Reproductive biology of the feral house mouse (Mus musculus) on subantarctic Macquarie Island

Reproduction of the feral house mouse (Mus musculus) was studied on subantarctic Macquarie Island and found to be seasonal. Females begin oestrus-cycling in early spring, following a minimum 3-month winter anoestrous period. By late spring all mature females are in breeding condition. Breeding is continuous through spring, summer and into autumn. Postimplantation loss occurs throughout the breeding season. Late autumn pregnancies may fail. Average litter size is 6-7 but litters as large as 10 have been found. Young born in the latter half of the breeding season attain sexual maturity at a later stage than those born in the early-spring-summer period and do not come into breeding condition until the following spring. Males show a slight cyclical change in testes weight, increasing from a winter minimum to a summer maximum, but are potentially capable of breeding throughout the year. Reproductive seasonality of the female determines breeding behaviour in this isolated subantarctic population of feral house mice. Seasonal reproduction is not well correlated with mean monthly ambient temperature, which varies by only 3O degrees C over the year. Food availability appears constant throughout the year with little interspecific competition for food or predation on the mice. Reproduction is suppressed over the short-day winter months. The possible interaction of photoperiod with other environmental and physiological variables in determining reproductive seasonality requires further research.