and Roberts 2006; Malo et al 2006; Minoretti and Baur 2006; Schulte‐Hostedde and Montgomerie 2006), but the causes of this variation are poorly understood (Ward 1998; Morrow and Gage 2001; Hettyey and Roberts 2006). This lack of knowledge is surprising given the considerable interest devoted to the evolutionary significance of sperm in sperm competition.…”