Body Size and Survival: Implications for an Overwintering Spider

Abstract

The relationship between body size and survival was investigated in overwintering specimens of the winter-active spider Pityohyphantes phrygianus in a natural population in SW Sweden. Size was measured by (1) the carapace width (independent of temporary changes in mass), and (2) the abdominal height (strongly correlated with mass and indicating nutritional condition). Measurements were taken before and after three winters. The mean carapace width increased in overwintering females in one of the three years, suggesting directional selection since moulting does not occur during winter. In the other two years the means were retained but the variance decreased during winter, suggesting stabilizing selection. The females increased their mean abdominal height throughout all three winters investigated; mean weight increases ranged from 5% to 98% of the average fresh mass in autumn. Regression analysis showed that the mass was correlated with the carapace width in autumn and spring. Females with large carapace width increased their weight more than did small females during one of the winters, but the survival of large females did not improve. During the only winter males were possible to analyse, their carapace width increased (directional selection), as did the abdominal height. In a field experiment during one of the winters no significant difference in winter survival was found comparing heavy (autumn weight >9.5 mg) and light (<9.5 mg) females. It was concluded that large size in itself did not increase the survival rate of overwintering spiders. However, the results suggested that both sexes may grow during winter and this may be important for their reproductive success in the following spring.