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Why Male Mice are scared of bananas

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.

Jeffrey Mogil, the study’s senior author and a professor in the department of psychology at McGill University said:

The whole thing came as a surprise, since we were not looking for this in particular and found it by accident. The pregnant females were in our lab for another experiment, and one of our grad students realized that the males began acting weird.

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.

Mogil said Rodents and a lot of mammals other than humans are reliant on their olfactory senses. 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.

That’s where the bananas come in.

The authors found that the compound n-pentyl acetate — found in female mice’s urine during later pregnancy and lactating — is similar to a compound found in a variety of fruits and used to produce banana extract. This is the chemical that caused extreme hormone changes in male mice.

Olfactory exposure to late-pregnant and lactating mice produces analgesia in male but not female mice in the radiant heat paw-withdrawal test. 

So when the team bought banana oil extract from the supermarket and placed it inside the cages of male mice, their stress levels increased significantly.

The stress response in the mice was found to be similar to the stress response when about to engage in a fight. The significance of this discovery is that even in the absence of an aggressive female, the mere threat of such aggression — communicated via the compound in bananas — is enough to stress out male mice.

The study found that virgin male mice were more likely to be stressed out by the presence of n-pentyl acetate, whether in bananas or mouse urine. This fits with their tendency to be more aggressive to infants than non-virgin mice, suggesting that they are more of a threat to infants than older males.

Mogil said mammals are signaling messages to one another more than we originally thought. We’re finding that their communications are a lot richer than we give them credit for.


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