Litter Size of Apodemus argenteus in Relation to the Population Cycle

Abstract

Litter size of the small Japanese field mouse, Apodemus argenteus, was examined in relation to its population fluctuation in a natural mixed forest of Hokkaido. Overwintered females had a slightly higher litter size than did current year's females, although the difference was not significant. There was a significant positive relationship between litter size and body size ; the correlation coefficient obtained for current year's females was higher than that for overwintered females. There was a significant negative relationship between litter size and population density ; a higher correlation was detected for overwintered mice than for current year's mice. Pregnant females were captured only in the increase phase, except for a few in the decline phase in May of 1979. Litter size significantly varied from year to year. Higher litter sizes were closely associated with the autumn breeding, a prolongation of the breeding season related to the population cycle.