The Effect of Size Dependent Mortality on Colour Morphs in Male Adders, Vipera berus

From 1983 to 1986 we studied, by capture-mark-recapture, male adders in a population on Hallands Vadero, an island in the Kattegat sea. About 50% of the males were melanistic. The mean length of the melanistic males was higher than of the normally coloured males suggesting a faster growth rate of the former. Most large males captured in spring 1985 were emaciated, most likely a result of shortage of prey in 1984. Between 1985-86 the population size decreased by half, and between these years the recaptured males were significantly smaller than those not recaptured, which indicates that the larger males suffered a higher mortality in 1985. We therefore propose that temporal variation in prey abundance could be an important factor in determining the frequency of melanism in adder populations.