La. Ebensperger et Ja. Simonetti, MICROCLIMATE AND MICROHABITAT SELECTION IN NOCTURNAL RODENTS OF CENTRAL CHILE - A TEST BETWEEN BIOPHYSICAL AND EXPERIMENTAL APPROACHES, Mammalia, 60(2), 1996, pp. 195-209
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.