Effects of altitude and temperature on organ phenotypic plasticity along an altitudinal gradient

Small mammals living in high-altitude environments must endure decreased am bient temperatures and hypoxic conditions relative to sea-level environment s. Previously, it was noted that heart, lung and digestive tract masses and blood hematocrit increase along an altitudinal gradient in small mammals. Increases in digestive organ mass were attributed to lower ambient temperat ures and greater food intake, and increases in lung mass and hematocrit wer e attributed to hypoxia, but these assumptions were not explicitly tested. In addition, it was not clear whether changes in heart and lung mass were a function of an increase in organ blood content or of an increase in organ tissue mass. We used captive deer mice (Peromyscus maniculatus sonoriensis) to determine the relative effects of ambient temperature and oxygen concen tration (P-O2) on organ mass and blood hematocrit along an altitudinal grad ient. We also exsanguinated hearts and lungs to determine whether changes i n mass were associated with the blood content or with increases in tissue m ass, We found that small intestine mass was, as expected, correlated positi vely with energy intake and negatively with ambient temperature. Heart mass was also negatively correlated with temperature, Lung mass and hematocrit were, as expected, positively correlated with altitude (and P-O2). Interest ingly, the masses of both small intestine and kidney were negatively correl ated with altitude, For kidney mass, this correlation was apparent in cold- exposed mice but not in warm-exposed mice. We also found that changes in bo th heart and lung mass were mainly a function of changes in tissue mass rat her than blood content. These data show that different abiotic variables ha ve different effects on organ masses at high altitude, but also that phenot ypic plasticity in response to cold temperatures and low oxygen pressures a t altitude is widespread across several different organ systems, suggesting a general elevated whole-body response.