Abstract

We report on body mass dynamics, field growth rates and age-specific growth curves for the leaf-eared mouse Phyllotis darwini (Waterhouse, 1837). Mark-recapture methods provided data for a population of P. darwini in a semi-arid region of Chile from 1987 to 1996. There were significant effects of sex, season, and slope exposure on body mass. In addition, we found significant effects of sex, mass class, and season on field growth rates. Individuals had highest growth rates during spring and summer and lowest during fall, males grew faster than females, and juveniles grew faster than adults. Growth rates of males were positively correlated with precipitation levels. This result suggests that higher somatic growth rates during periods of unusual high precipitation may account for population outbreaks observed in this species in semi-arid regions of Chile. Age-specific growth curves of body mass for both sexes showed a strong sexual dimorphism. This sexual dimorphism in body mass may he related to a polygynous mating system and strong male-male sexual competition during breeding periods.