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Can flies get drunk?

(Image: Nevit Dilmen/Wikimedia Commons)

(Image: Nevit Dilmen/Wikimedia Commons)

No, don’t be ridiculous

Why does that Bertram palm smell like a brewery? Producing abundant nectar, yeast-driven fermentation provides a heady brew that is wildly popular with the local Malaysian fauna. Amongst the visitors are treeshrews that imbibe copiously but curiously show few signs of intoxication. But this is the exception that proves the rule. Using leaves as natural sponges chimpanzees raid plastic containers storing the fermented sap of the raffia palm, sometimes ingesting copious amounts and with predictable results. Nor is this surprising given humans, or rather their distant ancestors, acquired a taste for alcohol back in the Miocene.

African folklore is rich in stories of inebriated elephants that have feasted on the rotting fruit of the Marula tree, a relative of the mango. Given the body mass of these pachyderms and the low alcohol content of the fruit, consumption would have to be prodigious to have much effect. But fruit-eaters (frugivores) should be careful, especially if they are flying. Having gorged themselves on the berries of the Brazilian pepper tree flocks of intoxicated Cedar waxwings have been observed crashing into windows and fences, with fatal results. So too dosing the food of fruit-bats with ethanol resulted in them not only flying more slowly but impairing their echolocatory clicking, perhaps analogous to the slurred speech of the truly plastered. So treeshrews may be unusual in keeping a steady head, but all these examples of animal intoxication involve either warm-blooded mammals or birds. Just like us. Drunk flies? Forget it!

Yes, of course

Rule number one: when it comes to alcohol intoxication never under-estimate our friend the fruit-fly (Drosophila). First of all we will need some laboratory kit, such as the aptly labelled inebriometer. Analyses of fly drunkenness can pursue several avenues, but let’s concentrate on how ethanol affects their walking. And the inebriometer? This allows a hundred or so flies to be accommodated in an equivalent number of tubes, along with a device to introduce ethanol fumes. The faster the flies walk the more often they break the infrared beam and this serves to register the activity of the increasingly intoxicated flies. Ready? In comes the ethanol and immediately the flies start to rush around, but only for a short time. Rates of running fall precipitously and then recover but not to the previous levels of activity. Now they peak, movements slow, followed by immobility, even death.

This overall response is astonishingly similar to how humans react to alcohol: first euphoria, then depressed passivity, finally “dead drunk”. But the parallels go much further. First, these responses involve brain chemistry feeding to behaviour and are not related to how alcohol is rendered harmless by metabolism. Second, it is common knowledge that the effects of alcohol on men and women differ, and not only on account of relative body mass. Chaps can drink more, but are at greater risk of alcoholism. Girls are more susceptible to a really stiff G&T. So too with flies. With the first flush of ethanol the males rush around sooner and take longer to calm down. Do the similarities end there? No! As any habitual drinker will tell you, it takes more booze to get the buzz and flies show the same pattern of tolerance. For them even a single swig is enough to set them on the long path to near-permanent befuddlement.

It all depends on the question

So is it all bad? If ethanol-related mutations in Drosophila have names like barfly, cheapdate, hangover and tipsy one might think only in negatives. Yet in the natural world fruitflies seek out rotting fruit and ethanol is usually on the menu. Indeed they can use alcohol for self-medication. And in other ways the parallels between us and flies are unexpectedly close. Both have a sweet tooth, but what happens when you test fruit-flies with many of the artificial sweeteners we slip into our tea and coffee? The comparisons are illuminating. Whereas our evolutionary cousins in the shape of New World monkeys, can only respond to some of those sugar substitutes, in fruitflies the reactions to these artificial sweeteners are much more similar to humans. This capacity has arisen independently, but why? Intriguingly it might be because Drosophila and Homo are not only “Out of Africa” but we tend to omnivory combined with a passion for fruit. We left Africa and in our baggage came the fruitfly, one of our closest commensals. Watch them buzz round our wine glasses. He murmured: “Enough of the Pouilly-Fuissé? On to the Gigondas?” She smiled: “Splash more, if you don’t mind ….”

Text copyright © 2015 Simon Conway Morris. All rights reserved.

Further reading
Carrigan, M.A. et al. (2015)  Humans adapted to metabolize ethanol long before human-directed fermentation.  Proceedings of the National Academy of Sciences, USA 112, 458-463.
Devineni, A.V. and Heberlein, U. (2012) Acute ethanol responses in Drosophila are sexually dimorphic.  Proceedings of the National Academy of Sciences, USA 109, 21087-21092.
Gordesky-Gold, B. et al. (2008) Drosophila melanogaster prefers compounds perceived sweet by humans. Chemical Senses 33, 301-309.
Heberlein, U. et al. (2004)  Molecular genetic analysis of ethanol intoxication in Drosophila melanogaster. Integrative and Comparative Biology 44, 269-274.
Hockings, K.K. et al. (2015)  Tools to tipple: ethanol ingestion by wild chimpanzees using leaf-sponges. Royal Society Open Science 2, 150150.
Keller, A. (2007) Drosophila melanogaster’s history as a human commensal. Current Biology 17, R77-R81.
Kinde, H. (2012) Strong circumstantial evidence for ethanol toxicosis in Cedar waxwings (Bombycilla cedorum). Journal of Ornithology 153, 995-998.
Morris, S. et al. (2006)  Myth, marula, and the elephant: An assessment of voluntary ethanol intoxication of the African elephant (Loxodonta africana) following feeding on the fruit of the Marula tree (Sclerocarya birrea). Physiological and Biochemical Zoology 79, 363-369.
Sánchez, F. et al. (2010)  Ethanol ingestion affects flight performance and echolocation in Egyptian fruit bats. Behavioural Processes 84, 555-558.
Wiens, F. et al. (2008)  Chronic intake of fermented floral nectar by wild treeshrews.  Proceedings of the National Academy of Sciences, USA 105, 10426-10431.

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