Age-dependence of body weight and linear dimensions in adult Mus and Peromyscus

Head plus body length (HBL), tail length (TL) and body weight (BW) were measured on two species of small myomorph rodents, Mus musculus and Peromyscus leucopus, throughout adult life. Both sexes of Mus show about a 15 per cent increase of HBL and TL between 5 and 24 months of age, and a BW increase of about 47 per cent. There is no age-trend of HBL in either sex of Peromyscus between 5 and 67 months of age, while TL increases about 7 per cent. In both species, cessation of skeletal growth is followed by a steady decrement of body mass, which averages about 0.23 per cent per month throughout adult life for the two sexes of Peromyscus, and over two per cent per month after cessation of skeletal growth in Mus. The weight loss in Mus in the last half of the lifespan is shown to be associated with age, rather than with the duration of subsequent survival. This evidence against the hypothesis that the late weight loss in Mus is due to terminal disease.