28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Diet Selection: The Nutritional Wisdom Hypothesis

C. R. Tracy, K. E. Nussear, T. C. Esque, K. Dean-Bradley, K. Castle, and L.A. DeFalco
Colorado State University, Fort Collins, CO

Classical foraging theory focuses on energy acquisition, and individuals of some species appear to forage so as to maximize net energy acquisition [e.g. Schoener 1971; Glander, 1981]. Herbivores have many nutritional needs (in addition to energy), and their requirements may not be met simultaneously when tortoises forage for energy [Zahorik, 1981; Rapport, 1981; Stephens, 1986]. The question remains, "how do herbivores meet their many nutritional needs?" "Nutritional wisdom" has been suggested as a trait by which animals obtain a nutritionally balanced diet [Richter, 1943]. The hypothesis proposes "specific hungers" permitting a sensing of nutrients in the diet, and selection of a diet with elements required [Richter, 1943]. However, few studies have demonstrated the ability to forage for particular elements although exceptions include foraging with appetites for sodium and water in rats and moose respectively [Zahorik, 1977; Rozin, 1977; Stephens, 1986].

An alternative hypothesis is that diet preferences are shaped through learned food aversions. This model posits that animals learn to avoid foods that can induce illness such as foods containing secondary compounds or diets deficient in, or excessive in, nutrients. While dietary aversions have been demonstrated in rats, coyotes, and other carnivorous animals, the mechanisms underlying this strategy may be difficult in herbivores. Many herbivores consume a variety of plant species over time, and the rates at which food passes through the gut can be up to several weeks, which makes it difficult for the animals to discriminate which of the food items eaten caused their illness. Thus, it seems unlikely that herbivores could discriminate which foods should induce an aversion [Zahorik, 1977]. Experience, or learned preference, has also been proposed as an explanation for diet selection for particular nutrients. However, studies have generally failed to demonstrate the ability to learn, or acquire diet preferences for specific nutrients [Zahorik, 1977].

While the mechanisms are not well defined, large mammalian herbivores have been shown to select diets more nutritious and less toxic than the range of available foods [Zahorik, 1977; Zahorik, 1981]. The idea that herbivores select a diet based on nutrient composition, or that they possess a nutritional wisdom, is frequently implied but seldom tested. Despite our primitive understanding of how herbivores forage and attain their nutritional needs, many investigators studies still invoke "nutritional wisdom" as a possible mechanism underlying diet selection in the desert tortoise [Nagy, 1986; Esque, 1994; McArthur, 1994].

We have conducted analyses of diet selection in the desert tortoise to address the question, "Can tortoises select diets with nutrient content differing from that which would be obtained if foraging occurred at random with respect to food availability?" We analyzed tortoise foraging by using plant biomass as an indicator of the relative availability of food species, and the proportion of bites of each food species as a measure of the diets of individual tortoises [Esque, 1994]. We compared diets selected by tortoises in relation to food available in the habitat. Then we compared the nutrient composition of the diets to the composition of diet that could be selected at random from the same habitat. From this analysis, we investigated whether it is possible to forage non-randomly with respect to nutrients by asking if a plant nutrient falls outside the 95% confidence limit for a random distribution of that nutrient. Secondly, we asked if there are plants that differ from the random distribution, and do tortoises preferentially select those plants. With high variation both among and within plants over time and space, it is unclear if tortoises could perceive differences and respond to those differences during and throughout the activity season. Additionally the quantity of nutrients within a plant correlate, and that tortoises foraging maximally for protein almost certainly maximize other nutrients and minimize fiber.