A drunken elk ended up hanging this week after climbing into an apple tree to munch on fermenting fruit. The inebriated elk tried to get one extra apple before last orders and ended up stuck in a tree after its legs gave way near Gothenburg in Sweden.
That some animals will consume enough fermented fruit to behave drunk is a much debated topic in the world of behavioural research. There is little evidence to suggest that animals actively seek out fermenting fruit because it contains ethanol however, ethanol is ingested by a variety of different taxa where it occurs naturally and is easily accessible.
We can see evidence of this when birds congregate in late summer to feed on fermenting fruit. The woozy birds will often be seen flying into windows showing a reflection of the sky. Further tests have shown high levels of ethanol in the blood of disorientated birds.
Ethanol is produced by the fermentation of sugar by yeast. In nature, this most commonly involves the simple sugars found in ripe fruit. Ethanol is metabolised by a complex enzymatic process and results in the formation of acetate, which is then incorporated into the body.
Toxins, like ethanol decrease net fitness in terms of reproductive success. Natural selection should favour individuals that avoid eating them versus those that do. With all this in mind, why would non-human animals appear to ingest alcohol by choice and often to the point of intoxication?
William McGrew, a professor at the University of Cambridge, investigated seven hypotheses to explain ethanol ingestion. Accidental occurrence for gustation or for nutritional reasons seemed the most realistic, but none of the seven could be falsified.
Supporting the hypothesis that animals ingest ethanol accidentally, is the fact that animals do not have the infrastructure to protect them from their own bad decisions. A drunken badger, who suffered from obvious disorientation and diarrhoea after eating overripe cherries, found this out the hard way by only making it halfway across a road in Germany before ‘passing out’. Bats are better equipped to protect themselves from such misadventures and studies have shown when fruit bats get accidentally intoxicated, they will actively seek out a type of sugar that helps them sober up faster.
Katharine Milton, a researcher looking into the evolutionary history of human fondness for ethanol, conducted a survey of primatologists covering 22 different primate species. She asked at which stage of ripeness monkeys preferred to eat fruit. None of the species preferred overripe fruit, suggesting that primates do not ingest ethanol for its taste.
Butterflies however, appear not only to ingest overripe fruit for its nutritional and energy value but also appear to enjoy it. The South American morpho will actively turn down an equivalent sugar-solution alternative for rotting fruit.
Whether animals ingest ethanol for its nutritional value, for gustation or by pure accident is unclear and without sufficient data we can only speculate. However, we can enjoy the notion that we are not the only ones that enjoy a tipple or two.