Researchers discovered that male mice are afraid of the scent of bananas when researching the reactions of male mice to pregnant and breastfeeding female mice.
A team from McGill University in Montreal, Quebec, studied stress hormone reactions in male mice when they were near late-pregnant female mice.
“In the presence of late-pregnant or nursing female mice, male mice exhibit stress and stress-induced [pain inhibition,” the researchers write in their paper, published in Science Advances. “We also observe in our own hands, as has previously been shown, that late-pregnant and breastfeeding female mice respond aggressively and with urine marking to gonadally intact stranger male mice.”
To defend their own DNA, male mice are known to be aggressive and infanticidal to pups. Heavy pregnant and lactating mice generate substances to keep these males away, as well as vigorously defending their young.
“Rodents and many mammals other than humans rely on their olfactory senses,” said the study’s principal author, Professor Jeffrey Mogil, to Live Science. “Urine scent-marking is well recognized, but what we’ve discovered here is a novel message in mammals that has never been described before.”
“There have been many examples of olfactory messages being conveyed from men to females, but there have been fewer examples of females sending them to males. The majority of these messages are about sexual conduct, but in this case, sex has nothing to do with it. The ladies are warning the males to stay away or else I will beat the heck out of you if you touch my babies.”
So, how do bananas fit into this? Are they also sending out a warning to male mice? Yes, by chance, but not for the purpose of defending their own mouse babies. The researchers discovered that the molecule n-pentyl acetate – prevalent in female mice’s urine, particularly later in pregnancy and lactation – was one of the compounds that had the greatest effect on hormone alterations in male mice.
“n-Pentyl acetate is extremely similar in chemical structure to isoamyl (or isopentyl) acetate, and both are found in a number of fruits and utilized to generate banana oil/extract,” the researchers notes in the report.
The researchers purchased banana oil extract from a shop and placed it in the cages of male mice to assess their stress levels, which increased dramatically in response. The researchers believe that the stress reaction in mice is similar to the stress response in humans before a fight.
“Although maternal aggression does not always prevent male intruders from committing infanticide, any threat of violence is likely to produce stress in both parties, and maternal aggression has been shown to directly produce stress-induced [pain inhibition] in males measured after the attacks,” the researchers wrote in their study.
“What we show here is that stress-induced [pain inhibition] may be observed in male mice even in the absence of genuine maternal aggression; the mere threat of such aggressiveness is sufficient, and this threat is communicated via volatile urine chemicals.”
The presence of n-pentyl acetate, whether in banana or mouse urine form, was found to be more stressful in virgin male mice. This is consistent with their proclivity to be more aggressive toward newborns than non-virgin mice, implying that they pose a greater threat to infants than older males.
The team stated, “The current data show that close proximity of male subjects to reproductively active females is a hitherto undiscovered stressor for the males, and that stress may even be generated by the presence of particular meals.”
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