UNIVERSITY OF CALIFORNIA PRESS, MAY 2014We are all familiar with the human drunk, and with the full range of her or his behaviors. These can range from the merely entertaining to embarrassing, damaging, and even death-eliciting consequences. But are there comparable outcomes in the animal kingdom? An Associated Press story published in 2002, for example, relates elephants marching through villages in Assam in search of illicit stills, which they broke open to quaff home brew. They then ran amok in a drunken rampage, even killing villagers. Similarly, numerous accounts of inebriated mammals and birds relate the consumption of either fermenting foodstuffs (such as bread dough) or alcohol-laden fruit, followed by apparently drunken comportment. This anecdotal and often humorous literature is, however, very difficult to interpret scientifically.
Are there any real data demonstrating alcohol intoxication in the wild? And are there any evolutionary expectations for animal physiology and behavior if low-level exposure to booze is an inevitable consequence of a fruit-based diet?
By many anecdotal accounts, drunkenness would seem to occur frequently in the animal kingdom. Cavorting groups of inebriated baboons, sozzled chimps falling out of trees, and birds too drunk to fly have all been described by naturalists, interested bystanders, and a more voyeuristically oriented popular media. Many stories hint at behavioral similarity of the drunken beast to inebriated humans. And sometimes the observers might have been drinking themselves, based on the tone of the reports. Most suggestive is a widely distributed sequence from the film “Animals are Beautiful People” (1974), which portrays staggering, cavorting, and apparently natural drunkenness in South African mammals ranging from baboons to elephants and zebra. It was subsequently revealed that these animals had been either fed excessively high levels of liquid alcohol, or had been injected with a veterinary anesthetic in order to elicit such behaviors, which otherwise have never been observed in the wild.
Nonetheless, some stories of alcoholic animals contain more than a hint of scientific truth. In 1990, veterinarians measured alcohol levels in two recently deceased cedar waxwings that had eaten hawthorn fruits, and then tragically fallen from a rooftop. Alcohol concentrations in the livers and crops of these birds were ten to one hundred times higher than those measured in control bird species, suggesting a high level of alcohol ingestion. Cedar waxwings seem to be at particularly high risk in this regard given their repeated appearance in the popular literature; multiple reports in North America have them flying drunkenly into windows and buildings. Fruit-eating birds in the temperate zone may also be particularly susceptible to inebriation when they consume berries fermenting in the spring thaw. A 2012 report from Cumbria in the United Kingdom similarly reported high levels of alcohol in dead blackbirds and redwings, consistent with lethal intoxication.
By contrast, tales of drunken animals in the tropics tend to focus on much larger species such as elephants, warthogs, and giraffes. In southern Africa, these animals are often reported to consume large quantities of fruit from a common and widespread tree called the marula. The yellow fruits of the marula tree are about 4 cm long and fall to the ground in large numbers when ripe, where they subsequently ferment and are consumed by the local fauna. Local peoples also use marula fruit for food and for moonshine production of fermented beverages. Elephants figure prominently in stories of marula-induced drunkenness, and the commercially produced South African Amarula Liqueur even features a proud pachyderm on the bottle's label. Marula fruit were also ostensibly the source of natural alcohol for the animals portrayed in the film “Animals are Beautiful People.”
But how many marula fruits must an elephant actually consume to be inebriated? In 2006, a team of physiologists estimated that the number would have to be excessively high. Given reasonable assumptions about alcohol content of the fruits, likely ingestion rates, and degradation of alcohol within the elephant's digestive system, this group concluded that the rates of natural dietary exposure to alcohol would be at least a factor of four below that required to effect overt intoxication. Even ingestion of a substantial 30 kilograms of marula fruit, corresponding to a daily meal of about 1% of body mass, would be inadequate. And this estimate makes the conservative assumption that the meal was not diluted by any water drunk over the same time period. Elephants are obviously very large, and unrealistically high numbers of fruit would be necessary to attain meaningful alcohol load. Under natural conditions, drunkenness in elephants, and presumably in other large mammals, would be very unlikely.
Nonetheless, biologists may well have missed the broader significance of even rare outbursts of drunken behavior, particularly given the diversity of fruit- and nectar-consuming animals that are exposed to alcohol. In the humid tropics, for example, many butterflies feed on fallen and decomposing fruits, rather than on nectar from flowers. Lepidopterists have traditionally used fruits and other fermenting substances to attract both butterflies and moths, and at least one published account suggests natural inebriation of a butterfly feeding on rotting fruit. Australian lorikeets have similarly been reported to become drunk while feeding on fermented nectar, ultimately becoming unable to fly. In fact, it might be particularly dangerous to drink alcohol while flying. A series of studies on fruitbats in the Negev desert suggested that, although they can sense alcohol in solution at very low levels, aqueous concentrations above 1% are actually avoided. Nocturnal flights to and from communal roosts are obligatory for these large bats, and any inability to fly would involve substantial risks for them to predators, and indeed for any flying animal that has but limited mobility on the ground. Behavioral responses to alcohol are thus likely to vary with the animal species in question, and with different aspects of its physiology and natural ecology.
Excerpted with permission from The Drunken Monkey: Why We Drink and Abuse Alcohol, by Robert Dudley. Available from the University of California Press. Copyright © 2014.