Abstract

We contrasted two approaches to test the hypothesis that microclimate influences selection of microhabitat by two species (Abrothryx longipilis and Phyllotis darwini) of nocturnal rodents of central Chile. Microhabitats analyzed included patches of continuous vegetation (mesic microhabitat), the most used by rodents, and patches with low vegetational cover, including the space beneath the shrubs (under-bush microhabitat), and the space among shrubs (open-space microhabitat). In the first approach, we compared the cost of thermoregulation expected in each microhabitat by computing the minimum energy expenditure and the fraction of the metabolic scope an animal would have to allocate just to maintain homeothermy. We also contrasted microhabitats in terms of the rate of cooling by aluminium cylinders. In the second approach, we compared field measurements of food intake and loss of body mass of animals maintained in wire-mesh cages during night with or without food. Figures from the animals' expected energetic cost suggested that energy savings per hour by preferring mesic rather than open-space microhabitats would be close to 20%, whereas energy savings by using mesic rather than under-bush sites could reach 18%. Although predicted energy savings tended to be low when the metabolic scope of species was considered, they remained over 5% in most of microhabitat contrasts. Rates of cooling of bare aluminium cylinders measured in the field showed that wind convection was higher in open-space sites compared to under-bush and mesic microhabitats. Although our simple biophysical approach predicted that microclimate should influence microhabitat use by Chilean cricetids, animals from both species exhibited similar rates of food intake and loss of body mass. Since both variables might affect individual chances of survival, we conclude thai differences in microclimate are not influencing the use of microhabitat by cricetid rodents in central Chile. Then, what is predicted by a biophysical approach, in terms of the importance of microclimate on microhabitat selection, is not be supported by direct measurements of fitness-related variables.