POSTINGESTIVE FEEDBACK AS AN ELEMENTARY DETERMINANT OF FOOD PREFERENCE AND INTAKE IN RUMINANTS

Ruminants select nutritious diets from a diverse array of plant specie s that vary in kinds and concentrations of nutrients and toxins, and m eet their nutritional requirements that vary with age, physiological s tate, and environmental conditions. Thus, ruminants possess a degree o f nutritional wisdom in the sense that they generally select foods tha t meet nutritional needs and avoid foods that cause toxicosis. There i s little reason to believe that nutritional wisdom occurs because anim als can directly taste or smell either nutrients or toxins in foods. I nstead, there is increasing evidence that neurally mediated interactio ns between the senses (i.e., taste and smell) and the viscera enable r uminants to sense the consequences of food ingestion, and these intera ctions operate in subtle but profound ways to affect food selection an d intake, as well as the hedonic value of food. The sensation of being satisfied to the full (i.e., satiety) occurs when animals ingest adeq uate kinds and amounts of nutritious foods, and animals acquire prefer ences (mild to strong) for foods that cause satiety. Unpleasant feelin gs of physical discomfort (i.e., malaise) are caused by excesses of nu trients and toxins and by nutrient deficits, and animals acquire avers ions (mild to strong) to foods that cause malaise. What constitutes ex cesses and deficits depends on each animal's morphology, physiology, a nd nutritional requirements. This does not mean that ruminants must ma ximize (optimize) intake of any particular nutrient or mix of nutrient s within each meal or even on a daily basis, given that they can withs tand departures from the normal average intake of nutrients (i.e., ene rgy-rich substances, nitrogen, various minerals, and vitamins). Rather , homeostatic regulation needs only some increasing tendency, as a res ult of a gradually worsening deficit of some nutrient or of an excess of toxins or nutrients, to generate behavior to correct the disorder. Extreme states should cause herbivores to increase diet breadth and to acquire preferences for foods that rectify maladies. From an evolutio nary standpoint, mechanisms that enable animals to experience feedback , sensations such as satiety and malaise, should be highly correlated with nutritional well being, toxicosis, and nutritional deficiencies, which are directly related with survival and reproduction.