Costs of reproduction in a population of European adders.

Eleven years of data on a small population of adders (Vipera berus) in southern Sweden provide quantitative information on the nature and degree of costs faced by reproducing animals. Reproduction imposes both an energy cost (measured by loss in body mass) and a mortality cost on adders of both sexes. The extent of the energy cost is broadly independent of levels of reproductive activity in males, but mortality costs are highest for large males, perhaps because they are more obvious to predators. In females, energy costs include a high 'fixed' (fecundity-independent) component, such that a large litter may cost little more to produce than would a small litter. Energy costs and mortality costs are separate in males, but inter-related in females. Mortality of reproducing females is high (40% per year), primarily because post-parturient females are emaciated and must forage actively, hence increasing their vulnerability to predators. Females producing relatively large litters (high Relative Clutch Mass) lose more body mass, and are less likely to survive after reproducing. The observed low reproductive frequencies of female adders may result from the presence of high fecundity-independent costs of reproduction.