MICROCLIMATE AND MICROHABITAT SELECTION IN NOCTURNAL RODENTS OF CENTRAL CHILE - A TEST BETWEEN BIOPHYSICAL AND EXPERIMENTAL APPROACHES

We contrasted two approaches to test the hypothesis that microclimate influences selection of microhabitat by two species (Abrothryx longipi lis and Phyllotis darwini) of nocturnal rodents of central Chile. Micr ohabitats analyzed included patches of continuous vegetation (mesic mi crohabitat), the most used by rodents, and patches with low vegetation al cover, including the space beneath the shrubs (under-bush microhabi tat), and the space among shrubs (open-space microhabitat). In the fir st approach, we compared the cost of thermoregulation expected in each microhabitat by computing the minimum energy expenditure and the frac tion of the metabolic scope an animal would have to allocate just to m aintain homeothermy. We also contrasted microhabitats in terms of the rate of cooling by aluminium cylinders. In the second approach, we com pared field measurements of food intake and loss of body mass of anima ls maintained in wire-mesh cages during night with or without food. Fi gures from the animals' expected energetic cost suggested that energy savings per hour by preferring mesic rather than open-space microhabit ats would be close to 20%, whereas energy savings by using mesic rathe r than under-bush sites could reach 18%. Although predicted energy sav ings tended to be low when the metabolic scope of species was consider ed, 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 pred icted that microclimate should influence microhabitat use by Chilean c ricetids, animals from both species exhibited similar rates of food in take and loss of body mass. Since both variables might affect individu al chances of survival, we conclude that differences in microclimate a re not influencing the use of microhabitat by cricetid rodents in cent ral Chile. Then, what is predicted by a biophysical approach, in terms of the importance of microclimate on microhabitat selection, is not b e supported by direct measurements of fitness-related variables.