Newly discovered mechanism in the throats of mice works like a supersonic jet engine

by Bryan Nelson

Mice are known for their squeaks, but scientists have just discovered how these diminutive rodents are also capable of making ultrasonic vocalizations far beyond the human capacity to hear. And the secret songs they sing usually take the form of love ballads for their mates.

Though researchers have known for a while that a fair amount of mouse communication happens at ultrasonic frequencies, they’ve only just figured out how the rodents do it. Using ultra-high-speed video recording at a whopping 100,000 frames per second, the team was able to see that a mouse is capable of pointing a small air jet, which comes from the windpipe, to blow against the inner wall of the larynx. This causes a resonance and produces an ultrasonic whistle.

“Mice make ultrasound in a way never found before in any animal,” said study lead author Elena Mahrt, from Washington State University, in a press release.

The mechanism is so bizarre that its closest analogue might be in human technology. Namely, what’s happening in the throats of mice is akin to a jet engine.

“This mechanism is known only to produce sound in supersonic flow applications, such as vertical takeoff and landing with jet engines, or high-speed subsonic flows, such as jets for rapid cooling of electrical components and turbines,” said study co-author Dr. Anurag Agarwal. “Mice seem to be doing something very complicated and clever to make ultrasound.”

Humans can’t hear these sounds, and maybe that’s the point. Singing in ultrasound allows mice to communicate at frequencies that many other animals can’t hear. That’s a boon when you’re often on the menu for most other larger predators.

Scientists think that the ultrasound whistles are actually mating calls, often sung by males to attract females; a mouse version of a “cat call,” perhaps. The sounds are also likely used for signaling territorial boundaries to rivals, though the full extent and use of the songs is still being studied. It’s even possible that this ultrasound mechanism was a prerequisite for the echolocation abilities seen in bats.

“Even though mice have been studied so intensely, they still have some cool tricks up their sleeves,” said senior author Dr. Coen Elemans.

Mice run for fun on wheels out in the wild.

By James Gorman

If an exercise wheel sits in a forest, will mice run on it?

Every once in a while, science asks a simple question and gets a straightforward answer.

In this case, yes, they will. And not only mice, but also rats, shrews, frogs and slugs.

True, the frogs did not exactly run, and the slugs probably ended up on the wheel by accident, but the mice clearly enjoyed it. That, scientists said, means that wheel-running is not a neurotic behavior found only in caged mice.

They like the wheel.

Two researchers in the Netherlands did an experiment that it seems nobody had tried before. They placed exercise wheels outdoors in a yard and in an area of dunes, and monitored the wheels with motion detectors and automatic cameras.

They were inspired by questions from animal welfare committees at universities about whether mice were really enjoying wheel-running, an activity used in all sorts of studies, or were instead like bears pacing in a cage, stressed and neurotic. Would they run on a wheel if they were free?

Now there is no doubt. Mice came to the wheels like human beings to a health club holding a spring membership sale. They made the wheels spin. They hopped on, hopped off and hopped back on.

“When I saw the first mice, I was extremely happy,” said Johanna H. Meijer at Leiden University Medical Center in the Netherlands. “I had to laugh about the results, but at the same time, I take it very seriously. It’s funny, and it’s important at the same time.”

Dr. Meijer’s day job is as a “brain electrophysiologist” studying biological rhythms in mice. She relished the chance to get out of the laboratory and study wild animals, and in a way that no one else had.

She said Konrad Lorenz, the great-grandfather of animal behavior studies, once mentioned in a letter that some of his caged rats had escaped and then returned to his garden to use running wheels placed there.

But, Dr. Meijer said, the Lorenz observation “was one sentence.”

For the experiment, the wheels were enclosed so that small animals could come and go but so that larger animals could not knock them over. Dr. Meijer set up motion sensors and automatic video cameras. Several years and 12,000 snippets of video later, she and Yuri Robbers, also a Leiden researcher, reported the results. They were released in the Proceedings of the Royal Society B.

Gene D. Block, chancellor of the University of California, Los Angeles, was not involved with the paper but knows Dr. Meijer and had seen the wheel set up in her garden. He said the study made it clear that wheel-running is “some type of rewarding behavior” and “probably not driven by stress or anxiety.”

Mice accounted for 88 percent of the wheel-running events, and spent one minute to 18 on the wheel. The other animals each accounted for less than 1 percent. Frogs, though there were very few, were seen to get on the wheel, get off and get back on.

Russell Foster, a circadian rhythm researcher at Oxford University, said he read the paper and sent it out to other scientists on behalf of the Proceedings and was delighted when peer reviews from other scientists were positive.

Marc Bekoff, a professor of ecology and evolutionary biology at the University of Colorado who is active in the animal welfare movement, said in an email that he thought the paper did show that wheel-running could be a “voluntary activity,” but that mice in labs may be doing more of it because of the stress of confinement.

“Wild bears will often pace back and forth,” he wrote, “but in captivity, the rate of doing it seems to be greatly heightened.”

As to why the mice, frogs or perhaps even slugs run, or move, on the wheel, Dr. Meijer said she thought that “there is an intrinsic motivation for animals, or should I say organisms, to be active.”

Huda Akil, co-director of the Molecular and Behavioral Neuroscience Institute at the University of Michigan, who has studied reward systems, said: “It’s not a surprise. All you have to do is watch a bunch of little kids in a playground or a park. They run and run and run.”

Dr. Akil said that in humans, running activates reward pathways in the brain, although she pointed out that there are innate differences in temperament in all sorts of animals, including humans. Rats that do not like to run can be bred. And plenty of people do all they can to avoid jogging, cycling and elliptical machines.

Presumably, the same is true of wild mice. While some were setting the wheel on fire with their exertions, others, out of camera range, may have been sprawled out on the mouse equivalent of a lounge chair, shaking their whiskers in dismay and disbelief.

Thanks to Dr. Nakamura for bringing this to the attention of the It’s Interesting community.

Pee marks the spot


Human beings tend to avoid places that smell of urine. But to mice, there is something positively addictive about the scent; they like to go back to a spot where they found the excretions again and again. Now, researchers have discovered that this behavior is triggered by a single protein in the urine of male mice.

Mice use scent to mark their territory, advertise their social dominance, and convey information about their health and reproductive status. But these are usually volatile pheromones that disperse quickly, and it has remained unclear what exactly stimulates a female to be attracted to a specific male.

Previous research had shown that female laboratory mice often return to a place where they have come across cage bedding soiled by males. Now, researchers at the University of Liverpool in the United Kingdom have confirmed this. Female mice spent five times as much time in a place where they had encountered a dish with male urine than at a place where they encountered water. Just 10 minutes of exposure to the urine was enough for the mice to show this place preference even after 14 days.

However, if the mice were prevented from by a mesh screen touching the urine with their nose, the place seemed to lose its attractiveness. “That suggested that the story was not as simple as everybody assumed and volatile pheromones were not responsible,” says behavioral ecologist Jane Hurst, one of the authors of the study. By separating the urine into different fractions, the scientists showed that a protein called darcin that they had identified in 2005—and which mice can only detect if their noses touch the urine—is responsible for the frequent visits. Pure darcin, produced in cell culture in the lab, elicited the same reaction, the authors report online today in Science.

“This is a really compelling story,” says Lisa Stowers, a neuroscientist at the Scripps Research Institute in San Diego, California. “Mammals were thought to be much more complex, but this study shows that a single chemical can lead [them to act] in a certain way.” The study is “very simple and elegant,” she adds. But it also raises new questions. For instance: There are many other ways a mouse could learn to return to a certain place. “So what is the benefit of evolving this [special] mechanism?”

Hurst says that what fascinates her is that the pheromones induce learning in the mice. And the animals do not only learn to be attracted to the place where they encountered the darcin. “They learn the odor cues of that specific male and are then attracted to it,” Hurst says. “Being familiar with a scent really seems to be important for whether a female is interested in a male.” The reason, Hurst suggests, is that dominant males, who make attractive mates, tend to leave the most marks in a certain territory.

The researchers showed that male mice, too, are attracted to a place if they have encountered darcin there, probably to foster a behavior called countermarking. “If males come across another male’s scent mark, they put their own, fresher urine there,” Hurst explains. This could also be the reason why some laboratory strains seem to have lost the ability to produce darcin: Because laboratory mice are usually group-housed, they have been selected to be less aggressive, and not producing darcin could help reduce tensions.

Thanks to Dr. Rajadhyaksha for bringing this to the attention of the It’s Interesting community.