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Most humans enjoy drinking alcohol from time to time. Could this appetite for alcoholic beverages be a legacy of our primate ancestors? That’s what a new study involving UC Berkeley biologist Robert Dudley suggests. Several years ago, this specialist already put forward the idea that our attraction to alcohol dates back several million years, when the first primates associated the smell of alcohol with ripe fruit.
In 2014, Dudley published a book entitled ” The drunken monkey : why we drink and abuse alcohol “, in which he exposed his idea for the first time: the fact that Man appreciates alcohol would be linked to the food preferences of our primate ancestors. Ripe fruits are sweeter and therefore taste better; maturation leads to alcoholic fermentation (the process by which sugars are transformed into ethanol). Thus, the smell of alcohol would have naturally guided the animals towards the ripest fruits, which are also the most nutritious. An idea commonly referred to as the “drunken monkey hypothesis”.
« Chimpanzees, our closest relatives, get about 90% of their caloric intake from ripe fruit. And where there is sugar in the tropics, there is alcohol », said Dudley in an interviewat the release of his book. Research has shown that primates sometimes consumed fruits with an alcohol content of up to 7%. But until now, scientists did not have data proving that monkeys and great apes preferentially ate these fermented fruits; they also did not know if they were able to digest the alcohol contained in these fruits.
Alcoholic fruits chosen for their caloric intake
A new study led by Christina Campbell, a primatologist at California State University at Northridge, provides new answers that support Dudley’s drunken monkey hypothesis. The researchers were particularly interested in the black-handed spider monkey (Ateles geoffroyi), a primate of Central and northern South America, which lives in trees. Monkeys of this species primarily consume ripe fruit and were therefore good candidates for this study of natural exposure to ethanol; they also have an excellent olfactory sensitivity to various alcohols.
By collecting remains of fruits eaten by these animals, the team found that their alcohol content was usually between 1% and 2% by volume. These ripe fruits came from the mombin plum tree (Spondias mombin); not only is this fruit one of the favorite foods of the spider monkey, but it has also been used for millennia by the indigenous populations of Central and South America to make the girla fermented alcoholic beverage.
« For the first time, we were able to show, beyond a shadow of a doubt, that wild primates, without human intervention, consume fruits containing ethanol said Campbell in a press release. Ethanol is a by-product of yeasts in fruit pulp, which metabolize sugar during the ripening process and can serve as a long-range olfactory signal to fruit-eating animals, including many primates.
Analysis of the urine of six different monkeys also revealed that it contained metabolic products of alcohol consumption, namely ethyl glucuronide and ethyl sulphate – which proves that these monkeys use fruit ethanol to produce energy.
These animals would thus turn more to fermented fruits, because they are more caloric and therefore more energetic. A phenomenon accentuated by deforestation, according to the specialist: monkeys having more and more difficulty finding their food, they try to get as many calories as possible from the food they find. Our appetite for alcohol would thus stem from a deep-rooted affinity of frugivorous primates for the ethanol present naturally in ripe fruit.
An unlikely state of drunkenness in monkeys
Our ancestors could also have chosen to preferably consume fruits loaded with ethanol, in order to absorb more calories. Contemporary patterns of alcohol consumption, in turn, may derive from these associations between ethanol and nutritional reward. ” Excessive alcohol consumption, as in diabetes and obesity, can then be viewed conceptually as a disease related to excess nutrition. “, underline the researchers in their publication.
It remains to be seen how much alcohol the monkeys absorb, and also, what are the effects of this consumption on their physiology and their behavior. We know the devastation caused by excess alcohol in humans, but what about non-human primates? According to Dudley, monkeys are unlikely to experience the same effects. ” They probably don’t get drunk, as their intestines fill up before they reach inebriated levels “, he explains.
This is what differentiates these animals from humans, who since the appearance of distillation have had access to liquid alcohol decoupled from its solid substrate. Thus, animals stop feeding when they are full, but humans can continue to drink until an alcoholic coma…
Apart from the fact that these alcoholic fruits provide the necessary energy for the monkeys, the biologist thinks that they could also have an antimicrobial effect. Yeast and microbes that help with fermentation may also help digest food, he says.
According to the researchers, many other species could regularly consume alcohol. ” Since positive selection on genes coding for ethanol catabolism has been extensive in mammalian species consuming fruits and nectar more generally, natural consumption of fermented carbohydrates is likely more widespread than previously thought. currently think so “, they note in their study. To better characterize the possible behavioral effects of ethanol on frugivorous primates, and its role in foraging, they plan to conduct further studies on a wider range of samples, involving several stages of fruit maturation.
Source : C. Campbell et al., Royal Society Open Science
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