Geographic variation in size and sexual dimorphism of North American weasels

Geographic variation in size (skull length) and sexual dimorphism in Mustela erminea, Mustela frenata and Mustela nivalis in North America is described and analysed in relation to latitude, longitude, climatic variables, and sympatry or allopatry of these species. Only erminea increases in size with latitude; it does so regardless of the presence or absence of frenata or nivalis. Latitude is a better predictor of size in erminea than available measures of climate, seasonality or prey size. There is no evidence for character displacement between any pair of species. The sexes covary in size in frenata and erminea , and probably in nivalis , although geographic variation in sexual dimorphism occurs in frenata and erminea. The principal cause of sexual dimorphism appears to be sexual selection for large size in males rather than the high energetic requirements resulting from an elongate body shape. However, prey size may constrain female size (and possibly also male size). Regional differences in the abundance of prey during the growth of young weasels may affect adult size much more in males than in females and contribute to geographic variation in sexual dimorphism.     

Variation in predation risk and vole feeding behaviour: a field test of the risk allocation hypothesis

Many prey animals experience temporal variation in the risk of predation and therefore face the problem of allocating their time between antipredator efforts and other activities like feeding and breeding. We investigated time allocation of prey animals that balanced predation risk and feeding opportunities. The predation risk allocation hypothesis predicts that animals should forage more in low- than in high-risk situations and that this difference should increase with an increasing attack ratio (i.e. difference between low- and high-risk situations) and proportion of time spent at high risk. To test these predictions we conducted a field test using bank voles (Clethrionomys glareolus) as a prey and the least weasel (Mustela nivalis nivalis) as a predator. The temporal pattern and intensity of predation risk were manipulated in large outdoor enclosures and the foraging effort and patch use of voles were measured by recording giving-up densities. We did not observe any variation in feeding effort due to changes in the level of risk or the proportion of time spent under high-risk conditions. The only significant effect was found when the attack ratio was altered: the foraging effort of voles was higher in the treatment with a low attack ratio than in the treatment with a high attack ratio. Thus the results did not support the predation risk allocation hypothesis and we question the applicability of the hypothesis to our study system. We argue that the deviation between the observed pattern of feeding behaviour of bank voles and that predicted by the predation risk allocation hypothesis was mostly due to the inability of voles to accurately assess the changes in the level of risk. However, we also emphasise the difficulties of testing hypotheses under outdoor conditions and with mammals capable of flexible behavioural patterns.