OPPOSING FITNESS CONSEQUENCES OF COLONS PATTERN IN MALE AND FEMALE SNAKES

Local populations of the adder, Vipera berus, are polymorphic for dors al colour pattern, containing both melanistic (black) and zig-zag patt erned individuals. Colour patterns in snakes influence crypsis and the rmoregulatory capacity and therefore may be subjected to natural selec tion. To find an explanation for the maintenance of this polymorphism I examined temporal and spatial variation in morph frequency, and test ed for differential selection among morphs using data from a six year capture-mark-recapture study. The data derive from six groups of islan ds in the Baltic Sea off the Swedish east coast, two mainland localiti es near the coast, and one inland locality. Morph frequency did not ch ange over time within a population but varied among populations: melan istic individuals were not found at the inland locality, but comprised from 17 to 62% of the coastal and island populations. Adders frequent ly moved between islands within a group, but the tendency to disperse was independent of morph. These results suggest that the polymorphism is stable and maintained by a deterministic process. Scar frequency wa s twice as high among melanistic as among zig-zag snakes, and melanist ic individuals were easier to capture, indicating that predation may b e higher on the melanistic morph. Colour morphs did not differ in body size, but analysis of recapture data shows evidence for differential survival among morphs. Zig-zag males survived better than melanistic m ales, but the relative survival rates of morphs were reversed in femal es. This difference was consistent through time and may be due to sexu al differences in behaviour, with melanism increasing predation intens ity when associated with male but not with female behaviour. Opposing fitness consequences of colour pattern in the two sexes may help maint ain colour polymorphism within populations of Vipera berus.