Preferences and Aversions

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Animals (and humans) have innate preferences and aversions for the taste of foods. Almost all animals from flies to people like the sweet taste of sucrose and other sugars; almost all animals will avoid and reject bitter tasting compounds: the prototypical bitter test solution is quinine (as in tonic water). These preferences and aversions are innate and instinctive: a newborn rat or human will consume sucrose solution, but avoid or reject (with much wailing) the taste of quinine. Because no prior experience or learning is required for this behavioral response to the taste, these are ‘unconditioned’ preferences or aversions.

There is a good evolutionary rationale for the existence of preferences and aversions. All animals can use glucose and other sugars as an energy source; therefore, a favorable response to the chemosensory detection of sugars leading to ingestion will be an advantage to any animal. Conversely, many toxins in the environment are bitter-tasting: the ability to detect and reject these toxins will also help insure survival. These attributes of foods are so nearly ubiquitous in the environment (sweet = calories, bitter = toxins), that natural selection has “hard-wired” almost all animals to respond automatically with ingestion or rejection, without having to go through a trial-and-error learning process.

There are some interesting exceptions that prove the rule. For example, species like koala bears and monarch butterfly caterpillars eat bitter-tasting foods almost exclusively. They have evolved to occupy specific ecological niches (eucalyptus trees and milkweed plants) and tolerate both the low-level of toxins and the bitter taste of these plants. But I bet both species love the taste of sugar!

(Box on the cockroaches from Gainesville….)

(Note: unless the animal can talk to us (i.e the animal is a human), we can’t actually tell if an animal “likes” or “dislikes” a food. Instead,we can only give our label to the observable behavior of the animal. So a food is defined as palatable or prefered if the animal will a) seek or work for access to a food, b) consume the food, and c) consume more of the food than another food, when given the choice. Of course, depending on the circumstances and the treatment of the animal, these responses are readily modified: so a food that is palatable by itself can be less prefered when a better alternative is available; and unpalatable (even toxic) foods will be consumed by a hungry animal.)

(Sclafani makes the distinction that intake of a food in a single-choice situation is an acceptance test; intake of a food in a two-choice situation is a preference test.)

Things to fill in: how to test for preferences and aversions; what neuroanatomical/neurochemical substrates might underlie the expression of innate preferences and aversions; plasticity of preferences and aversions