Male mice are terrified of bananas. Here's why.

S E E D E D   C O N T E N T

Scientists recently discovered something about male mice that's utterly bananas: The distinctive scent of a banana stresses them out.

Researchers from McGill University in Montreal, Quebec, learned about this unusual fruit aversion while analyzing spiking stress hormones in male mice when the males were close to pregnant or lactating females. The scientists reported in a new study that the males' hormonal shifts were triggered by the presence of a compound called n-pentyl acetate in the females' urine. It also happens to be the compound that gives bananas their distinctive smell.

"The whole thing came as a surprise, since we were not looking for this in particular and found it by accident," said Jeffrey Mogil, the study’s senior author and a professor in the department of psychology at McGill University. "The pregnant females were in our lab for another experiment, and one of our grad students realized that the males began acting weird," Mogil told Live Science.

In the paper, researchers wrote that "male mice, especially virgin males, are well known to engage in infanticidal aggression to advance their genetic fitness." As a way to keep these potential predators at bay, pregnant and lactating females rely on chemosignaling, or emitting chemical responses through their bodies, to send messages to the males to stay away from their offspring. 

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"Rodents and a lot of mammals other than humans are reliant on their olfactory senses," or sense of smell, Mogil said. "Urine scent-marking is well known, but what we’ve found here is a new message that has never been described before in mammals. We’ve seen a lot of olfactory messages being sent from males to females, but there are fewer examples of females sending them to males. Most of these messages have to do with sexual behavior, but in this case, sex has nothing to do with it at all. The females are telling the males to stay away, otherwise be prepared for me to beat the crap out of you if you touch my pups." 

After observing that stress levels in males rose in response to the chemicals in females' urine, Mogil and his team wondered if n-pentyl acetate from a different source would trigger a similar response. They purchased banana oil from a local supermarket and added the liquid to cotton balls, which they then placed inside the cages of male mice. The presence of the scent measurably increased the stress levels of the males — just like the urine had done in prior experiments — and the researchers suspect that this hormonal spike directly relates to the stress one feels when facing a possible fight.

Exposure to either urine or banana oil also had an analgesic, or pain-relieving, effect, decreasing the males’ sensitivity to pain, the study authors reported. Measured over time, the researchers learned that pain resistance in the male mice developed as quickly as five minutes after they smelled n-pentyl acetate, and abated 60 minutes after smelling it.

The study authors also discovered that the levels of stress-induced analgesia were significantly higher in virgin male mice, suggesting that unrelated males were bigger threats to the pups’ survival than the fathers were. The findings offer a glimpse into the invisible communication channels animals use to talk to each other, Mogil told Live Science. 

"Mammals are signaling messages to one another more than we originally thought,” he said. “We’re finding that their communications are a lot richer than we give them credit for."