The Washington PostDemocracy Dies in Darkness

The mystery of the great naked mole-rat migration

A naked mole-rat at the National Zoo in 2019. (Astrid Riecken for The Washington Post)
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Every so often, when night falls on Meru National Park in Kenya, an event takes place few people have ever witnessed — the migration of the naked mole-rat.

Naked mole-rats are mostly hairless, nearly blind subterranean rodents that resemble wrinkly pink sausages. From skin-covered eyes to sabre-tooth-like incisors built for gnawing through compacted soil, most everything about these animals is adapted to a life spent below ground, where they dwell in giant, queen-dominated colonies. Down there, the rodents have virtually no predators and no competition for the roots and tubers they eat.

Occasionally, one of these creatures peels off from its group and peeks out its head. After a few sniffs to make sure the coast is clear, the lone mole-rat leaves the safety of the only home it has known and waddles into the dark, dangerous night. Eventually, if the voyager isn’t devoured by a baboon or Kenyan sand boa, the mole-rat will stop wandering and start chewing — down, down into the rust-colored dirt. With luck, it will soon be joined by another naked mole-rat, preferably of the opposite sex, and the two will set about creating a new colony from scratch.

Scientists have been studying how naked mole-rats create new colonies for decades, even though none has seen the behavior in the wild. But nearly everything about the wild migration — how the sightless animals decide where to head and where to stop, and how they find each other — is a mystery, even to a researcher who has studied the animals for four decades.

Stan Braude has been catching, marking and releasing naked mole-rats and recently tested a tantalizing theory that the animals time their dark travels to coincide with the phases of the moon. They don’t.

“The data told us we were wrong,” said Braude, an evolutionary biologist at Washington University in St. Louis who published his results in the African Journal of Ecology. “Just when you think you’ve answered one question, a dozen more pop up.”

Some riddles about these enigmatic creatures do have answers. Naked mole-rats seem to have some resistance to cancer and can live 30 years or more in captivity, which is extremely long for a rodent. They are shorter than a dollar bill, and yet a single colony can create a tunnel network that snakes beneath an area the size of six football fields. They live in highly social societies subservient to an enormous, menacing mole-rat queen.

Originally, Braude set out to study the animals’ behavior in their native East Africa, but the mole-rats were so secretive he ended up embarking on a massive mark-and-recapture study — something like a naked mole-rat census. “By the early 2000s, we had close to 10,000 animals marked,” he said. That research has led to some naked mole-rat breakthroughs.

It used to be thought that naked mole-rats never surfaced at all. The assumption was that new colonies were formed by fission, or the rupture of an already existing colony. But the more time Braude spent trapping the rodents by sinking tubes into their burrows, the more he found individuals and small groups — just one, or two or three mole-rats where he expected 300.

Then, in 1996, a researcher in South Africa, Justin O’Riain, added another fantastically bizarre piece to the puzzle. By examining the physical and chemical characteristics of mole-rats that leave their burrows, he showed colonies sometimes produce animals that are quite different from the rest.

Such animals, O’Riain wrote, are “laden with fat,” surging with reproductive hormones, unwilling to mate with other members of their own colonies, and typically pretty unhelpful when it comes to the colony’s day-to-day chores. Such indolence would normally result in a swift mauling by the queen, but for some reason, the “disperser morphs,” as O’Riain dubbed them, got off scot-free.

In time, O’Riain would come up with another name for the unusual, portly mole-rats: Houdinis.

“Their desire to escape was overwhelming,” O’Riain, a behavioral ecologist at the University of Cape Town, said in an interview. Even after researchers put bricks on top of their enclosures, the Houdinis managed to get out in ways O’Riain still doesn’t understand.

In one trial, O’Riain removed run-of-the-mill naked mole-rats from captive colonies and placed them inside foreign colonies. As predicted, the accidental intruders seemed immediately to detect they were not at home, probably because each colony has a distinct odor profile, and they would try to back their way out.

“Naked mole-rats hate each other,” O’Riain said. Other experiments had revealed, gruesomely, that if researchers placed two naked mole-rat colonies side by side, the scene at the lab the next morning “would be a bloodbath.”

But the experiment yielded a different result if O’Riain dropped a Houdini into a foreign colony. Instead of cowering, the animal ran “at full speed into the middle of the colony and tried to copulate with anyone and everyone that it found,” O’Riain said.

And it was the same story if two Houdinis were placed together. “Instead of doing the norm for the species, which was fight to the death, they would copulate with each other” — and with gusto, O’Riain said.

“That nailed it,” Braude said. Between his data showing marked mole-rats turning up in colonies great distances from their birthplaces and O’Riain’s behavioral observations from captive populations, it became clear that naked mole-rats not only sometimes come to the surface but also produce pioneer mole-rats specially adapted for the journey.

For all his time studying these creatures, Braude has never seen a mole-rat doing its midnight shuffle. Partly, this is because the journey is undertaken only by a handful of animals, so the likelihood of catching them in the act is vanishingly low. But also, naked mole-rats are native to grasslands of Kenya, Somalia and Ethiopia — places where people don’t usually go for a stroll at night. “Because of lions,” Braude said.

Nearly two decades ago, he conducted a study that further proved the rodents sometimes come out at night. Mole-rats can’t jump, so by erecting hundreds of feet of four-inch-tall fencing that ended in submerged buckets, he was able to catch just under a dozen wandering mole-rats over the course of four years. Some had traveled well over a mile from their birthplaces.

This year, Braude reevaluated data from that study, as well as nearly a decade’s worth of captive colony data, to look for a link between when mole-rats wander and the phases of the moon.

He theorized the critters might prefer to come out in the dark of a new moon, because predators might have a harder time finding them. After all, the moon guides the behavior of plenty other animals, from oysters and coral reefs to wildebeest. But not, evidently, mole-rats.

Braude didn’t view it as a failure. There are so many mystifying facets of naked mole-rat life that simply crossing one possibility off the list is progress.

“It’s great that Stan’s still out there doggedly pursuing this because I do think it’s very interesting,” said O’Riain, “but I think it’s an extraordinarily difficult thing to do.”

This fall, Braude’s work will go on. Providing the coronavirus pandemic allows, he plans to return to East Africa to assist with a new naked mole-rat trapping project. Perhaps he’ll finally run into a naked-mole rat roaming through the night.

“I have lots of questions,” Braude said.

Read more:

Naked mole-rats are now even weirder: Without oxygen, they live like plants

No beauty, no pain: How naked mole rats handle the heat

Video: How do naked mole-rats choose their queen?