Alcohol content in ripe fruit can affect feeding behaviour.
Humans don't have the monopoly on drunken behaviour. New research shows that under-the-influence bats are more likely than their sober counterparts to eat junk food.
Knowing that fruit-eating bats frequently encounter fermenting fruits, Francisco Sánchez and his colleagues at the Jacob Blaustein Institutes for Desert Research at Ben-Gurion University of the Negev, Israel, decided to investigate what effects, if any, consumption of ethanol from fermentation had on feeding behaviour.
To explore this, they studied Egyptian fruit bats (Rousettus aegyptiacus). Twelve bats were placed in a cage and given feeding containers filled with a mixture of soy protein infant formula, sucrose, water and one of six concentrations of ethanol that ranged from 0% to 2%. Each day the bats were given a different concentration of ethanol in their food. Uneaten food was removed and measured.
The results showed that bats actively avoided concentrations of ethanol above 1%, yet below that threshold level their behaviour was unchanged. Wondering how hunger might affect this, the researchers ran a similar experiment with hungry bats.
The team had expected ethanol in low concentrations to increase the appetite of well-fed bats, because low levels of ethanol are thought to be used by fruit-bearing plants to encourage seed dispersers to eat more before they leave. They expected high levels of ethanol to decrease appetite because the chemical’s toxic effect. In hungry bats, the team expected intoxication from the ethanol to be more pronounced and hypothesised that this would reduce appetite.
But Sánchez and his colleagues report in the journal Naturwissenschaften1 that neither hypothesis proved to be correct. Well-fed bats did not eat more when consuming low levels of ethanol, and avoided eating high concentrations of ethanol altogether.
Starvation or intoxication
Hungry bats, in contrast, ate the same amount of food containing ethanol as they did normal food, regardless of ethanol content. “This is probably because starving to death poses a higher threat than intoxication,” says Sánchez. However, he is quick to add that hungry bats may be more willing to become intoxicated in captivity, where they know they are safe. “Wild tests are needed,” he says.
“We did not know that non-human mammals could respond to such low ethanol levels,” says physiologist Robert Dudley at the University of California, Berkeley. Some fruits develop concentrations as high as 8.1%. “With those fruits you can smell it, but that 1% concentrations are having an effect on bat behaviour is remarkable,” he says.
Ecologist Brock Fenton at the University of Western Ontario in London, Canada, thinks that body size in bats may be connected to alcohol tolerance because bigger animals would need to consume more ethanol to become intoxicated. “The larger size could be driven by fermentation," he says
In a follow-up study, Sánchez’s team found that when bats consumed ethanol together with the sugar fructose, they expelled the ethanol from their system more quickly than if they consumed sucrose or glucose. Under normal circumstances, bats prefer sucrose over fructose and fructose over glucose, but the team expected intoxicated bats to prefer fructose.
But that was not the case; instead, the bats’ desire for sucrose increased. “When consuming ethanol, they seem to prefer their favourite food, regardless of what benefits another food might offer them” comments Sánchez.
Why plants produce ethanol is still poorly understood. In the traditional view, plants tempt bats to disperse their seeds with fruit. “The yeasts that consume fruit sugars and produce ethanol have typically been viewed as parasitic on the system, but they might in fact be mutualistic or even symbiotic,” says Dudley. Ethanol is likely to prevent bacteria from spreading through fruit, or it could help keep fruit available for being eaten for longer.
Sánchez, F., Korine, C., Kotler, B. P. & Pinshow, B. Naturwissenschaften advanced online publication; doi: 10.1007/s00114-008-0359-y (2008).
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Kaplan, M. Drunk bats eat junk food. Nature (2008). https://doi.org/10.1038/news.2008.756