Population for small mammals are often characterized by wide temporal variations in the number of individuals. Variations in physiological responses to stress during reproductive periods often limit population distributions within species. For instance, lactation, the period during which the dam supports offspring through the production of milk, places particularly high water and energy metabolism requirements on small mammals that have large litters. The ability of the dam to make physiological adjustments to these requirements determines the success or failure in rearing the litter. Ultimately then, the size of the population is partly dependent on the response of the dam to the metabolically demanding period of lactation. The purpose of this project is to study the components of water metabolism during lactation and during non-reproductive periods to determine which and to what extent components are adjusted to the combination of water and lactation stress. The species to be studied are redback voles and white-footed mice. The voles are poorly adapted to dry environments and are often found in desert- like regions. The extent to which these two species adjust components of their water metabolism will thus help elucidate the role of the environment in shaping physiological responses of animals during lactation. Measuring the components of water metabolism represent the first complete quantitative analysis of water metabolism during lactation in a wild species. They are thus critical to eventual understanding of the population dynamics of small mammals.
