Reproduction of the feral house mouse (Mus musculus) was studied on su
bantarctic Macquarie Island and found to be seasonal. Females begin oe
strus-cycling in early spring, following a minimum 3-month winter anoe
strous period. By late spring all mature females are in breeding condi
tion. Breeding is continuous through spring, summer and into autumn. P
ostimplantation loss occurs throughout the breeding season. Late autum
n pregnancies may fail. Average litter size is 6-7 but litters as larg
e 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 un
til the following spring. Males show a slight cyclical change in teste
s weight, increasing from a winter minimum to a summer maximum, but ar
e potentially capable of breeding throughout the year. Reproductive se
asonality 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 vari
es by only 3-degrees-C over the year. Food availability appears consta
nt throughout the year with little interspecific competition for food
or predation on the mice. Reproduction is suppressed over the short-da
y winter months. The possible interaction of photoperiod with other en
vironmental and physiological variables in determining reproductive se
asonality requires further research.