Elephants Really Can’t Hold Their Liquor

Humans are not the only animals that get drunk. Birds that gorge on fermented berries and sap are known to fall out of trees and crash into windows. Elk that overdo it with rotting apples get stuck in trees. Moose wasted on overripe crab apples get tangled in swing sets, hammocks and even Christmas lights.

“You can’t just assume that humans are just like every other mammal and the physiological abilities of all these mammals are comparable,” said Mareike Janiak, a postdoctoral scholar in evolutionary anthropology at the University of Calgary and the lead author of the study. “Simply scaling up to body size doesn’t account for differences that exist between different mammal species.”

To test whether other species independently evolved the same adaptation, Dr. Janiak and her colleagues searched the genomes of 85 mammals that eat a variety of foods and located the ethanol-metabolizing gene in 79 species. But they identified the same or similar mutation as humans in just six species — mostly those with a diet high in fruit and nectar, including flying foxes and aye-aye lemurs.

But most other mammals did not possess the mutation, and in some species, including elephants, dogs and cows, the ethanol-metabolizing gene had lost all function.

“It was far more likely for animals that eat the leafy part of plants or for carnivores to lose the gene,” said Amanda Melin, a molecular ecologist at the University of Calgary and a co-author of the study. “The takeaway is that diet is important in what we see happening in molecular evolution.”

Some results were unexpected. Tree shrews, for example, drink “copious amounts” of fermented nectar with ethanol content equivalent to weak beer, Dr. Melin said, but they never show signs of inebriation. Yet tree shews do not share the same enzyme-producing mutation as humans. This implies that “there’s multiple, different ways to solve this problem,” she said.

Nathaniel Dominy, a biological anthropologist at Dartmouth College who was not involved in the research, said the new paper “highlights the novel adaptations of humans by putting our metabolic proficiency in broader evolutionary context.” He said it also “exemplifies the power of comparative biology” for teasing out the underlying function of specific genetic traits.

The elephant findings, in particular, are “interesting but confusing,” said Chris Thouless, the head of research at Save the Elephants, a nonprofit in Kenya. Forest elephants today regularly seek and eat fruit, but their ancestors became grass eaters around eight million years ago. Evidence indicates they then switched to a mixed diet around one million years ago.

“Maybe they lost the ability to efficiently metabolize alcohol, but either continued to have, or regained, a taste for and the ability to locate fruit,” Dr. Thouless said. He compared it with people who have very low tolerance for alcohol but still desire and drink it.

While the new study reveals the means by which elephants and other mammals may become inebriated, it does not explicitly confirm the phenomena in nature.

“The persistent myth of drunken elephants remains an open and tantalizing question, and a priority for future research,” Dr. Dominy said.

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