One of the very nice things about fruit is that it will often ferment all on its own. And a few animal species take advantage of this, deliberately seeking out fermented fruit with the objective of painting the jungle red and waking up in the natural equivalent of a ditch at the side of the road. This works well if you happen to be fairly safe from predators, but not many animals have this luxury.

Fruit and nectar eating bats certainly don't fall under the heading of "immune from predation," so researchers wondered what happened to fruit bats that ate from the fermented fruit. Considering body weight and the amount of energy required to keep a fruit bat going, the researchers figured that even the low alcohol content of fruit should still be enough to mean that a night's feeding involved a substantial amount of imbibing.

To test the fruit bat's alcohol tolerance, researchers from Canada gave two groups of bats, from a variety of different species, a sugar water drink. One of the group's drinks was spiked with 1.5 percent alcohol, and both groups were made to drink the same amount per gram of body weight. After waiting a short time, the bats were then released, whereupon they flew through an obstacle course. The researchers measured the time it took to get through the course and how often the bats collided with obstacles. In addition, the echolocation calls were recorded to see if those varied.

The second part of the experiment was determining the bat's blood alcohol concentration. The bats, unfortunately, refused a breath test, and it seemed a bit cruel to take blood samples, so the researchers used saliva swabs 15 minutes after consumption. This length of time allowed the excess alcohol in the mouth the wash out and the majority of the consumed alcohol to enter the blood stream.

The bat's saliva indicated that they had a range of blood alcohol concentrations, with a substantial portion having concentrations in excess of 110mg per 100ml of blood—enough to make most people observably intoxicated.

Despite the alcohol, the bats maneuvered through the course with pretty much the same speed as the control group. No bats collided with any obstacles and the echolocation calls had not changed. Clearly, the bats were not drunk.

The researchers speculate that there may be a combination of factors at work here. First of all, bats that do get drunk and run into stuff are likely to get eaten, so there is an advantage to being resistant to intoxication. However, there are three possible routes for this resistance to appear: fruit bats recognize the smell of fermented fruit and only partake of the fruit that is not yet too far gone; fruit bats are able to process alcohol effectively and quickly; and what we call the W.C. Fields effect: fruit bats build up a tolerance over a lifetime of hard drinking.

The researchers point out that the first option reduces the range of foods that a bat can feed on, and this is disadvantageous—though many animals are known to avoid fermented fruit. Furthermore, some bat species don't avoid fermented fruit and nectar, clearly precluding that as a good explanation. Although it is still possible that these species have higher tolerances to alcohol through some evolutionary adaption, the authors think that it is more likely that they are simply hard drinkers.

So, if you weigh 500g, don't get into a drinking competition with a fruit bat.

PLoS One, 2010, DOI: 10.1371/journal.pone.0008993