Morphological and physiological responses to altitude in deer mice Peromyscus maniculatus

Individuals within a species, living across a wide range of habitats, often display a great deal of phenotypic plasticity for organ mass and function. We investigated the extent to which changes in organ mass are variable, co rresponding to environmental demand, across an altitudinal gradient. Are th ere changes in the mass of oxygen delivery organs (heart and lungs) and oth er central processing organs (gut, liver, kidney) associated with an increa sed sustainable metabolic rate that results from decreased ambient temperat ures and decreased oxygen availability along an altitudinal gradient? We me asured food intake, resting metabolic rate (RMR), and organ mass in captive deer mice (Peromyscus maniculatus bairdii) at three sites from 1,200 to 3, 800 m above sea level to determine whether energy demand was correlated wit h organ mass. We found that food intake, gut mass, and cardiopulmonary orga n mass increased in mice living at high altitudes. RMR was not correlated w ith organ mass differences along the altitudinal gradient. While the condit ions in this study were by no means extreme, these results show that mice l iving at high altitudes have higher levels of energy demand and possess lar ger cardiopulmonary and digestive organs than mice living at lower altitude s.