Fd. Provenza, ACQUIRED AVERSIONS AS THE BASIS FOR VARIED DIETS OF RUMINANTS FORAGING ON RANGELANDS, Journal of animal science, 74(8), 1996, pp. 2010-2020
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.