Fat Cycles, Breeding and Population-Changes in House Mice

Abstract

Seasonal and annual changes of fat levels in wild house mice, Mus domesticus, were estimated from body water content, and examined in light of changes in reproduction and population levels. Breeding was inversely correlated with fatness. Mice bred continuously and fat levels remained low during the first winter which preceded a minor population outbreak. Breeding ceased completely in the second and third winters in which fat reserves were high and major population declines occurred. Fat was deposited primarily in late autumn and early winter when food supply and foraging conditions were deteriorating. Mice living in cereal crops also deposited some fat during the breeding season in summer. Most fat loss occurred after the winter population declines, but before breeding. Breeding and fat levels appear to be regulated independently in response to food intake, the inverse correlation between the two occurring because mice breed when the food supply is good and foraging conditions reliable, at which time fat reserves are a burden which confer little advantage. Maximum fat reserves were sufficient only to meet 2 days energy demand without other food intake. Fat reserves appear to serve primarily to meet short-term energy deficits when foraging conditions are unreliable.