Nine beers in one night could put even a seasoned drinker under the table. But the pen-tailed tree shrew in Malaysia consumes the equivalent of that in alcoholic nectar several nights a week, researchers have discovered, and six other species of animals there consume smaller amounts of alcohol as well. Unlike humans, the animals seem to suffer no ill effects from their habit. How they have evolved to tolerate alcohol could teach us something about the origins of human alcohol consumption and abuse, researchers say.
The pen-tailed tree shrew (Ptilocercus lowii) is a small, ratlike animal that inhabits the jungles of Southeast Asia. It feeds on the nectar of an ever-flowering plant, the bertam palm, which is a primary food source for many other species as well, says Frank Wiens of the University of Bayreuth in Germany. Wiens was observing tree shrews feed in the Malaysian jungle when he noticed an oddly familiar odor: "The palms smelled like a brewery," he says.
Wiens realized that some of the animals might be consuming huge amounts of alcohol, which prompted him and colleagues to spend more than 3 years in the field studying the ecology of the bertam palm. Yeast cells in the palm's flowers ferment its nectar, they discovered, which can contain up to 3.8% alcohol--among the highest concentrations ever found in natural foods. The researchers observed seven mammalian species feeding on the nectar. The pen-tailed tree shrews guzzled the stuff longer than they did any other food source, for an average of 138 minutes per night, in the process helping to pollinate the plants.
From the rate at which palm flowers were drained and refilled, the team calculated that, on average, the tree shrews meet or surpass the legal intoxication limit of 1.4 grams of alcohol per kilogram of body mass once every 3 days. (The average woman has to drink nine glasses of wine or beer in a 12-hour period to achieve that level.) The researchers couldn't check the animals' blood alcohol levels, but they did analyze their hair for ethyl glucuronide (EtG), a byproduct from breaking down alcohol. In the common tree shrew (Tupaia glis), they found an average of 1 nanogram of EtG per milligram of hair--almost 60 times the level that is considered a sign of excessive drinking in humans, the researchers report today in the Proceedings of the National Academy of Sciences.
How the tree shrews metabolize alcohol is still unclear, but Wiens and his colleagues suspect that they are much more efficient than humans; the tree shrews never moved as though they were inebriated. Most likely, the animals have coevolved with the bertam palm and are experts at living on alcohol. That's interesting, he says, because the pen-tailed tree shrew is believed to closely resemble the very first primates. Many researchers believe that consistent, easy access to alcohol is a relatively recent phenomenon, so the species as a whole hasn't evolved any protection against it. "This speaks against the current paradigm," Wiens says.
Robert Dudley, a physiologist at the University of California, Berkeley, who has studied the ecology of ethanol consumption, agrees that there has to be an evolutionary explanation for the tree shrews' alcohol tolerance, though more work is needed to understand exactly how they benefit from their heavy intake. The study also suggests that the tree shrew might be a better animal model for human drinking than lab rats, which do not naturally take alcohol, Dudley says. "I hope this breaks the alcohol profession out of the 'rats-drinking-liquid-ethanol model.' ... That's not going on in the real world."