Male mice are utterly terrified of bananas

Plus other weird things we learned this week.
a big pile of bananas
Deposit Photos

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FACT: Male mice are terrified of bananas

By Rachel Feltman

A while back, researchers at McGill University were studying pain sensitivity in mice and noticed something weird: When pregnant female mice—which were being used for another experiment—were kept close by, the male subjects started acting strangely. A grad student picked up on the fact that they were “aggressive” and had “super-high pain thresholds” when the pregnant females were in the area. 

So, not only is this a weird quirk that begs explanation, but it was also potentially messing up the study’s results—and maybe even the results of other, previously published studies where scientists had inadvertently skewed data by keeping pregnant mice around. The researchers decided to start a new experiment to investigate further. 

They zeroed in on the fact that soiled bedding from a pregnant female was enough to give males the superhuman (supermouse?) pain tolerance. A look at their hormonal levels also showed they were experiencing a spike in stress. They eventually isolated the chemical n-pentyl acetate, which can appear in female mouse urine, as the signal the males were reacting to.

Totally coincidentally, n-pentyl acetate is what gives bananas their signature odor. The researchers picked up some banana oil from a local supermarket and doused cotton balls in it to see if their presence would have the same effect. Sure enough, the banana funk raised stress hormones and lowered pain sensitivity. In both cases—urine and banana—the effect kicked in within five minutes and lasted about an hour. 

The researchers think this hormonal spike directly relates to a fight or flight response. Why would pregnant mice (and, as a result, bananas) have such an effect on young, healthy males? Because pregnant mice, generally speaking, can and will kick the absolute crap out of a young male mouse. 

Male mice, especially virgins (which researchers call “sexually naive”), have a tendency to try to kill babies. Rodents in general are more open to infanticide than we would like; females of many species will chow down on their children if something makes them smell unfamiliar, or if they have too many babies at once. Meanwhile, males of many species will go after pups that aren’t theirs. There have been some studies that suggest that introducing the smell of an unfamiliar male is enough to make certain rodent mamas stop caring for their pups, presumably because they assume some dude is going to come eat them and don’t want to waste energy on them in the meantime. 

By the way, young females without pups are also known to sometimes go bananas on other mice’s babies. They stop this behavior once they have kids of their own. Researchers have found that a whole region of the brain quiets down after a mouse gives birth, and that chemically blocking it can keep young ladies from getting infanticidal—while stimulating it can send any mouse on a baby-eating rampage. 

Research on the Chilean degu, an adorable rodent that nests in big social groups and does not put babies on the menu, suggests that there are genes that make a species more or less likely to go the infanticide route. Communal nesting may have evolved as an alternative to violent male competition. 

Back to mice: Studies have shown that a gene called Trpc2 is a big factor in determining whether a mouse is a parent-of-the-year—or at least a decent babysitter—or an infanticidal brute. Females who lack the Trpc2 gene act like males, which is to say they run around harassing pups and trying to mount other adult mice. When scientists engineered a male mouse with an activated Trpc2 gene, he reacted to the introduction of strange pups by building a nest and gently placing the foundlings inside. 

The protein that Trpc2 encodes is crucial in allowing animals to sense pheromones, which brings us back to the banana business. 

Basically, the researchers concluded that nursing mothers give off this chemical signal to say “don’t mess with me or my pups, or I will mess you up”—and males have evolved to actually listen, or at least get ready for the fight of their lives. 

Obviously this finding is kind of weird and funny, but it’s also important. Just like the researchers in this study, many scientists studying animal behavior in the lab are inadvertently introducing variables that mess up their data. The researchers point out that female pheromones, in particular, have been woefully underrepresented in the scientific literature—male animals are more likely to be studied, which is a known scientific bias and a huge problem in actually understanding how animals work. Plus, who knows: maybe even a grad student’s unfortunate choice of snack could insidiously skew an experimental result. 

FACT: One of our ancestors had extremely swole leg muscles

By Sara Kiley Watson

Human lineage split from chimpanzees (are our closest living relatives today) about seven million years ago, but early, primitive primates were evolving around 55 million years ago. Fast forward to 2 or 3 million years ago, and we’ve got the earliest tools being made, and also around the middle of when Australopithecus africanus’s reign, which had a bigger brain than its predecessors. Fast forward again to November 1974, and scientists discovered Lucy, a 40-percent complete fossil of a young female Australopithecus afarensis.

While the discovery of Lucy changed archeology forever, there are still loads of questions about who she was and how she lived. But recent research shows that Lucy was probably a lot more like modern humans than we realized when it comes to her ability to walk around on two legs—and she had the super-powered muscle mass in her legs to prove it.

FACT: Wildfires and fungal spores are dangerously intertwined

By Kasha Patel

If we learned anything from The Last of Us, it’s that some fungi can appear immutable and even indestructible. We take a tour on how fungal infections are linked with wildfire smoke, how fungi are adapting to warmer temperatures and why infections are hard to treat.