Natural selection on physiological traits associated with water and energy conservation in a desert-dwelling rodent, Merriam's kangaroo rat (Dipodomys merriami)

Animals that live in desert environments need to conserve both energy and water if they are to survive. This is particularly true of mammals, such as Merriam's kangaroo rats (Dipodomys merriami), because both energy and water use are increased in endotherms. It is unclear whether, for kangaroo rats, the conservation of one of these limited resources (energy or water) is more important than the other, and one objective of this study was to determine whether kangaroo rats would attempt to conserve water or energy when one or both resources was limiting. In an experiment designed to allow kangaroo rats to choose environmental temperatures that would facilitate either water or energy conservation, but not both, kangaroo rats consistently chose energy-conserving temperatures. This suggests that energy conservation may be more important to this species.; Kangaroo rats possess a suite of physiological traits that allows them to conserve both water and energy, and these traits are presumed to be adaptations. If so, then these traits, or the hormones that regulate them, should be subject to natural selection. The hormone thyroxine regulates physiological processes important for both energy and water use in kangaroo rats. I implanted kangaroo rats with time-release pellets that contained 5.0 mg thyroxine and pellets that did not contain thyroxine (placebo). These thyroxine-implanted animals had significantly elevated thyroxine levels in their blood, elevated basal metabolic rates, but no detectable increase in water loss. In a field study, thyroxine-implanted kangaroo rats did not have lower survival rates than placebo-implanted animals, suggesting that natural selection was not acting on thyroxine levels during this study. The field study was conducted during a very wet and mild summer following an El Niño event, and it is possible that selection only acts on thyroxine levels during periods of water or energy stress, such as during a drought. I conclude that the need to conserve energy in desert environments may have led to the physiological traits that kangaroo rats currently possess. Thyroxine is important in regulating one source of energy use, basal metabolic rate, and thyroxine may be a target of selection when animals are subject to periods of energy stress.