Postnatal growth and development in semi-captive muskrats, Ondatra zibethicus.

We studied postnatal growth and preweaning development of muskrats under semicaptive conditions in a temperate environment. The principal objectives were to quantify weaning age and investigate potential effects of sex and parturition date on growth rate. We also tested a previous hypothesis that juvenile muskrats inhabiting temperate environments exhibit a winter growth diapause. Developmental attributes varied significantly among 6 litters (n = 31 individuals), except for age at eye opening (median = 15 days). Analysis of arithmetic growth (g/day) indicated that muskrats were weaned between 26 and 28 days of age. The Gompertz equation was used to estimate asymptotic mass, growth rate constant, and age at inflection for 25 individuals from 5 litters. There were no sex differences in growth parameters after controlling for the effect of parturition date. The growth rate constant was positively correlated with parturition date. Consequently, juveniles attained similar asymptotic mass in autumn irrespective of birth date. Although most of the compensatory growth was attributed to the postweaning period, a significant portion occurred during lactation which suggested that females were investing more in offspring born later in the breeding season. Juvenile muskrats exhibited a winter growth diapause from October through April and did not attain adult body mass until 14-18 months after birth. We briefly discuss these life history traits in relation to habitat selection in muskrats.