INBREEDING DEPRESSION IN AN ISOLATED POPULATION OF ADDERS VIPERA-BERUS

Although inbreeding depression is well-studied in captive animals, ifs role in natural populations remains controversial. We provide informa tion on an isolated population of snakes (adders Vipera berus) that ha s been separated from neighbouring populations by the expansion of agr icultural activities in southern Sweden. Total adult population size i s < 40 adult individuals, and the mating system is such that a few mal es have disproportionate reproductive success and hence father most of the progeny each year. The isolation and small effective population s ize (< 15 adults) promote inbreeding. Compared to other nan-isolated S wedish populations of adders, the isolated population shows (i) a smal ler litter size relative to maternal body size, (ii) a higher proporti on of deformed and stillborn offspring; (iii) a lower degree of geneti c heterozygosity due to fixation or near-fixation of alleles; and (iv) a higher genetic similarity among individuals (as measured by DNA fin gerprinting). The incidence of inviable offspring was sharply reduced when we introduced males from other al eas into the isolated populatio n. These results suggest that the lower reproductive output and viabil ity of adders in the isolated population result from inbreeding depres sion. We also present data to falsify two alternative hypotheses: the characteristics of the isolated population are not due to environmenta l contaminants (metal and pesticide residue levels are low) or to poor food supply (adult adders are in good physical condition and their ne onates are of the same size as in other populations).