Poison shyness

Poison shyness is a behaviour whereby an animal that has ingested a toxic substance subsequently avoids ingesting that substance again. Animals learn an association between stimulus characteristics, usually the taste or odor, of a toxic substance and its toxicity. This allows them to detect and avoid the toxin when it is next encountered. Poison shyness occurs naturally in animals, usually generalists, that have evolved the behaviour to avoid toxicosis. It is often observed as bait shyness during attempts at pest control, when poisoned baits for insects and mammals are ingested at sublethal doses; the target species subsequently detect and avoid these poisoned baits. This learned behaviour is technically known as conditioned food aversion learning.

For any organism to survive, it must have adaptive mechanisms to avoid toxicosis. In mammals, a variety of behavioral and physiological mechanisms have been identified that allow them to avoid being poisoned. First, there are innate rejection mechanisms such as the rejection of toxic materials that taste bitter to humans. Second, there are other physiologically adaptive responses such as vomiting or alterations in the digestion and processing of toxic materials. Third, there are learned aversions to distinctive foods if ingestion is followed by illness.

A typical experiment tested food aversion learning in squirrel monkeys (Saimiri sciureus) and common marmosets (Callithrix jacchus), using several kinds of cues. Both species showed one-trial learning with the visual cues of color and shape, whereas only the marmosets did so with an olfactory cue. Both species showed a tendency for quicker acquisition of the association with visual cues than with the olfactory cue. All individuals from both species were able to remember the significance of the visual cues, color and shape, even after 4 months. However, illness was not necessarily prerequisite for food avoidance learning in these species, for highly concentrated but non-toxic bitter and sour tastes also induced robust taste aversion learning and retention.

Conditioned taste aversion has been widely used as a method of pest control and conservation. These aversions have been induced in both predator and prey species.

Rodents: Rats and mice develop bait shyness very readily; it can persist for weeks or months and may be transferred to nontoxic foods of similar types. Thus, if poisons are used for control they must provide no sensation of illness following ingestion. For this purpose, baits containing anticoagulants such as Warfarin were long used; they kill relatively slowly through internal bleeding, which is not associated with ingestion. More recently a highly potent toxin attacking the central nervous system, bromethalin, has been used. Again, with sub-lethal doses of this chemical, the animal cannot learn the association between the odour of the food and its toxicity, thereby preventing poison shyness from developing.

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