Food selection in an herbivorous rodent: balancing nutrition with thermoregulation

Ecology, Oct, 1997 by Hugo Torres-Contreras, Francisco Bozinovic

Observed variation in food choice may not only reflect changes in food quality and thermoregulatory constraints on degus, but it may also depend on digestive strategies (Afik and Karasov 1995). Bozinovic and Martinez del Rio (1996) postulated that knowledge about the mechanisms of nutrient assimilation may provide a straightforward explanation of diet choice in nature. We suggested that digestive characteristics, such as retention time and apparent digestibility, may greatly contribute to the net profitability of the food items within microhabitats (but see Kaiser et al. 1992). Penry (1993) pointed out that constraints on food selection that result from limitations on digestive processing rate (limitations on the retention time for digestion) are common. In fact, variation in digesta retention time appears to be an adaptation in individuals that experience wide variation in food quality (Penry 1993).

Our data suggest that the degu, an herbivorous rodent, does not select food based on a single criterion, but, when given a choice, individuals select high-quality food. Consequently, we are able to predict that, depending on the abundance and availability of food resources in the environment, degus will select food by using multiple criteria, such as thermal factors, nutrient and energy content of foods, and their own metabolic and nutritional requirements. We found that drymatter and energy intake were negatively and significantly correlated with apparent digestibility of dry matter and energy. Interestingly, the total amount of food eaten, and probably the food selected, varied as a function of food digestibility and mean retention time. There are probably limits on an individual's capabilities to reduce retention time (or increase throughput time) and still meet its nutritional/energetic metabolic requirements. Consequently, degus ate less food when apparent digestibility was higher, and vice versa. Thus, degu food preferences may be determined, in part, by the efficiency and rate at which they assimilate nutrients.

As demonstrated by Martinez del Rio and Karasov (1990), these rates of gut processing are determined by the interaction between intestinal hydrolysis, intestinal nutrient uptake, mass of digesta carried, and retention time. It is possible that degus on high-fiber diets retain digesta for a shorter time because fiber stimulates contraction of the gut wall musculature. This factor may also contribute to determining dietary preferences, if environmental temperature is not physiologically risky. There are many possible multiple-criteria selection procedures. One is that animals may select food based on a primary criterion, but, when two diets are equal for that criterion, a secondary criterion may be used. The alternative is that both criteria are always important, for example, when an animal tries to attain sufficient energy and protein from food that is rich in only one of these resources.

Because consumption of different dietary items depends on the ecological context in which foraging takes place, because food quality influences degu preferences and digestion, and because thermoregulation may constrain foraging behavior (Karasov 1986), both food quality and the ecological physiology of animals should be considered in explaining temporal and spatial processes of foraging ecology and microhabitat use. Consequently, when individuals are subject to changes in the environment, coupled with their own physiological constraints, they should respond behaviorally to such temporal and spatial changes, minimizing the time required to obtain nutrients and energy, increasing the amount of energy/nutrients gained in a fixed time, or both. Degus may respond to the variation in food quality in a labile manner, being time minimizers trying to select and digest the highest quality food items possible, but also avoiding hyperthermia during summer. During winter, however, they are reducing thermoregulatory costs and selecting high-quality food, apparently becoming both time minimizers and energy maximizers. Our laboratory results dealing with behavioral and physiological processes may explain the previously described patterns of microhabitat use observed in degus in the field. We think that physiological performance and constraints force small mammals to select high-quality food with a higher assimilation rate, and to restrict their foraging to microhabitats where suitable foods are available in sufficient supply and where environmental temperature allows a thermoregulatory homeostasis.