Residents of Gilbert, Minnesota, were alarmed earlier in October when birds crashed into their windows and passed out on their lawns. Each year, cedar waxwings gorge on berries before migrating south for the winter. But this time, the fruit had fermented earlier than usual, and the birds that ate it got intoxicated.
While rare, reports of drunk birds鈥攅specially fruit-eating species like waxwings鈥攇o back decades. found the same birds got hammered off Hawthorne fruits. In 2012, scientists in California of cedar waxwings that had met a tragic fate: fatally crashing into windows and fences after feasting on the fruit of the Brazilian peppertree. They concluded that, indeed, 鈥渇lying under the influence of ethanol鈥 had caused the birds鈥 deaths. This month, New Zealand named the kereru its , in part because of its fondness for getting tipsy on fermented fruit.
Aside from occasional reports like those, little data exists on which animals can and do get drunk, according to Robert Dudley, a biologist at UC Berkeley and author of . Some animals are inclined to imbibe, albeit at a pace moderated by limited availability of alcohol鈥攁nd it seems these boozy creatures can, in fact, handle their liquor.
Drinking and flying is no problem for some airborne mammals, for example. In , scientists fixed sugar water and ethanol cocktails for six species of bats that are known to feed on fermenting fruit and nectar. (When naturally occurring yeasts ferment the sugar in ripe fruit, the process creates ethanol, a volatile chemical compound and the same alcohol found in beer, wine, and spirits.) The intoxicated bats had no difficulty navigating aerial obstacles even with blood alcohol levels that would render us unfit to drive鈥攕ome even had a blood alcohol content over 0.3 percent. (The scientists waited until the bats sobered up before releasing them back into the wild.)
The pen-tailed tree shrews of Malaysia spend their nights sipping berta palm nectar鈥攚hich has alcohol levels up to 3.8 percent, similar to a light beer鈥攚ithout consequence. The amount of alcohol the shrews drink in a night is the equivalent of us drinking nine glasses of wine in 12 hours. , scientists found that the shrews鈥 alcohol levels reached what in humans would 鈥渋ndicate life-threatening chronic drinking and strong behavioral deficits.鈥 But these mouse-resembling critters don鈥檛 stagger around, black out, or get into brawls. 鈥淭hey should be getting overly drunk, and they don鈥檛,鈥 Dudley says.
Many fruit-eating animals, in fact, eat low levels of ethanol daily.听For some听mammals and insects, especially those living in the tropics,听fermenting fruit on the forest floor is a major food group.
In the Caribbean, vervet monkeys get sloshed on stolen tourist cocktails.
Over millions of years, these animals might have evolved to clue in on ethanol. While other aromas might vary from apple to orange, ethanol doesn鈥檛鈥攁ll fermenting fruit off-gasses this molecule. So听ethanol can be a 鈥渦biquitous and reliable鈥 indicator that fruit is ripe enough to eat, says Dudley. Ethanol is also calorie dense, with nearly twice the amount of energy as the same amount of carbohydrates. The common fruit fly prefers to eat and lay eggs in . Dudley thinks humans evolved a similar attraction to ethanol.
Scientists have found that around the same time that our ancestors began to walk on two feet, about 12 million years ago, they also evolved better enzymes to metabolize alcohol, perhaps in order to eat extra-ripe fruits they picked up from the ground. Our central nervous system also rewards alcohol drinking for reasons unknown, making us feel happy and social after a glass or two.
It鈥檚 hard to say if animals who seem to enjoy getting drunk enjoy it for the same reasons we do. In the Caribbean, for example, vervet monkeys get sloshed on stolen tourist cocktails. While they seem to like the taste of the drinks, it鈥檚 unclear whether they have a fuzzy-feeling psychoactive reward similar to our own, Dudley says. But in one curious parallel, male fruit flies when recently rejected for mating by females.
According to Dudley, alcoholism can be seen a 鈥渄isease of nutritional excess.鈥 We evolved to sense and digest low levels of ethanol, but the distillation process has made the formerly limited chemical available in excess.
As for cedar waxwings, however, you probably don鈥檛 have to worry about them developing a taste for alcohol and going after your booze. Their sense of smell is much worse than that of mammals, and they mostly target food by sight, Dudley says. For them, getting drunk is more accident than evolutionary attraction gone awry.