While studying the reaction of male mice to pregnant and lactating female mice, researchers made a related and somewhat odd discovery: male mice are terrified of the smell of bananas.
A team from McGill University in Montreal, Quebec, were looking at spikes in stress hormone responses in male mice when near late-pregnant female mice.
"Male mice display stress and stress-induced [pain inhibition] in the close proximity of late-pregnant or lactating female mice," the team write in their report, published in Science Advances. "We also observe in our own hands, as has been seen before, that late-pregnant and lactating female mice respond to gonadally intact stranger male mice with aggression and urine marking."
Male mice are well-known for being aggressive and infanticidal to pups, in order to protect their own genetics. Heavily pregnant and lactating mice – as well as defending their infants aggressively – emit chemicals to warn these males away.
"Rodents and a lot of mammals other than humans are reliant on their olfactory senses," the study's senior author Professor Jeffrey Mogil told Live Science. "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."
So where do bananas come into this? Are they too emitting a warning to male mice? Coincidentally, yes, though not for the reason of protecting their own mice pups. The authors found that the compound n-pentyl acetate – found in female mice's urine, especially during later pregnancy and lactating – was one of the chemicals that produced the largest effect in hormone changes in male mice.
"n-Pentyl acetate," the team explains in the study, "is very similar in its chemical structure to isoamyl (or isopentyl) acetate, and both are found in a variety of fruits and used to produce banana oil/extract."
The team bought banana oil extract from the supermarket and placed it inside the cages of male mice to measure their stress levels, which increased significantly in response. The team believe that the stress response in the mice is similar to the stress response when about to engage in a fight.
"Although maternal attack does not always succeed in preventing 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 team wrote in their study.
"What we demonstrate presently is that stress-induced [pain inhibition] in male mice can be observed even in the absence of actual maternal aggression; the mere threat of such aggression is enough and that this threat is communicated via volatile urinary compounds."
The study found that virgin male mice were more likely to be stressed out by the presence of n-pentyl acetate, whether in banana or mouse pee form. 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.
"The current findings suggest that close proximity of male subjects to reproductively active females is a previously unknown stressor for the males," the team concluded, "and that stress may even be caused by the proximity of certain foodstuffs."