Diet Selection by a Heteromyid Rodent: Role of Net Metabolic Water Production

Kangaroo rats (family Heteromyidae) are primarily granivores. These desert rodents do not drink free water, but depend on performed water in their diet and water produced by oxidation of food (metabolic water). Oxidation of different nutrients produces different net amounts of metabolic water. At low humidities, oxidation of carbohydrates produces a net metabolic water gain, lipid metabolism results in a net water loss, and protein metabolism produces a severe water loss, mainly through urination. Under humid conditions, carbohydrate metabolism again produces a net water gain, lipid oxidation produces a large net metabolic water gain, and although this is slightly reduced, protein metabolism still results in a large water loss. Diet composition therefore influences heteromyid water balance. Preference experiments involving semisynthetic diets that were energetically equivalent tested the influence of net metabolic water production on diet preferences of Dipodomys spectabilis. At low humidities, kangaroo rats in a positive water balance preferred: (1) intermediate—protein, (2) high—lipid, and (3) high—carbohydrate diets. When water stressed, the rodents preferred low—protein diets, and the preference for high—lipid diets decreased. At high humidities, the rodents always preferred high—lipid diets. These results indicate that when energy contents were equivalent, kangaroo rats usually preferred diets that produce the greatest net metabolic water gain.