Listen: Mice ‘argue’ about infidelity in ultrasound

If you came across California mice in the wild, you wouldn’t hear a thing. Their jabber is ultrasonic—humans hear it only when it's slowed to five percent its original speed. But that’s when the imperceptible squeaks morph into a vocal range that’d put Mariah Carey to shame.

Mice, you see, regularly vocalize to communicate in many different situations—which researchers did not know until recently.

“It’s an under-appreciated part of biology of one of most diverse groups of mammals,” says Matina Kalcounis-Rueppell, a professor of biology at University of North Carolina, Greensboro who discovered about a decade ago that these mice vocalize.

These sounds range from coos to startling barks. New research published in Frontiers in Ecology and Evolution shows that when these monogamous mice are separated from their mate and then reunited, the animals sometimes don’t handle it well—revealing a new side to their social lives and behavior.

Here are some of the mouse calls recorded by Josh Pultorak, who recently earned his PhD with principal investigator Catherine Marler at the University of Wisconsin-Madison in the course of this research. The first sounds, short tweets, are considered friendly, and the most common. The second, slightly longer calls appear when the mice are getting “lovey-dovey”, says Pultorak. The third whale-like yelps are also friendly and connote a strengthening relationship.

But the last group of calls, which sound like angry barks, are unambiguous. They roughly translate to “get the hell out of here,” says Pultorak.

These angry calls were observed after mouse couples were separated, placed with other mice, and then brought together again. About half of the mice responded this way.

Even in the case of such “arguments,” the mouse couples eventually returned to a sense of normalcy. But mice that more quickly returned to friendly-sounding calls, or avoided such rancor altogether, were more likely to have and raise healthy offspring.

For these mice, “there’s some benefit to riding the wave out,” says Pultorak.

“It’s really exciting to see that the quality of social communication plays a role in determining mating success,” says Sarah Keesom, a biologist at Utica College in New York who wasn’t involved in the research.

Mouse social lives

Native to the coastal stretch between San Francisco and Baja, the California mouse is kind of a jerk. They’re territorial, aggressive and solitary, says Pultorak, until they find a mate. The mice bond for life, and unlike most rodents, the dad helps raise the pups. Without him there, the babies are less likely to survive, says Pultorak.

Their picky social dynamics and essential parent relationship makes them ideal for examining the function and importance of monogamy in mammals, how it might be challenged by “infidelity,” and what role communication and even reconciliation may play.

“Like it or not, we have a very similar basic system, hormonally and brain-wise,” Pultorak says. Mice provide insight into mammal behavior stripped of all the cultural layers humans add.

To understand the role of separation on social bonds, Pultorak and his colleagues split some couples by putting one in an arena with a member of the opposite sex. They also left some pairs together and separated some couples into solitary cages. When reunited, the team looked for which couples sniffed and followed each other—signs they were getting along—and which pairs wrestled or bit. They also listened to their super-slow calls.

A whole new world of research

To be clear, the research team doesn’t know whether or not the mice actually mated with someone new, and Pultorak doesn’t think the mice can tell if their partner had a fling. Rather, he says, because they’re solitary, single-bond creatures, their behavior probably shows whether or not that bond has dissolved in the time apart.

Before Pultorak’s work, researchers didn’t even know for sure that these different calls had specific social connotations. When Kalcounis-Rueppell first discovered these sounds, she had a hunch that was the case. But this research has proved that suspicion true, and expanded to show how essential they are to mating success.

That’s part of why researchers hope to keep studying the mouse calls. So little is known, and yet it promises to change or drastically add to much of what we understand. Kalcounis-Rueppell likens the study of such communication to research on birdsong; Surely, it would be ridiculous to try to understand birds without listening to them, and the same is true for mice.

“This opens up possibilities for asking all kinds of behavior questions,” Kalcounis-Rueppell says. “And this is just one mouse-like rodent species out of 1,300.”