As early as the 19th century, elephants have had a bit of a reputation for getting “drunk” from the fruit of the marula tree in Africa. Whilst various studies have questioned these stories due to the volume of fruit they’d need to consume, other animals have also reportedly been found inebriated (from raccoons to bees).
To provide some insight into these tipsy tales, a team of researchers have looked into the genetic variations of the ADH7 gene of 85 mammals, as previous studies have linked mutations in the gene with the ability of a species to metabolize ethanol – the form of alcohol contained in beverages and that also naturally occurs from the fermentation of fruit sugars.
“We focused on the ADH7 gene because previous research (led by co-author Dr Matthew Carrigan) had found that humans have a mutation in this gene that makes the protein 40 times more efficient at metabolizing ethanol,” Dr Mareike Janiak, a corresponding author of the study from the University of Calgary, Canada, told IFLScience. “This evolved around 10 million years ago in our common ancestor with chimpanzees and gorillas, potentially as an adaptation for eating fruit.”
Curious as to whether these mutations were found in other mammals, Janiak and her colleagues mined data on the genomes of 85 species, across 21 orders, and with a variety of diets. One of the hypotheses the team were interested in was whether a diet high in fruit resulted in the selection on the ADH7 gene, seen in humans, to help them more readily metabolize ethanol. The team found the ADH7 gene sequences in 79 of the 85 mammals they investigated.
“We found that many mammals that eat mostly meat or leaves actually didn’t have a functional ADH7 gene and they may have lost this gene because of the lack of fruit in their diet,” Janiak explained. However, some animals such as fruit and nectar bats, whose natural diet contains at least some ethanol, may have acquired the same human change in ADH7 to prevent them being “drunk and disorderly.”
“Being inebriated would be especially bad news for a flying mammal, so being able to better metabolize ethanol could be an important adaptation,” Janiak remarked.
Although it may not feel like it after a big night out, humans are up there with other African great apes as one of the mammals capable of quickly metabolizing ethanol. Others include aye-ayes, a primate found in Madagascar who are known to drink nectar, and koalas, whose eucalyptus diet contains a lot of toxins.
At the other end of the scale are those species who no longer have a functional ADH7 gene, Janiak explained. This includes bovids, carnivores, some rodents such as guinea pigs, dolphins, whales, and our old friends elephants. “Many of these species just don’t consume fruit, fresh or rotten, so they’re probably rarely exposed to ethanol,” Janiak said. “But anecdotes about elephants and horses eating rotting fruit are common.”
Whilst the lack of a functional ADH7 gene would make it easier for an animal to become inebriated, one of the study’s conclusions, published in Biology Letters, pointed out the tendency of humans to anthropomorphize animal behavior – likening an animal’s innocent unsteadiness to our drunken sways. Therefore, each story of animal intoxication should take into consideration each species’ unique physiology, rather than referring to human's drunken experiences, the authors warn.
Probably for the best – I'm not sure an elephant flailing round a pole in the middle of a club would end well.