When a naked mole-rat encounters another, the accent of their tweets can reveal whether they are friends or enemies.
These social rodents are famous for their wrinkled, hairless appearance. But go around one of their colonies for a while and you’ll notice something else: they’re a talking group. Their underground burrows resonate with squeaks, growls, squeaks and almost constant screams.
Now, computer algorithms have uncovered a hidden order within this cacophony, researchers report on Jan. 29. These distinctive squeaks, which pups learn when they are young, help xenophobic rodents, mostly blind ones, to discern who they belong to, strengthening the bonds that maintain cohesion in these highly cooperative groups.
“Language is really important for extreme social behavior, in humans, dolphins, elephants, or birds,” says Thomas Park, a biologist at the University of Illinois at Chicago who did not participate in the study. This work shows that naked mole rats (Heterocephalus glaber) also belong to those ranks, Park says.
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Naked groups of mole rats look more like colonies of ants or termites than mammal societies. Each colony has a single breeding queen that suppresses the breeding of tens to hundreds of non-breeding working rats digging elaborate underground tunnels in search of tubers in East Africa (SN: 18/10/04). Food is scarce and rodents vigorously attack intruders from other colonies. Although researchers have long observed mouse squeaking, few have studied it.
“Naked mole rats are incredibly cooperative and incredibly vocal, and no one has really analyzed how these two characteristics influence each other,” says Alison Barker, a neuroscientist at the Max Delbrück Center for Molecular Medicine in Berlin.
For starters, she and her colleagues harnessed the computing power of machine learning to analyze more than 30,000 “soft squeaks,” a common vocalization, of seven lab colonies over two years. The analysis revealed that each colony had a unique sound, which varies mainly in frequency and how much that frequency changes in a single tweet.
Naked mole-mice also pick up on these differences, responding to the sounds of their own colony with frequent tweets, but largely ignoring foreign dialects, the researchers found. “That surprised us and suggests that the soft tweets could indicate that a naked mole rat belongs to the colony,” Barker says. Naked mole rats also do not respond only to voices they have heard before, as artificial calls agreed upon with a specific dialect also elicited an answer.
A bit of luck allowed Barker and his colleagues to test whether these dialects are learned or genetically coded. Most colonies reject foreigners, but sometimes offspring from other groups can be adopted by one colony (SN: 20/10/20). Several laboratory populations produced new litters at the same time, which allowed researchers to switch three young to new colonies. If the dialect comes from genetics, these outsiders should still look like outsiders. But if dialects are learned, transplanted puppies should look like their new siblings.
The latter was true. And the closer a puppy moved to the birth, the more it adjusted to the dialect of his new home.
“A sample size of three is small, but they are really hard experiments to do,” says Chris Faulkes, an evolutionary biologist at Queen Mary University in London who did not participate in the study. Still, he says the results strongly suggest that dialects of naked mole rats are learned, similar to those of humans, cetaceans, and some birds (SN: 02/07/20).
Although the sound of a colony is distinctive, it is not fixed. Researchers found that in periods of anarchy, when a queen dies and is not yet replaced, dialects began to dissolve, becoming much more variable. Once a new queen emerged, the colony became coherent again, suggesting that in addition to suppressing reproduction, queens also somehow control the voice of a colony.
Barker says the dialects probably play a role in maintaining the "exquisite cooperation" of naked mole rat societies. But they also reflect how vocal communication is another means by which queens suppress the individual interests of colony members for the good of the group.
“We tend to think of this communication and cooperation as positive aspects of naked rat-mole culture, but individuals are rigidly controlled in their behavior by the queen,” Barker says. "It gives them a huge survival advantage, but it's a bit like living in an oppressive regime."