Why Oriental hornets can’t get drunk

Like people, many animals enjoy having a drink every now and again. Rather than sip brandy or Chablis, though, they feed on fermented fruit, but the effects are the same. Though alcohol is rich in calories, it muddles minds and shortens lifespans. It presents a serious risk to animals and most avoid drinking to excess.

Then there is the Oriental hornet, a bug native to northern Africa and south-western Asia, which loves fermented fruits and is so attracted to human-made alcoholic beverages that it will aggressively fight for them with a sting that is akin to being jabbed with a searing hot tack. Everyone has assumed that the rules for the hornet were the same as those for other species—that the alcohol poses them problems. But new research reveals that the rules do not apply. The bugs are immune to becoming drunk and do not have their lives shortened by booze, either.

The problems from alcohol come from the yeast that makes it. The fungus guards the fruits, vegetables and grains that it grows on by producing a toxin, ethanol, to ward off competitors. Very few animals can endure consumption of food rich in ethanol. To date, the champions of this particular contest have been tree shrews and fruit flies, which can periodically ingest concentrations of up to 3.8% and 4% ethanol respectively without suffering ill effects (humans can tolerate far more: wine is typically 10% alcohol and spirits are around 40%). With this in mind, Sofia Bouchebti at Ben-Gurion University of the Negev and Eran Levin at Tel Aviv University found themselves perplexed by Oriental hornet oenophilia.

Since the existing literature on the predilection of Oriental hornets for strong human drinks was sparse, Dr Bouchebti and Dr Levin decided to explore how much ethanol the bugs could cope with. They put small groups of the insects into testing boxes where they were fed sugar water for seven days that was laced with 0%, 1%, 10%, 20%, 60% or 80% ethanol. The hornets fed the 80% solution were, in effect, a negative control and were not supposed to survive. All the same, they did.

Stunned, the researchers ran experiments to monitor longevity and nest-building behaviour. They report in Proceedings of the National Academy of Sciences this week that even as the hornets gorged on the 80% ethanol solution they maintained normal lifespans, built ordinary nests and behaved as they usually did. Unlike all other known animals, they were unfazed by chronic consumption of exceedingly high levels of ethanol. Follow-up work with chemically labelled ethanol that was fed to the insects revealed that the key to their extraordinary abilities is the presence of multiple copies of the gene for alcohol dehydrogenase, which supports the production of enzymes that break ethanol down into components that can be metabolised.

As to why Oriental hornets have multiple copies of this pivotal gene, Dr Bouchebti speculates that it has to do with a close relationship that the hornet has with the fungus Saccharomyces cerevisiae, also known as brewer’s yeast. During the winter, the queens of many social wasps, like the Oriental hornet, are known to harbour the yeast on their bodies along with other microbes. Then, when spring arrives, they transfer it to their newly hatched workers who then spread it to the fruits they come into contact with. It would seem that evolution has driven hornets carrying extra copies of the alcohol dehydrogenase gene to be selected for over time, so they can continue to make use of these food sources even after they are rendered utterly inedible to other species by the ethanol released by the yeast.

In essence, the relationship between the yeast and the hornets seems to be symbiotic; the hornets carry the yeast to things it can feed on and the yeast then preserves these food sources for these hornets, and only them, to feast upon. Whether any other species of social wasp has such a remarkable tolerance for alcohol remains to be determined; but Dr Bouchebti and Dr Levin think it is likely, given the fondness that so many of them seem to have for brewer’s yeast. ■

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