Scientists Thought This Amphibious Mouse Was Extinct. After Almost a Century, It's Reappeared.

The Ethiopian water mouse (Nilopegamys plumbeus) has never been a household name—even in Ethiopia.

In fact, when the species was discovered in 1927 and formally described the following year, a single specimen was all researchers had to go on. And after that discovery, the mouse was never seen in the wild again, and when most of its habitat was lost to agriculture, it was presumed extinct.

So it came as a total shock when Mengistu Wale, an ecologist at the Ethiopian Biodiversity Institute, checked one of his traps during a biodiversity survey and saw a large, brown, beady-eyed amphibious rodent staring back at him.

“When I saw it for the first time, I was very excited,” says Wale. “I was 95 percent sure it was Nilopegamys.” Unlike many other native rodent species, Nilopegamys has rather conspicuous white spots behind its ears—a dead giveaway to its identity.

As an expert on Ethiopian mammals, Wale was well aware of the water mouse and concerned that it had seemingly disappeared. And for this reason, he often places traps along waterways and within wetland habitats, just in case. But even he didn’t expect to come face-to-face with the lost mammal.

Amazingly, Wale and his colleagues then captured two more specimens, and by comparing the animals’ features to those in the 1928 description, they believe they can say with confidence that the rodents belong to the long-lost species. The team plans to announce their find tomorrow, but genetic tests are also underway that will solidify the discovery.

“If confirmed, this would be remarkable news,” says Thomas Giarla, a mammalogist from Siena University who was not part of the biodiversity survey. “While I look forward to seeing the detailed verification data, Nilopegamys is distinctive enough that an expert visual assessment may well hold up,” he says. “If validated, it’s a thrilling reminder of how much tropical biodiversity remains hidden in plain sight!”

Part of what made the discovery so unexpected is where it happened. The Ethiopian water mouse was originally found in Northern Ethiopia, but past searches of the original collection site had all been “fruitless,” says Giarla. Instead, the new specimens were discovered in the Transition Zone of the Yayu Coffee Forest Biosphere Reserve, an area in South-Western Ethiopia that’s one of the few remaining areas of intact Afromontane rain forests, as well as one of the last places on earth where wild arabica, one of the plants that produces coffee beans, grows.

“What was quite fascinating was it was not in the middle of the reservation, but it was in fact close to the agricultural landscape next to the village,” says Alena Fornuskova, a mammalogist at the Institute of Vertebrate Biology of the Czech Academy of Sciences. “It was in the stream where people were washing their clothes on the exact same spot.”

With so few specimens and no natural history observations, it’s difficult to say much about a rodent that spends its life submerged in murky waterways. Even the team’s local guides, who are well-versed in many native, crop-raiding rodents, knew nothing of the water mouse. However, the mammal’s anatomy does hold some interesting clues about its life history.

Dense, silky fur may help the animal stay afloat while in water, for instance, while large hind feet may act like paddles. What’s more, “a large brain, swollen muzzle and extensive vibrissae [or whiskers] which may form a sonar system for detecting aquatic invertebrates,” according to the IUCN SSC Small Mammal Specialists Group.  

The scientists believe the water mouse may survive by hunting insects and possibly fish, but Wale says it was lured into the trap with regular ol’ peanut butter.

The fact that the rodent was found near people suggests that it may not be as vulnerable to development as once thought, says Fornuskova. However, habitat remains a major concern for the species—the northern habitat where the species was originally found seems to have been nearly entirely converted to areas more conducive to livestock tending.

When people think about rare or endangered species, rodents don’t tend to be top of mind. But there are more than 2,000 rodent species on the planet today, and nearly one-quarter of those are of conservation concern.

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And Ethiopia is a good example of how these threatened creatures may survive out of sight, even when experts are looking for them—a few years ago, researchers also discovered a new genus of rodent called Chingawaemys in the nearby Chingawa Forest.

With limited funding, conservationists can only spend so much time looking, as well. The expedition that re-discovered the amphibious rat was the first comprehensive faunal survey of its kind in the biosphere reserve, says Getachew Mulualem, a mammalogist also with the Ethiopian Biodiversity Institute. “There is still a lot of biodiversity hidden in these forests that we have yet to document and understand,” he says.

While no one had seen the Ethiopian water mouse in nearly a century, the species was still classified as critically endangered by the International Union for Conservation of Nature.

“Small, semi-aquatic mammals are notoriously elusive because standard mammal surveys tend to rely on dry-land traps, so aquatic species easily go undetected,” says Giarla, who, in 2021, extracted DNA from the original Nilopegamys specimen to help figure out its place in the rodent family tree.

But now that it’s back, the researchers hope to learn more about the water mouse’s lifestyle, using metabarcoding techniques to discover what it ate recently, says Fornuskova.

And the mouse may be one of many surprises hiding in plain sight in Ethiopia’s remote forest ecosystems, Mulualem notes. “If we conduct more comprehensive surveys across additional localities and habitats, I am confident that many more surprises, including potentially new species and important range extensions, are waiting to be discovered.”