Ever since childhood, when he saw his father descend into alcoholism, evolutionary physiologist Robert Dudley has been curious about humans’ strong attraction to booze.
The notion crystallized one day 18 years ago in the monkey-filled jungles of Panama, when he observed an abundance of rotting fruit littering the forest floor, fragrant with the smell of alcohol. Perhaps, he thought, the odor of alcohol in fermenting, overripe fruit actually draws monkeys to the trees where the nourishing food, normally hidden among the lush greenery, is most abundant. Maybe human attraction to alcohol is not unique in the animal world, and actually has a survival advantage.
Dudley, who specializes in the biomechanics of flight, spent the ensuing years accumulating evidence for this hypothesis, which he presents in a new book, “The Drunken Monkey, Why we drink and abuse alcohol” (UC Press 2014). He recently discussed his motivations for writing the book, the evidence that our attraction to alcohol is an evolutionary adaptation, and what this finding means for efforts to prevent alcohol abuse.
Q: What are you arguing in this book?
The argument here is that our attraction to alcohol goes back about 18 million years, to the origin of the great apes, if not 45 million years with the origin of diurnal fruit-eating primates. If you look at gibbons, chimps, gorillas and orangutans, they are just dedicated fruit eaters. Chimps, our closest relatives, are getting about 90 percent of their caloric expenditure from ripe fruits. And where there is sugar in the tropics, there is alcohol. Now, not a lot; it depends on how much you consume. They are not drinking down gin and tonics, but they are getting a long, sustained, low-level exposure.
So, I hypothesize that social facilitation of communication and food sharing and all these bright warm fuzzy feelings we get when we have a drink have basically evolved to facilitate rapid identification of fruit at a distance – you smell a plume, go upwind, and you get to the fruit. Fruit flies do it, we just don’t know if primates do or not. But they might. And once you get the fruit, you consume as much as possible before others do, or you share it with your close relatives, which is a well-documented behavior. The positive psychoactive effects of alcohol may simply exist to enhance the efficacy of these behaviors and, ultimately, they are the targets of natural selection.
It was kind of a fun realization that there is an ancestral, almost neurological bias associating ethanol with nutritional reward and caloric gain.
Q: Do primates get a buzz on?
No one knows. Primates are so hard to study in the field. We know they like alcohol. Darwin comments on consumption of alcohol by captive primates, which is well known. But captive monkeys eat diets imposed on them, so their preference for alcohol may not relate to natural preferences.
Q: What about other animals?
I get a lot of anecdotes about drunken animals. But it is so hard to know whether it’s natural, or whether they are drunk on the alcohol or the alkaloids in the fruits. There are no data.
We know, however, that other animals are exposed to alcohol. It has been known for a long time that fruit flies are attracted to alcohol vapor. They try to find fermenting fruit and lay their eggs on it, because that is what the larvae will cruise around in. The larvae also eat the yeasts that are producing the alcohol. And there is crazy stuff, like, male fruit flies that have just been rejected by a suitor are more likely to move to a high-alcohol containing fruit substrate. There are a lot of analogies between fruit fly and human behavior.
And there was a recently published paper on tree shrews and slow lorises in Malaysia, where there is a lot of alcohol in fermenting nectar of a night-blooming Malaysian palm, and where they are drinking lots of it. But they are never getting drunk. We have millions of species of nectar feeding insects and lots of birds, mammals and insects that feed on fallen and overripe or rotten fruit. All these nectar feeders are potentially getting exposed to chronically low levels of alcohol.