Effects of Variations in the Availability of Food and Home Sites and of Culling on Populations of House Mice Housed in Out-Door Pens

Abstract

In field populations of house mice (Mus musculus) seasonal and year-to-year variation in numbers are usually attributed to variations in the availability of food and home sites, and the effectiveness of agents affecting life-spans. Variations in numbers similar to those observed in the field also occurred in a mouse population housed in an out-door pen in which the number of home sites was limited but food supplies were abundant but these variations were due to seasonal variations in the effectiveness of self-regulating mechanisms. To examine the effects of food supply, amount of shelter and of one agent affecting life-spans (culling) on mouse numbers in confined populations, numbers were monitored in a control pen and in three experimental pens in which food supply was halved, home sites were doubled and mice were culled at four weekly intervals. The effects of the treatments were followed for four and a half years. Although supplies of food and shelter remained constant throughout the study, mouse numbers in the control pen were high in summer and low in winter and the heights of the summer peaks varied from year to year. Halving the amount of food available reduced the mean size of the population by 37% by reducing numbers in summer. Doubling the number of home sites increased mean size by 13% by increasing numbers in winter. Culling about 60% of the mice at monthly intervals reduced mean size by 13%. All experimental treatments reduced the amplitude of the seasonal fluctuations and between-year differences in the heights of the summer peaks. In the control pen more young were recruited in summer than in winter. Halving the amount of food available reduced the numbers recruited in summer by increasing the number of pups dying between birth and weaning. Doubling the number of home sites and culling increased the numbers recruited in spring, summer and autumn by increasing the number of females breeding. These increases were associated with decreases in the mean sizes of the populations living in the home sites. In the control pen the mean life-spans of the males and females which survived until they were weaned were 4.9 and 6.4 months respectively. Halving the amount of food available had no effect on mean life-spans but doubling the number of home sites reduced them significantly. It was concluded that self-regulating mechanisms continued to affect numbers in all three experimental pens but that numbers were also affected by the availability of food in summer in both the pen in which food supplies were reduced and the pen in which the number of shelters was doubled. It was also concluded that although mouse numbers were reduced by culling, the indirect effects of this treatment on recruitment tended to compensate for its direct effect on numbers.