POPULATION CHARACTERISTICS OF HOUSE MICE LIVING IN ENGLISH CORN RICKS: DENSITY RELATIONSHIPS

SUMMARY This paper has presented certain demographic data of forty populations of house mice from English grain ricks. The populations are grouped into four density classes on the basis of the number of weaned mice per cubic metre. The resulting demographic averages of various density classes are then compared. Relatively little reproductive alteration occurred in rick populations in relation to density. The major “density effects ” so often observed in laboratory studies did not appear or were only moderately expressed in these data. Mortality rates of the populations revealed no dramatic or conspicuous change throughout the density classes studied (as evidenced by weight ratios and sex ratios). Fecundity of both sexes remained high throughout all density classes. Fertility revealed a significant decline in the highest density class, but it still remained at high levels. Pregnancy rates declined from 58 per cent in the low density class to 43 per cent in the very high density class. Average litter size fell from 6·23 to 5·11. Embryonic and fetal resorption increased significantly in the highest density class (14 per cent and 27 per cent). The general rate of reproductive increase declined with increasing density. The incidence of wounded males increased significantly in the highest density class (0·8 per cent to 21·1 per cent). Adrenal weights did not reveal adrenal hypertrophy in crowded populations. The relative stability of rick populations with increasing density is probably a reflection of the high quality of the habitat. Density effects are undoubtedly minimized by the favourable abundance and distribution of food and cover. It is of equal interest that individual populations in this relatively uniform habitat did show wide variations in their population size and structure. The fact that this variation is not obviously related to density, food, or cover, focuses attention on factors outside the normal realm of physical habitat as major population influences.