GROWTH-RATE IN DIFFERENT COLOR MORPHS OF THE ADDER, VIPERA-BERUS, IN RELATION TO YEARLY WEATHER VARIATION

Many ectothermic vertebrates such as snakes convert solar radiation into body heat by basking; and in these animals several behavioural and physiological traits including growth are temperature dependent. I performed a five-year capture-mark-recapture study on an insular population of adders. Vipera berus, in order to: examine yearly variation in individual growth rate in relation to weather conditions and prey abundance; test for a difference in growth rate between melanistic and zig-zag patterned adders with different thermoregulatory capacities: examine whether individual differences in growth rates were consistent among years. Size-specific growth rates varied significantly among years. Growth rates were lowest in the two years that had the largest proportion of cloudy days during the growing season, and the lowest air temperature in the beginning of the growing season. The abundance of field voles, Microtus agrestis, was measured as an estimate of prey availability in two years only. Adders grew faster in the year with the lower vole abundance, but with warmer temperature and less clouds. These results indicate that adverse weather conditions may limit individual growth rate in adders. presumably by preventing them from attaining body temperatures required for optimal foraging performance and growth. Although melanistic snakes have faster heating rates and higher body temperature than zig-zag snakes, growth rates did not differ significantly between colour morphs, but melanistic snakes grew slightly more slowly in four out of the five years. In male adders individual relative growth rates measured in different years were significantly correlated. In females the correlation was positive but not significant, nor did it differ significantly from the correlation in males. This result shows that despite environmentally induced yearly variation in growth rate some individuals always grow fast relative to other individuals in the population.