Sugar over Spirits

Matthias Laska

A spider monkey shows off its taste receptors

In overripe fruit, microbes ferment sugars into alcohol, reaching concentrations of 0.05 to 3 percent. The so-called “drunken monkey” hypothesis posits that alcoholism developed in our primate ancestors from their attraction to fermenting fruit, perhaps, some propose, as a source of extra calories. A new study suggests, however, that monkeys enjoy alcohol for its sweetness rather than its calories.

Biologist Matthias Laska of Linköping University in Sweden and colleagues at Linköping and at Universidad Veracruzana in Mexico tested the ethanol preferences of eight fruit-eating spider monkeys (Ateles geoffroyi) across three experiments. The researchers assessed the spider monkeys’ threshold for tasting ethanol. When presented the option to drink either plain water or water spiked with ethanol at concentrations from 0.05 to 3 percent, the monkeys could detect ethanol—starting at 0.5 percent (compared to human detection at 1.34 percent)—and favored the ethanol solutions over plain water.

In the next experiment, the team offered monkeys the choice between sucrose sugar solutions spiked with ethanol or plain sucrose solutions. When the spiked and plain sucrose solutions were equally sweet, the monkeys preferred the spiked drinks. But when the plain sucrose solutions were sweeter than the spiked solutions, the animals clearly preferred extra-sugary, alcohol-free solutions over less-sugary ethanol solutions, even when the latter had triple the calories.

To more closely approximate what spider monkeys might naturally encounter, the team pureed three ripe fruits of varying carbohydrate and caloric levels—mango, papaya, and cantaloupe—and then offered the monkeys the drinks with and without 3 percent ethanol. Here, the monkeys preferred the spiked purees over the plain purees, and they significantly favored spiked mango, which had the most carbohydrates, over spiked melon, which had the least.

Because ethanol binds to sweet-taste receptors in mammals, the team suggested that spider monkeys may like ethanol because it enhances sweetness. Laska concluded that the results do not support either the idea that monkeys like alcohol for the extra calories or what he called the “rather absurd” drunken monkey hypothesis that the evolutionary roots of alcoholism lie in primates’ attraction to overripe fruit. As alcohol content rises in fermenting fruit, he emphasized, nutritious carbohydrates, consumed by microbes, decline. Plus, microbes manufacture various toxins, which might be a nasty chaser. (Chemical Senses)