Many fruits eaten by mammals in a tropical forest contain alcohol and this may be one way that 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 looking at what role alcohol plays in the interactions between plants and animals, says Julia Casorso at the University of Calgary in Canada.
Casorso and her colleagues collected fruits at different stages of ripeness from more than 70 plant species in a Costa Rican tropical dry forest, both directly from trees and from the ground. To measure the alcohol concentration of the fruits, they placed them in plastic bags for an hour and then sampled the air inside the bag with a breathalyser.
Based on existing knowledge of the kinds of fruits that animals eat, they classified smaller, brightly coloured fruits as bird-dispersed and heavier, duller fruits as mammal-dispersed. Soft, succulent fruits were considered mixed-dispersal. 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 per cent of the fruit species, and those likely to be dispersed by mammals had higher levels of alcohol. The highest concentration of alcohol, detected in a hog plum (Spondias mombin), was more than 1.5 per cent.
Mammals might find alcohol attractive because it indicates a ripe, sugary fruit that provides more nutrition, the researchers say. Previous studies have found that animals such as lemurs and monkeys have a preference for fruit with a higher alcohol content.
“Mammals in particular use their sense of smell to find food,” says Casorso. “Ethanol is one odour amongst the many aromas that fruits produce and the fruits might be using alcohol to attract dispersers.”
Although plants don’t produce alcohol themselves, they evolved to grow large, fleshy fruits with fermentable sugars in the early Cretaceous Period. Around the same time, yeasts evolved to ferment sugars and produce ethanol. Yeasts also benefit from the interaction with mammals, because the animals help to disperse their spores.
While the alcohol level in individual fruits is quite low, Casorso says that small-bodied mammals may accumulate quite a significant amount of blood alcohol 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.