ACQUIRED AVERSIONS AS THE BASIS FOR VARIED DIETS OF RUMINANTS FORAGING ON RANGELANDS

Ruminants eat an array of plant species that vary in nutrients and tox ins. This sense, but no theories adequately explain this diversity. So me maintain it reduces the likelihood of overingesting toxins, whereas others contend it meets nutritional needs. Nevertheless, herbivores s eek variety even when toxins are not a concern and nutritional needs a re met. I offer another explanation for this behavior, one which encom passes the avoidance of toxins and the acquisition of nutrients. A key concept in this theory is aversion, the decrease in preference for fo od just eaten as a result of sensory input (a food's taste, odor, text ure, i.e., its flavor) and postingestive effects (effects of nutrients and toxins on chemo-, osmo-, and mechano-receptors) unique to each fo od. Aversions are pronounced when foods contain toxins or high levels of rapidly digestible nutrients; they also occur when foods are defici ent in specific nutrients. Aversions occur even when animals eat nutri tionally adequate foods because satiety (satisfied to the full) and su rfeit (filled to nauseating excess) represent points along a continuum , and there is a fine line between satiety and aversion. Thus, eating any food is likely to cause a mild aversion, and eating a food too fre quently or in excess is likely to cause a strong aversion. Aversions a re involuntary and are not the result of conscious decisions by an ani mal. Aversions yield benefits (e.g., obtain a balanced diet, reduce in gestion of toxic foods, optimize foraging and rumination times, sample foods, maintain a diverse microflora in the rumen) that are often mis taken as the cause of varied diets. In this article, I discuss the sub tle ways in which aversions diminish preference and cause animals to e at a variety of foods.