What singing mice tell us about human neuroscience

In Central America, the Alston’s singing mouse has taken scientists by surprise. Carl Zimmer explains what this means for humans

A male sings in response to a female’s odour (Country Mouse Studios/YouTube)

High in the mountains of Central America lives a little-known creature called Alston’s singing mouse. This rodent, which spends its life scuttling around the floor of the cloud forest, may not seem as if it has much to tell us about ourselves.

But the mouse produces remarkable songs, and researchers have discovered some profound similarities to our own conversations. This ability may be linked evolutionarily to the ancient roots of human language.

Scientists have struggled for more than a century to work out the origin of language in our mammal ancestors.

“Until very recently, there was still this belief that human speech and mammalian vocalisations are two completely different things,” says Steffen R Hage, a neurobiologist at the University of Tubingen in Germany.

No other mammal has a brain capable of doing what is required for human language – from understanding the rules of grammar to coordinating quick, complex commands to muscles in the mouth and throat.

Early studies suggested that mammals used much simpler brain circuits for communicating.

If a monkey were to be confronted by another monkey, the thinking went, fear-processing centres of the brain would signal a cluster of neurons in the stem of the brain. The brain stem would then send commands to the mouth and throat to produce a call.

But it turns out monkeys can control their sounds in ways that early researchers did not recognise. Scientists can train monkeys to call only when they see a cue on a computer screen, for example. To exert this control, monkeys use clusters of neurons in the outer layer of the brain known as the cerebral cortex.

We have similar brain clusters, and they are essential for language. This likeness between humans and monkeys means that the building blocks of language evolved in our distant primate ancestors.

When scientists examined mice – far more distantly related to us than monkeys, obviously – they found no evidence of this sort of control. House mice, the favourite species of scientists, produce simple ultrasonic squeaks.

In 2011, Michael A Long, a neuroscientist at NYU Medical School, first heard about Alston’s singing mice and realised that when it comes to sound, they are a lot more interesting than lab mice. Singing mice produce arias of loud chirps that can last as long as 16 seconds, and each mouse produces its own distinctive song.

“This is their bar code that says, ‘This is me,’” Long says.

Alston’s singing mice sometimes belt out a song when they’re alone, but they are especially vocal when other mice are around. Males sing as a way to fight over territory with other males, and both males and females sing to one another during courtship.

Working with Steven M Phelps, a biologist at the University of Texas at Austin, Long set up a home for the mice in his lab to study their brains.

“They’re kind of divas,” he says. “They need exercise equipment in their cages and specialised diets. But they thrive here.”

One day, Andrew M Matheson, one of Long’s graduate students, noticed something odd about two male mice in neighbouring cages. Instead of singing over each other, they sounded as if they were having a conversation.

Long and his colleagues eventually discovered that Matheson’s hunch was correct. The singing mice never overlapped: each mouse would wait for the other to stop, and then start up within a fraction of a second.

“They’re polite in conversation,” says Arkarup Banerjee, a postdoctoral researcher in Long’s lab.

To Long, these patterns were strikingly similar to human conversation. “We’re tuned to be exceptional communicators,” he says. “It’s like a hitting a tennis ball right across the net, back and forth. And neuroscience hasn’t caught up with how the brain does this.”

So the researchers began probing the brains of the mice, searching for the neurons that led them to be “polite” raconteurs.

In one experiment, the researchers cooled down patches of mouse brain by a few degrees, slowing the neurons. One patch in the mouse cortex is essential for controlling their singing, the scientists found. If this section is cooled, the mouse sings extended songs, adding on extra notes.

The researchers also injected nerve-blocking drugs into this brain patch and then played a recording of another male. Drugged males often failed to sing back. And when they did, they were slow to begin, taking seconds to start their own song.

Long thinks this region of the mouse cortex is crucial to the mice’s special communication. “We think of it as a conductor,” he says. “It allows the animals to sing in this turn-taking way.”

The study was published in Science. Hage, who was not involved in the research, says the results were both surprising and persuasive.

They show for the first time that mammals other than primates can use the brain cortex to control their sounds. What’s more, Hage says, the findings raise the possibility that the common ancestor of humans and rodents, which lived some 100 million years ago, already had that ability.

“It’s a feature that, in the end, is crucial for the evolution of human speech,” Hage says.

The experience taught Long that there are perils in depending too much on one type of mice while ignoring the rest of biodiversity. “This exposes the major blind spot of betting everything on a single species,” he says.

It is possible that the circuits in Alston’s singing mice and humans are so similar that they are influenced by the same genes. That may make the mice good models for studying how autism leads people to have trouble with conversations – something that Long describes as “a black box”.

Long is now gearing up to genetically engineer Alston’s singing mice with some of the mutations linked to autism.

“We’re going to try to understand how they affect communication in a simpler system, so that we can get to the heart of what’s really going on,” he says.

© New York Times