Responses to lactation and cold exposure by deer mice (Peromyscus maniculatus)

Recently, much interest has been expressed in understanding how animals use phenotypic plasticity of tissue size and function to meet increased metabo lic demands. We set out to learn (i) whether female deer mice (Peromyscus m aniculatus) given lactation (two to seven pups per litter), cold (5 degrees C), or cold plus lactation as energy demands display phenotypic plasticity in organ size and function; (ii) whether that plasticity is similar to labo ratory mice given the same demands; and (iii) whether lactational performan ce in deer mice is derived from limits on central or peripheral organs. We found that deer mice responded to lactation by increasing digestible food i ntake and increasing the masses of the stomach, small intestine, cecum and liver, and the length of the small intestine. Heart mass was lower in lacta ting than in nonlactating mice. Cold exposure also caused increases in dige stible food intake and increases in the masses of the small intestine, kidn ey, and heart. We conclude that deer mice display organ tissue plasticity i n response to both lactation and cold exposure in a similar manner to labor atory mice. We also conclude that deer mice are not limited by central proc essing organs because they are able to increase digestive organ size contin uously with increased energetic demands.