In the first controlled study of its kind, a team of scientists at Dartmouth College has found that two species of non-human primates – the slow loris (Nycticebus coucang) and the aye-aye (Daubentonia madagascariensis) — can discriminate varying concentrations of alcohol, and further, that both species prefer the highest available concentrations.

Alcohol is widespread in nature, existing in fermented nectars, saps and fruits.

Recent reports suggest that alcohol is a supplemental source of calories for some non-human primates.

For example, slow lorises, nocturnal lorisoid primates endemic to Southeast Asia, consume the fermented nectar of the bertam palm (Eugeissona tristis) with a mean alcohol concentration of 0.6% (0 to 3.8%).

A similar behavior is hypothesized for aye-ayes, nocturnal lemurs endemic to Madagascar.

“Aye-ayes are essentially primate woodpeckers,” said study co-author Prof. Nathaniel Dominy, from the Department of Biological Sciences and the Department of Anthropology at Dartmouth College.

“They have an elongated, bony finger for detecting and extracting grubs from decaying tree trunks.”

“It is puzzling that they can digest alcohol so efficiently.”

In the wet season, aye-ayes devote as much as 20% of their feeding time to the nectar of the traveler’s palm (Ravenala madagascariensis).

“If the nectar is fermented, then the hyper-efficient alcohol digestion would make ecological sense,” said study lead author Samuel Gochman.

“Since we didn’t have access to such flowering trees for the study, instead, we tested whether aye-ayes are attracted to alcohol in a nectar-simulating solution of sucrose.”

At the Duke Lemur Center in Durham, N.C., the scientists conducted multiple-choice feeding experiments with two aye-ayes, Morticia and Merlin, and a slow loris, Dharma, to test for an aversion or preference to varying concentrations of alcohol in simulated nectar.

The alcohol concentrations were low (0 to 5.0%) to reflect levels found in nature.

Each liquid treatment, together with two controls, was placed in a circular array of small-recessed containers in a round resin outdoor table.

The position of the liquids was randomized and behavioral data were collected blind to the contents, to avoid observational bias.

Each of the two aye-ayes participated in a trial once a day for 15 days for a total of 30 trials.

The slow loris participated in a trial each day over five days for a total of five trials, as time was limited.

The team found that the aye-ayes could discriminate between tap water and the varying concentrations of alcohol, and that they adjusted their intake accordingly.

Further statistical analysis showed that the aye-ayes preferred the highest concentrations of alcohol.

Unexpectedly, the aye-ayes continued to probe the containers with the highest concentrations long after they were emptied, suggesting that they wanted more.

The five trials with the slow loris were too few to yield statistical results, but the pattern of discrimination and preference was practically identical.

None of the animals exhibited signs of impaired coordination or behavior, as intoxication was not part of the study.

The findings were published online today in the journal Royal Society Open Science.

“This project has definitely fueled my interest in human evolution. Our results support the idea that fermented foods were important in the diets of our ancestors,” Mr. Gochman said.

Some biologists have suggested that our genetic mutation for efficient alcohol digestion, which is shared with chimpanzees and gorillas, is linked to the consumption of fermented fruits on the forest floor, a dietary behavior that could have pre-adapted humans for the Neolithic Revolution.

And some archaeologists have argued that making beer was our primary motivation for harvesting and ultimately domesticating cereals, the plant that give rise to complex societies.

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Samuel R. Gochman et al. 2016. Alcohol discrimination and preferences in two species of nectar-feeding primate. R. Soc. Open Sci. 3: 160217; doi: 10.1098/rsos.160217