Hit me with the strong stuff. Aye-ayes, a tiny crazed-looking lemur species, prefer fake nectar with higher concentrations of alcohol – as does the slow loris, a small primate with huge eyes from Southeast Asia.
Chimps have been caught gulping fermented nectar in the wild before, but it wasn’t clear if primates just tolerated the alcohol or actively sought it out, says Robert Dudley at the University of California in Berkeley.
“This is the first study – albeit using captive primates – to show that there is a preference for higher levels of alcohol,” says Dudley, who wasn’t involved in the study.
Aye-ayes usually snack on beetle larvae that they pick out of rotting wood with their bizarrely long middle fingers, but they also spend 20 per cent of the wet season drinking nectar from the traveller’s palm. The nectar could easily become fermented if natural yeasts carried by pollinators enters it, although this has not yet been confirmed in the wild.
The slow loris spends much of its time guzzling bertam palm nectar, which has documented alcohol concentrations in the wild of up to 3.8 per cent.
The researchers offered two aye-ayes and one slow loris five cups with mixtures of sugary water and alcohol similar to what they would find in flowers – between 0 and 5 per cent alcohol for the aye-ayes and between 0 and 4 per cent for the loris. A cup of tap water served as the control. The primates drank about twice as much of the boozier liquids.
“Aye-ayes used their fingers to compulsively probe the cups long after the contents were emptied, suggesting that they were extremely eager to collect all residual traces,” says Nathaniel Dominy at Dartmouth College in Hanover, New Hampshire, an author of the study.
Primates that like alcohol have an advantage, says Samuel Gochman at Dartmouth College. Alcohol forms vapours easily, and the smell of these vapours might help animals locate edible fruits and flowers.
It’s possible they also get a dietary benefit from ingesting it, since alcohol slows down metabolism and promotes fat storage, says Matthew Carrigan at Santa Fe College in Gainesville, Florida.
This fits with Dudley’s drunken monkey hypothesis – the idea that primates that could handle their drink had an evolutionary edge.
This is also backed by the fact that aye-ayes have a highly functioning version of an enzyme called alcohol dehydrogenase that lets them detoxify alcohol, as did the last ancestor of humans, chimps and gorillas, which evolved the enzyme independently. This probably happened when apes began walking on the ground and encountering more overripe fruits, Carrigan says.
Usually, animals that eat fermented foods don’t get drunk because the alcohol levels in the foods are too low, says Dudley. Even though many modern humans have inherited the ancient ability to break down low levels of alcohol, the invention of hard liquor created something much more concentrated and capable of causing drunkenness.
Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.160217