IS THE ANNUAL CYCLE IN BODY-WEIGHT OF POUCHED MICE (SACCOSTOMUS-CAMPESTRIS) THE RESULT OF SEASONAL-CHANGES IN ADULT SIZE OR POPULATION-STRUCTURE

The annual cycle in body weight of pouched mice (Saccostomus campestri s) was examined among 104 specimens which were collected throughout th e year in the Transvaal province of South Africa. Each specimen was as signed to one of five age classes using toothwear characteristics alth ough none of them belonged to the youngest age class. There was no sig nificant effect of sex on body weight but older individuals were signi ficantly heavier than younger ones. Pooled data from both sexes displa yed seasonal variation in body weight with significantly heavier anima ls 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 s eason compared to the wet season, while older individuals maintained a relatively constant body weight throughout the year. These results su ggest 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 wi nter as an adaptation to limit their energy requirements when food ava ilability declines.