June 7, 2022

Scientists found bananas stress out male mice. A compound inside the fruit could be why.

  • Scientists say late-pregnant and lactating female mice have a compound in their urine that creates stress for male mice.
  • The compound found in their urine is similar to a compound in banana oil.
  • When scientists ran tests to see how banana oil impacted male mice, they showed “significant analgesia” or pain inhibition.

They’re tasty. They’re healthy. But apparently, bananas are pretty stressful for male mice.

Scientists at McGill University accidentally discovered that bananas create stress in male mice due to a compound inside of the fruit, according to an article published in the peer-reviewed journal Science Advances.

Jeffrey Mogil, E. P. Taylor Chair in Pain Studies at McGill University, said his team was doing experiments with male and female mice when a student noticed something odd.

“The male mice that were there for a completely separate experiment were acting weird,” he recalled.

He initially thought it was ridiculous, but years of research have taught him that when students notice things, they’re often true or worth looking into.

“Let’s present male and female mice with all kinds of different mice,” Mogil told his student. “Regular males, regular females, females that are pregnant, early on females that are pregnant. We did it sort of systematically and lo-and-behold, late pregnant and lactating female mice are the only ones that stress out male mice.”

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Male mice aggressive towards pups that aren’t theirs

When late-pregnant or lactating female mice were within about a foot of the male mice, the males showed signs of stress and ran around their cages more than normal.

Mogil said male mice are aggressive towards baby mice, or pups, that aren’t theirs. They often kill them so female mice can birth their pups instead.

That’s where a compound called pentyl acetate comes into play. It’s found in the urine of late-pregnant and lactating female mice, Mogil said.

In some cases, the presence of pentyl acetate in female mice’s urine prevents the males from killing the pups. But either way, the males get stressed by the thought of having a fight because that could lead to injury or death, Mogil said.

Where bananas come in, Mogil said, is chemical makeup.

The compound pentyl acetate, found in late-pregnant and lactating female mice’s urine, has a similar chemical structure to isoamyl acetate. Both pentyl acetate and isoamyl acetate are found in fruits and are used to produce banana oil/extract.

To test out the male mice’s reaction to isoamyl acetate and bananas, the team used supermarket banana oil. They also measured the mice’s stress hormones in their blood, as well as pain inhibition.

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So, are they really afraid of bananas?

When male mice were near the banana oil, the team noticed “significant analgesia,” or pain inhibition.

But does this mean male mice are afraid of the bananas?

“The word afraid is a little bit less precise, but I think it’s just as accurate,” Mogil said. “What they’re actually stressed by or afraid of is the prospect of the fight with the female mice. It’s just that the chemical that the female mouse is putting in her urine just happens to be the same chemical that makes bananas smell like bananas.”

He also said the research has its limitations, like how the study focuses solely on mice and has a small sample size.

Brian Trainor, professor and vice chair of the department of psychology at the University of California, Davis, said while the researchers showed male mice responded to a chemical found in bananas, they did not actually show that the mice were afraid of the odor.

What the researchers showed, he said, is that a chemical found in bananas reduces pain responses.

“Stressful situations can reduce pain responses, which occurs when the body releases pain-killing chemicals like beta-endorphin,” Trainor told USA TODAY. “Beta-endorphin is in the same category of chemicals as morphine. Showing that a chemical reduces pain perception does not mean that animal is afraid, because beta-endorphin can be released after other activities such as exercising.”

Trainor said more behavioral tests would be needed to prove that the mice are afraid of the banana odor.

Mogil thinks the findings are funny, but useful.

“It’s another example of the fact that mice communicate with each other and they have more richer communication than we think … We may have been underestimating mice and animals of that sort.”

Saleen Martin is a reporter on USA TODAY’s NOW team. She is from Norfolk, Virginia and loves all things horror, witches, Christmas and food.

Saleen Martin, sdmartin@usatoday.com, Twitter: @Saleen_Martin

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