Nocturnal Primates Are Big Drinkers

Aye ayes and lemurs prowl the night in search of spiked nectar.

Nocturnal primates not only regularly consume alcohol in fermented nectars, saps and fruits, but they also seem to love it, finds new research on two night-prowling primate species: the aye-aye and the slow loris.

Aye ayes (Daubentonia madagascariensis) are lemurs native to Madagascar that have an elongated, bony finger for detecting and extracting grubs from decaying tree trunks. They also commonly eat seeds, fruits, nectar and fungi. The slow loris, which hails from Southeast Asia, has a similar omnivore diet and is, as its name suggests, a slow mover.

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Aye ayes, as well as chimpanzees, bonobos, gorillas and humans, all have a genetic mutation that improves breakdown of alcohol in the body. Together, the findings suggest that the common ancestor of humans and great apes regularly consumed naturally fermented foods and liquids, perhaps after the goods wound up on the forest floor.

The two aye ayes and slow loris involved in the new study, published in the journal Royal Society Open Science, seemed to feel no pain.

"The animals in the study appeared to like alcohol very much, suggesting some kind of sensory reward, although no signs of inebriation were observed," lead author Samuel Gochman, a sophomore at Dartmouth College, told Discovery News.

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Gochman, senior author Nathaniel Dominy and co-author Michael Brown conducted multiple-choice feeding experiments with two aye ayes, "Morticia" and "Merlin," as well as a slow loris, "Dharma," at the Duke Lemur Center to test for an aversion or preference to varying concentrations of alcohol in simulated nectar. The alcohol concentrations were low -- 0 to 5 percent -- reflecting levels found in nature.

Dominy, an anthropologist and evolutionary biologist at Dartmouth, said that in nature, "some of the highest (alcohol) recordings from a wild plant come from overripe palm fruits in Panama. The levels were about 6 percent."

The researchers found that primates could detect varying concentrations of alcohol, and that they adjusted their intake of the nectar with the highest amounts. Morticia and Merlin even continued to probe the containers with the greatest amounts of alcohol long after they were emptied, implying they wanted more.

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"The prevailing assumption is that alcohol is toxic, negatively affecting motor control, survival, and fitness," Gochman said. "However, calories are scarce in the environment, and alcohol is a rich source of calories for primates with high metabolisms, so there may be nutritional benefits to consuming moderate amounts of alcohol that outweigh the costs, especially if a species has evolved a digestive system that can break it down hyper-efficiently, as ours does."

It could even be that fermentation evolved, like fragrance, to attract consumers to slightly boozy edibles. Dominy explained that if consuming more nectar or fruit benefits the plant, such as via better pollination and seed dispersal, then fermentation could bring mutual benefits to the plant and consumer.

Nocturnal primates may be particularly good at finding fermented foods and liquids, since they rely on smell to find food at night. Dominy says that "alcohols are lightweight molecules that can travel far from their source, so it is possible that nocturnal primates can better detect and navigate towards fruits or nectars on the basis of smell."

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The behavior could be extra advantageous for these night-loving primates, he said, since they have high metabolic demands and must fuel their bodies with calories, which alcohol is a rich source of in the wild.

Still, day-active (diurnal) primates must eat a fair amount of natural alcohol too, since all fruit contains at least trace amounts of alcohol. Primates like gibbons have a diet that is up to 95 percent fruit, and given the commonness of fermented fruits, a gibbon's regular alcohol intake could be fairly significant.

Fermented food consumption must date back to long ago, given the evolved genetic mutation, A294V. Gochman said this mutation results in a 40-fold improvement in the digestive efficiency of the ADH4 enzyme, one of several alcohol-digesting enzymes.

Robert Dudley, professor and chair of the University of California at Berkeley's Department of Integrative Science, is the author of the book The Drunken Monkey: Why We Drink and Abuse Alcohol (University of California Press, 2014).

Dudley told Discovery News that the new study "nicely documents behavioral tendencies of two basal primate species to prefer higher concentrations of alcohol within solution."

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Dudley added, "It also indirectly supports the drunken monkey hypothesis, which postulates that modern-day drinking behaviors reflect ancient ecological and dietary exposure of hominids (i.e., our ancestors), and possibly of all primates, to this most widespread of the psychoactive compounds consumed by humans today."

Although we have evolved to detect, tolerate and like alcohol, based on so many years of consuming naturally fermented liquids and foods, the problem is that we now make such consumables ourselves with alcohol levels far greater than 5–6 percent. As a result, "we are producing amounts and concentrations that surpass our digestive tolerance," Dominy said.

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Now the question is: How much alcohol, if any, should humans consume?

While the answer remains unknown for now, Dominy said "there is this idea called hormesis, (which holds) that a low level of toxin is good for our bodies, as it keeps our immune system in good working order, and it is this idea that lies behind the French paradox."

The French Paradox says that moderate consumption of alcohol -- such as in most wines or in fermented products like kombucha, tempeh, kimchi, sauerkraut and more -- can contribute to good health.

Dominy suggested that an area of future research could focus on the ADH family of enzymes across primates or on the ALDH enzyme family, which helps to clear aldehyde from the body. Aldehyde, he said, "is actually what makes people intoxicated."