Female mice that are heavily pregnant or have recently given birth produce a banana-smelling chemical in their urine that stresses out males, possibly to stop them from killing their pups.
Jeffrey Mogil at McGill University in Montreal and his colleagues discovered this behaviour by accident. “We were doing experiments with pregnant female mice and noticed that male mice that were being used for other experiments in the same room were acting a bit crazy,” he says.
To explore further, they tested the stress levels of male mice when they were placed in a cage near that of another male mouse or a female that was either not pregnant, newly pregnant, heavily pregnant, had recently given birth and was lactating, or had given birth in the past and was no longer lactating.
The male mice showed reduced pain sensitivity and elevated corticosteroid levels – which are both signs of stress – when they were caged near female mice that were heavily pregnant or lactating, but not when they were near the other mice.
The researchers discovered that this was because heavily pregnant and lactating females produced a chemical in their urine called amyl acetate, which smells of bananas. This wafted into the males’ nearby cages and made them stressed when they sniffed it.
Just exposing the males to this chemical alone made them stressed, even when there were no pregnant or lactating females around.
Females probably release this chemical when they are about to have pups or have just had them to let males know, “if you come any closer, I’ll beat the crap out of you”, says Mogil.
This is because male mice try to kill pups that have been fathered by other males, he says.
In line with this, pregnant and lactating females left more urine marks when they were exposed to stranger males than when they were exposed to the father of their pups.
“Females are known to unleash serious aggression if males try to attack their pups so we think that when males smell this chemical in their urine, the prospect that there might be a fight causes their stress response,” says Mogil.
The researchers didn’t test if sniffing this chemical did in fact stop males from killing pups because it would be unethical to conduct that kind of experiment, says Mogil.
The findings have implications for other mouse research since some scientists may unwittingly be using stressed mice in their experiments if they house male mice near pregnant females, says Mogil. This could be one reason why different labs sometimes get different results from the same experiments, he says. “It’s something we need to pay more attention to.”
Journal reference: Science Advances, DOI: 10.1126/sciadv.abi9366