Is sperm really so cheap? Costs of reproduction in male adders,<i>Vipera berus</i>

TLDR

Reproduction is energetically expensive for males as well as for females, yet evolutionary biologists have traditionally considered sperm production costs trivial compared to overt reproductive behaviours such as mate‑searching, courtship, copulation and male–male combat. The study uses adders (Vipera berus) as a model to quantify the relative costs of spermatogenesis versus overt reproductive behaviours, because these activities occur sequentially, are separated by skin sloughing, and males do not feed during either phase. The authors measured male mass loss rates during the immobile spermatogenesis phase and the active reproductive phase, using these rates as an index of energy expenditure. Surprisingly, mass loss rates during the immobile spermatogenesis phase are at least as high as during the active reproductive phase, are not significantly correlated with male size or reproductive activity, and suggest that sperm production is a major energy cost for reproducing male adders, making this species a useful system for further study.

Abstract

Reproduction is energetically expensive for males as well as for females, but evolutionary biologists have typically regarded the energy costs of sperm production as trivial compared to the energy costs of overt reproductive behaviours, such as mate-searching, courtship, copulation and male–male combat. Adders (Vipera berus) offer an ideal model system in which to quantify the relative costs of spermatogenesis (and associated physiological preparations for reproductive activity) versus the costs of overt reproductive behaviours, because (i) these two activities occur sequentially rather than simultaneously, and are separated by a clear indicator (sloughing of the skin); and (ii) males do not feed during either of these phases. Hence, the rate of mass loss by males can be used as an index of energy costs. Surprisingly, the rate of mass loss is at least as high during the phase when males are immobile, building up sperm supplies, as when they move about and engage in reproductive behaviour. Rates of mass loss are not significantly correlated with male size or measures of the male's participation in reproductive activities. Our data suggest that sperm production may be a major energy cost to reproducing male adders, and that this species offers a useful system in which to further investigate this possibility.