Reproductive Biology and Development of the Water Rat, Hydromys chrysogaster, in Captivity

Abstract

In a captive colony of H. chrysogaster most litters were born between September and March, although some were born in every month except June. Most females had regular oestrous cycles in the breeding season but there were isolated instances of oestrus in every month. In mature males, testes were scrota1 and there was full sperm production all year. There was some regression in weight of male accessory reproductive glands in the autumn and winter. Oestrus lasted 10 days (range 7-17), and its stages: pro-oestrus 1 day, oestrus 2 days, metoestrus 2 days, anoestrus 5 days. Some instances of delayed implantation were suspected. Gestation length was 34 days (33-41). Parturition was followed the next day by a postpartum oestrus, lasting 1 day. Lactation anoestrus was at least 3 weeks. Litter size was 3.29 ± 1.26 (1-7) and the number of litters per breeding season was 2.6 ± 0.97 (1-5). In the latter half of pregnancy there was a linear relationship between the crown-rump length of foetuses and gestation length. Young were born naked, blind and with the pinnae folded forward with edges attached to the head. The upper incisors emerged at 4 days, the lower incisors at 6 days, the auditory meatus opened at about 10 days, the eyes opened at about 14 days, some solids were eaten and young were more independent at about 3 weeks of age, and they were weaned at about 29 days. Phases in the rate of increase in weight were associated with each of these developmental stages. Females were first capable of breeding at 124 days (433 g); most matured about 240 days in the season following that of their birth. The testes descended in males between 90 and 120 days (475 g) but full sperm production did not occur until about 130-140 days. Females could breed for three seasons (until about 3.5 y old). Placental scars were visible for up to 6 months, but had sometimes disappeared at 4 months post partum. Growth curves of tail and weight were sigmoid, and those of other body measurements were exponential, as in the Rattus group of Australian rodents, and differing from the pseudomyine rodents. Body measurements showed a sigmoid relationship to the linear equivalence of body weight. Developmental events occurred earlier, in relation to growth, in Hydromys and Rattus lutreolus than in Pseudomys novaehollandiae.