THE FASTING ENDURANCE HYPOTHESIS - THE CASE OF 2 RODENT SPECIES FROM CENTRAL CHILE

Recently, Millar and Hickling (1990) have restated a model in which th e interplay between food requirements, fasting endurance, and food ava ilability would explain the evolution of both large and small body siz e in mammals in either unpredictable or unproductive environments. One main assumption of the model is that within species, large individual s have greater fasting endurance capacities than small ones. Evidence supporting such an assumption is only indirect. Field experiments, usi ng two sigmodontine rodents (Abrothrix longipilis and Phyllotis darwin i) from central Chile, whose food resources fluctuate temporally, prov ide a preliminary test for evaluating the realism of this assumption a nd the applicability of the model itself. Different sized adult animal s were left individually in wired-mesh cages during 12-h periods (20:0 0-08:00 h), without food, in the microhabitats currently used by them. The fraction of body weight loss during these periods was inversely c orrelated to body weight in P. darwini, suggesting that fasting endura nce is greater in larger animals. Large and small sized individuals of the syntopic species, A. Longipilis, exhibited similar losses of body weight, suggesting no relationship between fasting endurance and body size. Although part of present results supports Millar and Hickling's model, other suggest that the role: of fasting endurance during the e volution of mammalian body size might be limited even in environments having fluctuating food resources for the focal species. Some drawback s of the model as well as ways to test it further are discussed.