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Humans, chimpanzees, gorillas, bats and other mammals with a diet that contains lots of fruit and nectar may be the best in the animal kingdom at metabolising their alcohol.
Fruits and nectars are rich sources of energy that many animals rely on. However, they produce ethanol by natural fermentation, which can lead to alcohol concentrations as high as 3.1 per cent in nectars and 8.1 per cent in fruits, says Mareike Janiak at the University of Calgary in Canada.
This means it was beneficial for fruit and nectar-eating animals to evolve the ability to break down alcohol quickly and avoid becoming drunk, Janiak says. “Being able to eat a lot of fruit or nectar without being subject to the effects of ethanol would certainly open up an important food resource,” Janiak says.
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To see how many animals have managed to do this, Janiak and her colleagues studied a gene called ADH7 in 85 different mammal species. ADH7 codes for an enzyme called aldehyde dehydrogenase 7, which helps remove the intoxicating chemicals that alcohol is converted to in our bodies.
They found that mammals that regularly consume fruit or nectar are more likely to have a version of ADH7 that makes them extremely efficient at processing alcohol. This includes bonobos, aye-ayes, as well as chimpanzees, gorillas and humans – primates that shared a common ancestor “at least 10 million years ago, long before we began fermenting beverages on purpose”, says Janiak.
Fruit and nectar bats are also efficient at processing alcohol, Janiak says. “Being inebriated would be bad news for a flying mammal, so being able to better metabolise ethanol could be an important adaptation for them.”
Natural selection
In contrast, mammals whose diets generally lack fruit or nectar, such as cows, horses and elephants, are poor alcohol metabolisers, because these animals lost their functioning version of ADH7.
Previous studies have suggested poor alcohol processors – such as elephants – would never be able to consume enough fermented fruit to get drunk. But this study suggests it may be possible, says Nathaniel Dominy at Dartmouth College in New Hampshire. “The persistent myth of drunken elephants remains an open and tantalising question, and a priority for future research.”
Journal reference: Biology Letters, DOI: doi/10.1098/rsbl.2020.0070
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