Is the annual cycle in body weight of pouched mice (Saccostomus campestris) the result of seasonal changes in audit size or population structure?

Abstract

The annual cycle in body weight of pouched mice (Saccostomus campestris) was examined among 104 specimens which were collected throughout the year in the Transvaal province of South Africa. Each specimen was assigned to one of five age classes using toothwear characteristics although none of them belonged to the youngest age class. There was no significant effect of sex on body weight but older individuals were significantly heavier than younger ones. Pooled data from both sexes displayed seasonal variation in body weight with significantly heavier animals in the wet season (December‐March) than during the dry season (June‐September). However, there was no significant difference between the age structure of the population at these times. Instead, pouched mice in the two younger age classes were significantly lighter in the dry season compared to the wet season, while older individuals maintained a relatively constant body weight throughout the year. These results suggest that the annual cycle in body weight of S. campestris is caused by a reduction in body weight of young animals which lose weight in winter as an adaptation to limit their energy requirements when food availability declines.