Inbreeding depression in an isolated population of adders Vipera berus

Although inbreeding depression is well-studied in captive animals, its role in natural populations remains controversial. We provide information on an isolated population of snakes (adders Vipera berus) that has been separated from neighbouring populations by the expansion of agricultural activities in southern Sweden. Total adult population size is < 40 adult individuals, and the mating system is such that a few males have disproportionate reproductive success and hence father most of the progeny each year. The isolation and small effective population size (< 15 adults) promote inbreeding. Compared to other non-isolated Swedish populations of adders, the isolated population shows (i) a smaller litter size relative to maternal body size; (ii) a higher proportion of deformed and stillborn offspring; (iii) a lower degree of genetic heterozygosity due to fixation or near-fixation of alleles; and (iv) a higher genetic similarity among individuals (as measured by DNA fingerprinting). The incidence of inviable offspring was sharply reduced when we introduced males from other areas into the isolated population. These results suggest that the lower reproductive output and viability of adders in the isolated population result from inbreeding depression. We also present data to falsify two alternative hypotheses: the characteristics of the isolated population are not due to environmental contaminants (metal and pesticide residue levels are low) or to poor food supply (adult adders are in good physical condition and their neonates are of the same size as in other populations).