Pork plums grown in tropical forests can have an alcohol content of more than 1.5 percent
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Many fruits eaten by mammals in a tropical forest contain alcohol, and this may be one way plants entice animals to disperse their seeds.
Ethanol occurs naturally in fruits as a product of fermentation by wild yeasts. Until now, there have been few large-scale studies of alcohol’s role in plant-animal interactions, says Juliet Casorso at the University of Calgary in Canada.
Casorso and her colleagues collected fruit at various stages of ripeness from more than 70 plant species in a Costa Rican tropical dry forest, both directly from trees and the ground. To measure the fruit’s alcohol concentration, they placed it in plastic bags for an hour and then sampled the air in the bag with a breathalyzer.
Based on existing knowledge of the types of fruits that animals eat, they classified smaller, brightly colored fruits as bird-dispersed and heavier, duller fruits as mammal-dispersed. Soft, juicy fruits were considered mixed spread. After excluding fruits for which the sample size was too small, 37 species were included in the final analysis.
The researchers found detectable levels of alcohol in 78 percent of fruits, and those likely dispersed by mammals had higher levels of alcohol. The highest concentration of alcohol detected in a pig plum (spondias mombin), was more than 1.5 percent.
Mammals may find alcohol attractive because it indicates a ripe, sweet fruit that provides more nutrition, the researchers say. Previous research has shown that animals such as lemurs and monkeys prefer fruit with a higher alcohol content.
“Mammals in particular use their sense of smell to find food,” says Casorso. “Ethanol is one of many aromas that fruit produces, and the fruit may be using alcohol to attract dispersers.”
Although plants themselves do not produce alcohol, they evolved into large, fleshy fruits containing fermentable sugars in the early Cretaceous. Around the same time, yeasts evolved to ferment sugars and produce ethanol. Yeasts also benefit from interacting with mammals, as the animals help spread their spores.
While the alcohol content in individual fruits is quite low, Casorso says small-bodied mammals can accumulate a significant amount of alcohol in the blood by consuming large amounts of fruit. As a result, many mammals have evolved enzymes that process alcohol and the intoxicating chemicals it is converted into.
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