The biology of the Water Rat Hydromys chrysogaster Geoffroy (Muridae: Hudromyinae) in Victoria.

Abstract

In the wild population of water rats studied, males were more numerous and larger than females. In a population of 589 water rats, 323 were males and 266 females. Whole weights ranged from 170 to 1275 g for males and 156 to 992 g for females. Males were age-grouped into adults (including subadults) and juveniles on the basis of combined testes weights. Sexual maturity is reached when a whole weight of between 400 and 600 g (14-21 oz) is attained. Females were age-grouped on the stage of maturity of the reproductive tract. Female rats can reproduce when a whole weight of 425 g (15 oz) is attained. Mating takes place in late winter and continues through spring. The breeding season extends from September to January, with the peak in early spring. Females are in anoestrus in late summer, autumn, and early winter. The number in each litter varies from one to seven: the usual number is four or five. Growth is rapid; young can reach adult size in less than one year. The structure of the population varies throughout the year. The proportion of adults (including subadults) is high in winter and spring and falls in summer and autumn when juveniles appear in numbers. The fecundity of water rat's is lower than normal for murid species, but fertility is high and a substantial population turnover occurs each year when conditions for breeding are favourable. It is considered therefore that the water rat has potentiality for management as an economic fur-bearing animal.