UCalgary anthropologists discover link between mammals and higher concentrations of ethanol in fruit

A group of UCalgary anthropologists has produced two studies showing the links between mammals and ethanol concentration and digestion.

Ethanol, more commonly known as alcohol, is the active ingredient in alcoholic beverages that causes drunkenness. Ethanol is also present in wild fruits, fermenting when naturally occurring yeasts in the fruit interact with sugars in the fruit to make ethanol.

In the first study, published in the Proceedings of the Royal Society B, Julia Casorso, BSc’21, Allegra DePasquale, a PhD candidate in the Department of Anthropology and Archaeology, and Dr. Amanda Melin, PhD, an associate professor in the Department of Anthropology and Archaeology, found wild fruits primarily eaten and dispersed by mammals had higher ethanol concentrations than those eaten and dispersed by birds.

Fruit selection varies between mammals and birds

“This is quite notable because mammals rely on their sense of smell and touch to find and select ripe, sugary fruits, while birds rely more on their excellent colour vision,” explains Casorso, the lead author of the study. “These mammal-dispersed fruits and bird-dispersed fruit will differ in their traits, which reflects the ways they find and select fruits.”

Mammal-dispersed fruits are typically bigger, duller in colour and have a hard husk or thick skin that the mammals must use their hands and teeth to get at the flesh inside, like a grapefruit or mango. Bird-dispersed fruits are smaller, brightly coloured and can be eaten in one bite, like a cherry or blueberry.

Casorso says since mammals tend to eat and disperse fruits with higher ethanol concentrations, it suggests they may be using ethanol odours in the fruits to select the best ripe fruits.

First to focus on ethanol trait

Research had previously been done looking into other traits when it came to fruit dispersal, but this was the first time systematically looking at ethanol as a trait.

“Ethanol has historically been a very important substance,” says Melin. “It’s used for celebration, it’s used in rituals, and for many thousands of years humans have deliberately been fermenting alcohol.”

The research was conducted based on the “Drunken Monkey” hypothesis by a scientist named Robert Dudley, who observed that ethanol was naturally present in a lot of fruits, and hypothesized that humans and other primates were predisposed to like ethanol because it was a cue of fruit ripeness.

“The research had mostly been anecdotal up until this point, but we did a systematic study using a new field-friendly method,” says Melin.

Matthew Carrigan, a co-author of the study, developed a novel method to test the fruits by utilizing police-grade breathalyzers to test the concentration levels.

While the research for the study was done in Costa Rica, DePasquale says a wider geographic study needs to be conducted for ethanol concentrations, as has been done for other dispersal traits.

“I think we need more research, but I would expect similar findings in other systems because I think the selective pressures are similar,” she says.

DePasquale says this is because they also found that plant relatedness affected ethanol production. Because the fruit-producing Costa Rican plants have similar relatives in ecosystems all over the world, those relatives may also tend to have similar ethanol concentrations in their fruits.

Ethanol is very volatile so it produces a strong smell, and the team believes that smell, to which some mammals are very sensitive, plays a role in why mammals are attracted to the higher-concentration fruit.

“This could be evidence of a mutually beneficial plant-yeast-mammal relationship that enhances seed dispersal,” says DePasquale.

However, it is not just smell that would play a role in this, as other adaptations would have to happen in the mammalian digestive system to efficiently break down the ethanol.

Gene mutations play a role

This is where the second study comes in. Swellan Pinto, MA’22, Mareike Janiak, a recent postdoctoral scholar at UCalgary, and Melin led this large collaborative study inspired by research which showed humans, chimpanzees and gorillas all share a mutation in a gene that is responsible for breaking down ethanol after it is consumed.

“Basically, we and our close primate cousins can break ethanol down more efficiently,” Melin explains.

In the study, Melin and her team looked across a whole range of mammals and found evidence to suggest there are mutations in mammals, especially in fruit-eating bats, that allow for more efficient ethanol break down.

From a nutritional perspective, ethanol can indicate how much sugar is in a fruit, which is the big reward mammals are going for. Ethanol may also have a nutritional reward, being quite caloric.

More efficient metabolism of ethanol is also beneficial for mammals as they won’t be as prone to its inebriating effects.

“Think of bats — being able to metabolize ethanol efficiently allows them to fly and travel through tree canopies without being basically drunk,” says Casorso.