Annual variation in growth rate and body condition of adders, Vipera berus: effects of food availability and weather

Abstract

The objective of this study was to see if year-to-year variation in size-specific growth rate and body condition of adders, Vipera berus, was correlated with any of four environmental variables: mean summer temperature, number of sunny days during summer, number of days when the temperature was over 15 °C during summer, and vole density as an index of food abundance. Growth rate and body condition were measured during 5 years in two insular populations of adders (at localities A and I) in central Sweden. At locality A, vole density remained constant among years (range 0.86–3.00/ha), whereas at locality I it changed significantly (0.67–22.67/ha). The two localities were approximately 2 km apart and therefore experienced the same weather conditions. At locality A, growth rates (after sex and snout–vent length were controlled for) remained stable, whereas at locality I they varied significantly among years and showed a significant positive correlation with yearly estimates of vole density. I conclude that the growth rate was affected positively by food abundance, but less so by weather. Body condition of male adders (females were excluded because the sample size was small) varied significantly among years in both populations. At locality A, body condition was positively correlated with number of sunny days, whereas at locality I, where vole density changed over the years, body condition was positively correlated with both vole density and number of sunny days. This suggests that both food abundance and weather may be important in determining body condition.