Mechanisms of learning in diet selection with reference to phytotoxicosis in herbivores

Abstract

Our objective is to develop explanations for why herbivores ingest poisonous plants by first discussing how herbivores learn to select diets, by then considering mechanisms that enable herbivores to ingest phytotoxins, and by finally developing hypotheses about why herbivores overingest phytotoxins. Animals learn about foods through 2 interrelated systems: affective and cognitive. The affective system integrates the taste of food and its postingestive feedback; this system causes changes in the intake of food items, depending on whether the postingestive feedback is aversive or positive. The cognitive system integrates the odor and sight of food and its taste; animals use the senses of smell and sight to select or avoid specific food items. We further divide cognitive experiences into 3 categories: learning from mother, learning from conspecifics, and learning through trial and error. Physiological and conditional responses enhance the ability of animals to ingest phytotoxins. Physiological mechanisms include binding the compound before it can exert its action, metabolizing the compound so it cannot exert its action, and tolerating the compound. Conditional responses complement physiological responses and further decrease herbivore susceptibility to toxins by preparing the animal for the effects of the toxin. Herbivores are likely to overingest poisonous plants when any of the aforementioned systems fail. For example, the affective system is likely to fail when phytotoxins circumvent the emetic system, when aversive postingestive consequences are delayed temporally and positive consequences during the delay are pronounced, and when toxicosis is accompanied by a change in environmental context. Likewise, cognitive systems are likely to fail when animals are unable to distinguish subtle molecular changes that render nontoxic plants toxic, when toxins in 2 or more plants interact, and when herbivores are unable to differentiate nutritious from toxic plants as a result of being placed in an unfamiliar environment. We conclude that a thorough understanding of affective and cognitive systems, and the specific conditions under which these systems fail, will be necessary in order to understand why herbivores ingest foods that do them harm.