The Influence of Food Habits on the Energetics of Eutherian Mammals

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Abstract
The level of energy expenditure in eutherian mammals is correlated with the type of food consumed, especially in medium—sized to large species; some foods (including vertebrates, herbs, and nuts) permit high basal rates, while other foods (including invertebrates, fruit, and the leaves of woody plants) require low basal rates. Various properties of a food, including a low digestibility, a high content of plant secondary compounds, the inability to separate food from non—nutritive particles, and a seasonal reduction in availability, may require a consumer to have a low basal rate of metabolism; all of these factors are exaggerated in importance by the high food intake of large mammals. In small species most foods, except flying insects and seed crops in the desert, permit high basal rates and effective endothermy. Climatic factors modify these relationships to a limited extent, while a specialization in moderately—sized species to arboreal habits leads to reduced activity, smaller muscle mass, and lower basal rate than is found in terrestrial species using the same food resources. Mixed diets generally produce basal rates intermediate to those expected from the component specialist diets. The correlation of basal rate with food habits has the following demonstrable consequences for mammals. (1) The scaling of basal rate usually follows the Kleiber relation, unless food habits require low basal rates, or temperature regulation requires high rates. (2) Mammals that feed on foods associated with high basal rates have a higher reproductive output than mammals of similar size that use foods requiring low basal rates. (3) Predator avoidance is correlated with level of energy expenditure and food habits, terrestrial species with low rates of metabolism relying principally on burrows or passive integumental structures, such as shells, plates, or spines. (4) Most living "conservative" mammals use foods that require low basal rates of metabolism, a condition that protects conservative species from replacement by "advanced" eutherians, which are also required by these foods to have low basal rates.