The naked mole-rat is a seemingly strange creature. This blind, cold-blooded mammal burrows underground in a network of subterranean tunnels — ruled by a queen overlord — rarely making its way to the surface. It has scrunchy skin, minuscule ears, and is, well, naked.
But despite these bizarre traits, the naked mole-rat is more humanlike than it appears. New research suggests how they communicate hides clues for better understanding the origin of human language. Meanwhile, their xenophobic tendencies hint at something complicated, and unexpected — culturally transmitted dialects. These dialects contribute to maintaining social order, but they also lead to aggression against outsiders.
This is the first such discovery in a rodent. The finding was published Thursday in the journal Science.
Some background — Scientists have some understanding of naked mole-rat socialization: they live their lives in colonies, they eat each other's poop, and they call to each other and convey information through "chirps." This team wanted to find out more by studying the rodents' distinct vocalizations.
They hypothesized the chirps might serve a social function. Previous studies have found that due to limited food supplies, the queen limits breeding in colonies, producing both tightly-knit social communities — and a strong fear of outsiders.
Professor Gary Lewin, head of the Molecular Physiology of Somatic Sensation Lab at the Max Delbrueck Center for Molecular Medicine in the Helmholtz Association (MDC), said in a press statement: "We wanted to find out whether these vocalizations have a social function for the animals, who live together in an ordered colony with a strict division of labor."
Previous studies have found that due to limited food supplies, the queen limits breeding in colonies, producing both tightly-knit social communities — but also a strong fear of outsiders
"You might even say that these animals are extreme xenophobes," co-author Gary Lewin, head of the Molecular Physiology of Somatic Sensation Lab at the Max Delbrueck Center for Molecular Medicine, said.
"This behavior is probably a result of the permanent shortage of food in the dry plains of the naked mole-rat's East African habitat."
When dealing with members of their own colony, however, the rodents work together in peace.
How they did it — Although the naked mole-rat can produce 17 distinct sounds, the researchers were most interested in studying the "soft chirp" they use when responding to other noises.
Over a two-year period, the scientists recorded more than 36,000 sounds from naked mole-rats in seven colonies. They used a machine-learning algorithm to analyze eight different features of the soft chirp in order to "predict the identity of individuals within a colony."
The researchers also tested for colony-specific dialects, using the same method. They then placed the naked mole-rats in two interconnected chambers with audio equipment that could playback and record noises.
What's new — Researchers learned a few surprising things about naked mole-rats during their study, including how the soft chirping sounds contribute to the social order of the colony.
For example, individuals within each colony produce distinct soft chirp vocalizations — the algorithm picked up on those individual sounds with a reliable degree of accuracy.
Each colony also has its own unique soft-chirp signal or "dialect." This suggests the blind naked mole-rats can produce distinct chirping sounds, allowing them to distinguish members of their clan from outsiders.
Naked mole-rats also exhibited a distinct, powerful preference for the sounds of their own colony, reflecting the rodent's xenophobic behavior towards foreign members of their species.
Digging into the details — The team also got the chance to test their vocalization hypothesis during times of social unrest. During the study, one of the colonies lost two of its queens in a violent upheaval.
By recording soft chirps during these periods of naked mole-rat anarchy, the researchers found that the noises varied more in frequency in the queen's absence. The algorithm also couldn't predict the colony's dialect as accurately during periods of anarchy.
This suggests the queen maintains the colony's language — her very presence a driver of dialect cohesiveness.
The researchers also wanted to know whether these colony-specific vocalizations remain strong across numerous generations. The scientists write:
"If naked mole-rats use distinct colony dialects to differentiate themselves from neighboring colonies or as a mechanism for ensuring conformity within the colony, such dialects must be maintained across generations."
To test this, the team transferred abandoned rodent pups from their home colonies to new ones. All of the abandoned pups, in turn, adopted the chirping dialect of the new colony, suggesting the unique language does transfer across generations.
Why it matters — Although other animals, such as bats and primates, also communicate through acoustic vocalizations, scientists have not previously observed this behavior in rodents.
In a related commentary, Rochelle Buffenstein, a senior principal investigator at Calico, writes: "This is an astonishing feat for a rodent and is in stark contrast to the majority of mammalian vocalizations, which are innate, immutable, and genetically inherited.”
The study could also help scientists better understand the development of human language. According to the research, the scientists found similarities between the way naked mole-rats and humans both communicate to signal individual and group identity. The study team writes:
"With a simple vocal greeting, humans convey individual identity (distinctive voice) and cultural identity (dialect usage); here we show that naked mole-rats also signal social membership with dialect usage."
What's next — The researchers hope to build on this study's surprising findings in future work.
Specifically, they want to identify whether they are truly capable of complex acoustic learning, similar to songbirds, or whether they're simply "exceptionally good usage learners" like non-human primates — communicating based on social conditioning and observation.
Such findings could help scientists better understand the development of different types of social communication in the animal kingdom.
Abstract: Naked mole-rats (Heterocephalus glaber) form some of the most cooperative groups in the animal kingdom, living in multigenerational colonies under the control of a single breeding queen. Yet how they maintain this highly organized social structure is unknown. Here we show that the most common naked mole-rat vocalization, the soft chirp, is used to transmit information about group membership, creating distinctive colony dialects. Audio playback experiments demonstrate that individuals make preferential vocal responses to home colony dialects. Pups fostered in foreign colonies in early postnatal life learn the vocal dialect of their adoptive colonies, which suggests vertical transmission and flexibility of vocal signatures. Dialect integrity is partly controlled by the queen: Dialect cohesiveness decreases with queen loss and remerges only with the ascendance of a new queen.