Fruit cocktail

In "The Drunken Monkey Hypothesis" (12/04-1/05) Dustin Stephens and Robert Dudley suggest that human alcoholism is rooted in the evolutionary history of primates. They reason that frugivorous primates evolved mechanisms for tolerating dietary ethanol because low levels of alcohol are characteristic of ripe fruits.

It is compelling supporting evidence for the hypothesis that approximately 10 percent of the total soluble protein in the human liver is alcohol dehydrogenase, one of the enzymes that metabolizes alcohol. But it is also important to consider how primates recognize ethanol, if it is a major foraging cue. Matthais Laska of the University of Munich and others report that macaques possess an exquisite olfactory sensitivity to alcohol. Vicktoria Danilova and Goran Hellekant of the University of Wisconsin--Madison have shown that nerves that convey taste and somatic sensations from the mouth respond to ethanol. The responses are far greater in primates than in other mammals. A tendency towards tipsiness, then, could well be an evolutionary hangover.

Nathaniel J. Dominy

University of California

Santa Cruz, California

The key assumption of the argument by Dustin Stephens and Robert Dudley is that contact between prehistoric primates and ethanol is necessary for alcoholism. On closer examination, however, that seems unlikely.

Alcoholism is an addiction; it is characterized by dependence, the development of tolerance, and a loss of control over intake. People become addicted to many different substances that our primate cousins never came in contact with, such as methamphetamines, heroin, and crack cocaine. What appears necessary for addiction is that a substance interacts in specific ways with neural pathways involved in motivation, reward, and reinforcement. Those pathways, of course, also have an evolutionary history, but one that long predates frugivorous primates. A thoughtful critique of the drunken monkey hypothesis was recently published by Katharine Milton ("Ferment in the Family Tree" Integrative and Comparative Biology, 44: 304-314, 2004).

Justin S. Rhodes and John C. Crabbe

Portland Alcohol Research Center

Portland, Oregon

On our property in the Hudson Valley, my wife and I have several old mulberry trees with berries that ripen in July. As the sun warms the berries, many fruit-eating birds that rarely visit our seed feeders come and feast. It seems as though the berries begin to ferment in the sun. The young robins are particularly affected. They eat their fill and then try to fly, usually flapping and landing close to the tree, seemingly quite drunk. We have not noticed the same phenomenon in more mature birds.

David Ginsberg

New York, New York

ROBERT DUDLEY REPLIES: Alcohol is unique among the addictive substances in that all fruit-eating animals, including human ancestors, regularly consume low-levels of alcohol while feeding on ripe and overripe fruit. Justin S. Rhodes and John C. Crabbe are correct in suggesting that the neural pathways associating alcohol ingestion with reward precede the origin of primates. As Dustin Stephens and I stated in our article, fruit flies track ethanol plumes to find fruit, and the flies' fecundity and longevity are enhanced at low levels of alcohol exposure.

Recent work by Ulrike Heberlein, an anatomist at the University of California, San Francisco, has demonstrated that fruit flies also have molecular pathways of inebriation similar to those of humans. Her finding shows that fruit flies are good models for studying drug abuse in people. Identifying the evolutionary origins of such physiological and behavioral responses to alcohol is an important goal for basic research in the biology of addiction.

COPYRIGHT 2005 Natural History Magazine, Inc.
COPYRIGHT 2008 Gale, Cengage Learning