Food, Sex, Time, and Effort in a Small Mammal: Energy Allocation Strategies for Survival and Reproduction

The relationship between food energy and work effort (foraging) is crucial to small mammals such as the house mouse (Mus domesticus). Energy allocation processes were studied by using a special caging system in which animals were required to work for 45 mg food pellets by running on an activity wheel. Two experimental themes were investigated: 1) When weaning female mice were forced to work harder for less food, their highest energy allocation priority was to maintain fat reserves; body growth was next in importance while achieving puberty had the lowest prority of all. The relative insensitivity of fat deposition to high foraging costs suggests a strategy for survival whereby dispersing animals maintain emergency fat reserves at the expense of growth and fertility. In male mice, however, reproductive development is independent from body growth. Both sexes employed fundamentally different energy allocation strategies during peripubertal development. Some of these sex differences were gonad-dependent, while others were not. The time spent foraging while exposed to cold ambient temperature is also critical during peripubertal development, and this relationship probably determines whether or not house mice will breed continuously or seasonally in feral habitats. 2) Deer mice (Peromyscus maniculatus) and house mice were challenged to produce litters at increasingly greater work requirements. Deer mice supported heavier litters and produced more pups at weaning than house mice, mainly because deer mice were more efficient at food use. With regard to circadian organisation, deer mice accomplished almost all of their locomotor tasks during darkness while house mice exhibited day-to-day flexibility in locomotor activity, especially during lactation. Deer mice attempted to wean five or six pups, regardless of how severe the feeding conditions; however, pups became progressively stunted as females worked harder and obtained less food. In contrast, house mice had more pups at birth than deer mice, but females killed and cannibalized offspring throughout the first 12 days of lactation. Most surviving house mouse pups thus attained similar body weights at weaning. These divergent energy allocation patterns may reflect strategies arising from opportunism (house mice) versus seasonality (deer mice).