Energy Assimilation, Nitrogen Requirement, and Diet in Free-Living Antelope Ground Squirrels Ammospermophilus leucurus

Abstract

Antelope ground squirrels (Ammospermophilus leucurus, 80-100 g) consumed vegetation, seeds, arthropods, and vertebrate flesh throughout the year in the Mojave Desert near Barstow, California. Apparent energy assimilation efficiency (100 x ) on each type of food ranged from 55% on mature plant leaves to ~90% on vertebrate flesh and seeds. Apparent assimilation of nitrogen (N) ranged from 63% on plant fruiting parts to 87% on arthropods. For the foods studied here, antelope ground squirrels apparently assimilated as much of the energy and N in their food as do other rodents, even those that specialize on some of these food types. However, an important difference between omnivorous rodents, such as A. leucurus, and herbivorous rodents may be the ability of the latter to survive on diets with lower dry-matter digestibilities. The N requirements of free-living A. leucurus were estimated based on a feeding trial with captives fed a protein-free diet. The results suggested that even though free-living small mammals have rates of energy expenditure two to three times basal, their N requirements can be reliably predicted based on laboratory measurements made under more basal conditions. The assimilable N requirement of free-living, adult, nonreproductive A. leucurus is about 500 mg N kg⁻¹ day⁻¹. These ground squirrels require plant food with at least 13-16 mg N (g dry mass)⁻¹ for N balance if they consume only leaves. However, the small quantities of arthropods and vertebrate flesh which they consume throughout the year ensure that they are in N balance if plant N contents fall below that level.