No legal limit for bats?

  • A bat in the hand

    Timeline, 2010: People with a blood alcohol level of 0.3 percent are undeniably kneewalking, dangerously drunk. In fact, in all 50 states in the US, the cutoff for official intoxication while driving is 0.08, almost a quarter of that amount. But what has people staggering and driving deadly appears to have no effect whatsoever on some bat species.

Why, you may be wondering, would anyone ask this question about bats in the first place? Bats are not notorious alcoholics. But the bat species that dine on fruit or nectar frequently encounter food of the fermented sort, meaning that with every meal, they may also imbibe a martini or two worth of ethanol.

Batty sobriety testing

Recognizing this exposure, researchers hypothesized that the bats would suffer impairments similar to those that humans experience when they overindulge. To test this, they selected 106 bats representing six bat species in northern Belize. Some of the bats got a simple sugar-water treat, but the other bats drank up enough ethanol to produce a blood alcohol level of more than 0.3 percent. Then, the bats got the batty version of a field sobriety test.

Bats navigate by echolocation, bouncing sound waves off of nearby objects to identify their location. To determine if the alcohol affected the bats’ navigation skills and jammed the sonar, the researchers festooned a ceiling with dangling plastic chains. The test was to see if the animals could maneuver around the chains while under the influence of a great deal of alcohol. To their surprise, the scientists found that the drunk bats did just as well as the sober ones.

Some bats hold their drink better than others

Interestingly, the bats did show a human-like variation in their alcohol tolerance, with some bats showing higher levels of intoxication than others. But one question that arises from these results is, Why would bats have such an enormous alcohol tolerance?

As it turns out, not all of them do. These New World bats could, it seems, drink their Old World cousins under the table. Previous research with Old World bats from Egypt found that those animals weren’t so great at holding their drink. Thus, it seems that different bat species have different capacities for handling—and functioning under the influence of—alcohol.

One potential explanation the investigators offer for this difference is the availability of the food itself. In some areas, fruit is widely available at all times, meaning that the bats that live there are continually exposed to ethanol in their diet. Since they can’t exactly stop eating, there may have been some selection for those bats who could get drunk but still manage to fly their way home or to more food. In other bat-inhabited areas, however, the food sources vary, and these animals may not experience a daily exposure to intoxication-inducing foods.

Alcohol driving speciation?

This study may be one of the first to identify a potential role for alcohol in the speciation of a taxon. Bats as a group underwent a broad adaptive radiation, meaning that there was a burst of speciation as different bat species evolved in different niches. Factors driving this burst are thought to have included different types of fruit; for example, tough fruits require different bat dentition features compared to soft fruits. Now, it seems that alcohol availability may also have played a role in geographical variation of alcohol tolerance in bats. Bats with greater tolerance would have been able to exploit a readily available supply of alcohol-laden foods.

What’s next in drunk-animal research? The investigators who made this unexpected bat discovery have a new animal target—flying foxes, which aren’t really foxes at all but yet another species of bat that lives in West Africa. We’ll have to wait and see how these Old World bats compare to the New World varieties when it comes to holding their liquor.

Flying drunk no problem for bats

Drunk New World bats fly fine under the influence

People can’t do it. When we drink, alcohol impairs all kinds of functions, including our ability to drive or walk a straight line. Bat researchers in work published in the online open-access journal PLoS ONE hypothesized that the same rule would apply to bats: the frugivorous (fruit-eating) types often encounter fermented fruits, meaning that frequently, a meal for a bat comes with the alcohol equivalent of a dry martini.

Sonar unaffected

And the humans–not for the last time–were wrong. Bats flying under the influence of a blood alcohol measuring three times the human legal limit maneuvered just fine in their human-imposed drunk tests. The test consisted of plastic chains suspended from the ceiling, requiring the bats to make their way around and through without a collision. Whether they’d imbibed sugar water or grain alcohol, the world’s only flying mammals performed equally well.

Bats may build up a tolerance

Not all bat species have this capacity. It seems that bats, like people, may vary in their alcohol tolerance. In addition, bat species like the New World bats in this study that encounter fermented fruits all the time may have a better tolerance for alcohol than bats who imbibe only occasionally. Old World bats, it appears, are less able to hold their liquor compared to their New World, daily imbibing cousins.

Alcohol: a previously unidentified force of natural selection?

Humans may have long been aware that alcohol can drive certain choices. And now, the bats may confirm that. According to the study authors, sensitivity to ethanol may have determined which bat species developed where. Just as types of fruit may have influenced the speciation of bats, the bat ability to tolerate–or not–ethanol may also have affected bat adaptive radiation.

Ideas for questions

Bats navigate by sonar, while humans rely primarily on inputs including vision to maintain balance and walk a straight line. Do you think that this difference might help explain why these New World bats don’t show the effects of alcohol in their navigation? Why or why not?

The paper refers to the bat “adaptive radiation.” What is an adaptive radiation, and what are the conditions that are required for one to occur? How did bat speciation exemplify this process?

Other frugivorous or omnivorous species encounter fermented foods, as well. One hypothesis, the Drunken Monkey hypothesis,  is that the smell of fermenting fruit drove primate evolution. Can you find other research describing the influence of ethanol on animals?

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