Ethiopian Amphibious Rat: Only one specimen—and lost since 1927

A particularly remarkable novelty

It is known from a single specimen: an adult male that the American zoologist Wilfred Hudson Osgood collected on March 20, 1927, in the Ethiopian highlands—in a small mountain stream near the source of the Little Abbai. This animal later became known as the Ethiopian amphibious rat or forest stream mouse.

The discovery was made during an expedition by the Field Museum in Chicago to what was then Abyssinia. Osgood apparently realized quickly that he was not dealing with an ordinary rodent. Even before the entire collection of small mammals had been fully evaluated, he published the scientific description of the new species. He called it “a particularly remarkable novelty” that differed clearly from all known forms, and highlighted its “distinct adaptations to life in water”.

In fact, the Ethiopian amphibious rat combines several traits that are unique in this combination among African rodents. Its fur is extremely dense and soft, its ears are greatly reduced, and its unusually large, broad hind feet probably helped it swim. Osgood therefore compared it with aquatic rodents from other continents—such as the South American fish-eating rats of the genus Ichthyomys or the water rats of New Guinea and Australia (Hydromys). For Africa, however, he emphasized that he knew of no truly comparable animal.

The skull also provided clues to the species’ unusual way of life. Osgood was struck above all by its characteristic shape: above the eyes the skull is slightly depressed, while the snout curves upward. He knew similar structures from other aquatic rodents. He therefore suspected that the shape of the skull was connected with movement in water; it may have made it easier for the animal to hold its head while swimming so that the nostrils remained above the water surface.

It is also striking that the Ethiopian amphibious rat does not appear to be closely related to the African shaggy swamp rats (Dasymys), although these are also adapted to wet habitats. For Osgood, this made it plausible that a close association with water must have evolved independently in different African rodent lineages.

In his original description, published in 1928, Osgood therefore not only described a new species, but also placed it in its own genus: Nilopegamys plumbeus. This genus has remained monotypic to this day. No other animal has ever been assigned to it, and no second specimen of the Ethiopian amphibious rat itself has ever been found.

Ethiopian amphibious rat – fact sheet

alternative names	Ethiopian water mouse, Ethiopian water rat
scientific names	Nilopegamys plumbeus, Colomys goslingi plumbeus
English names	Ethiopian amphibious rat,
time of extinction	after 1927
causes of extinction	probably habitat loss
IUCN status	critically endangered (possibly extinct)

Range and habitat

Little can be said about the range of the Ethiopian amphibious rat beyond what goes back to a single moment in 1927. The known locality lies in the highlands of northwestern Ethiopia, in the former province of Gojjam, between Sakalla and Njabarra. The coordinates are given as approximately 11°07′20″ N, 37°12′40″ E.

It is a landscape of highland ridges, spring-fed streams and wet depressions. The locality was near the source of the Little Abbai, also called Gilgel Abay—a tributary of Lake Tana in the headwater system of the Blue Nile. The Ethiopian amphibious rat therefore did not live on a large river, but on a small stream at around 2,600 meters elevation.

Osgood (1928) described the immediate capture site as follows:

“The type specimen (…) was caught in a small, clear stream, probably unnamed, which flows into the Little Abbai not far from its source. The trap (…) was set in a narrow walkway that led from the water over a tiny island—little more than a plant-lined stepping stone.”

This brief description is almost all that is known about the original habitat of the Ethiopian amphibious rat. It was probably tied to cool, structured highland streams: shallow water, densely vegetated banks, small islands, moist hiding places between plants and stones. For a small mammal living near water, such places are ideal because they provide cover, access to water and probably food.

Whether the species occurred only in this one stream or also inhabited other spring-fed waters nearby remains unknown. At the time, Osgood had no opportunity to search comparable sites for the species. Later researchers likewise failed to find another specimen. In 2021, the ecologist Tom Dando estimated the possible range of the Ethiopian amphibious rat at less than ten square kilometers.

The loss of taxonomic independence

When Osgood formally described the Ethiopian amphibious rat in 1928, it did not fit into any of the known genera in his view. Its traits were so unusual that he established a separate genus for it: Nilopegamys. This assessment was initially adopted by later authors, including Glover M. Allen (1939) and John Reeves Ellerman (1941).

But the taxonomic independence of the species did not remain unchallenged. In 1966, the British mammalogist Robert William Hayman reached a completely different assessment: in his view, Nilopegamys plumbeus could not be distinguished from the semiaquatic African forest stream mouse (Colomys goslingi) of Central Africa. Especially because of similarities in coloration and fur pattern, it could be “nothing more than an Ethiopian form of this species.” Hayman therefore placed Osgood’s species in the genus Colomys. Nilopegamys plumbeus thus became Colomys goslingi plumbeus—a subspecies.

From today’s perspective, this decision seems remarkable because Hayman had never examined the only known specimen of the Ethiopian amphibious rat himself. His assessment was based on notes and photographs of the holotype, which the Belgian mammalogist Walter Verheyen had made. What is even more interesting is that Verheyen himself came to a completely different assessment in unpublished notes: he recorded several differences between Nilopegamys and Colomys that argued against treating them as the same (Kerbis Peterhans & Patterson, 1995).

Nevertheless, Hayman’s classification prevailed. For decades, the Ethiopian amphibious rat effectively disappeared taxonomically into another species. It was no longer regarded as an independent representative of a distinctive Ethiopian lineage, but as a local subspecies of the more widespread African forest stream mouse. In 1983, this view was again supported—despite examination of the type specimen—by Fritz Dieterlen, who revised the genus Colomys and continued to treat plumbeus as one of six subspecies of C. goslingi.

It is understandable why this assessment lasted for so long. Nilopegamys was still only known through the holotype. There were no series of animals, no observations in the field and hardly any comparison material from similar habitats. Osgood’s original description was never significantly expanded and remained the primary basis, but it was of limited use for modern comparisons. Under such conditions, external similarities could easily be overvalued—especially since Osgood himself had not made a direct comparison with Colomys (Kerbis Peterhans & Patterson, 1995).

It was only in the following decades that the state of evidence changed. New museum expeditions produced additional material from African forest rodents, which also had long hind feet and were adapted to streams and moist habitats, including representatives of the genera Colomys, Malacomys and Deomys. This made a broader comparison possible for the first time: Was Osgood’s water mouse really just a divergent form of Colomys? Or was it an independent lineage?

Julian C. Kerbis Peterhans and Bruce D. Patterson addressed this question in 1995. Their study was clear: Nilopegamys differs distinctly from Colomys in several external, cranial and anatomical traits. The differences affected not only coloration, but also body proportions, hind feet, skull structure and other features that are crucial for systematic classification.

This restored the Ethiopian amphibious rat’s independence. What had been listed for decades as Colomys goslingi plumbeus was now again regarded as Nilopegamys plumbeus: a distinct species in its own genus.

For Kerbis Peterhans and Patterson, this reassessment was more than a mere correction of the taxonomy. It showed that this was not an outlying population of a Central African species, but a distinct taxon of the Ethiopian highlands—one that underscores “the distinctiveness and endemism of Ethiopia’s highland fauna.”

Wader or swimmer?—Adaptations to life in water

Almost nothing is known about the behavior of the Ethiopian amphibious rat. No living animal was ever observed, no track documented, no foraging filmed or described. Everything that can be said about its way of life must be inferred from the single specimen collected and its location—from body structure, skull features and the few details about the type locality.

Yet experts can still draw conclusions from such traits: the Ethiopian amphibious rat was not an ordinary inhabitant of wet banks, but a rodent strongly adapted to water. Kerbis Peterhans and Patterson showed in 1995 that the species was more specialized in several respects than the African forest stream mice of the genus Colomys, with which it had long been confused or associated.

The hind feet are especially striking. They are large, broad and powerful—an indication that they were used not only for moving along stream banks, but also played a role in swimming. The animal also had especially dense, soft fur, which likely protected its body from cooling in the cold water of highland streams. The ears, by contrast, are greatly reduced; this can reduce heat loss, but also reduce drag while moving in water.

Further details point in the same direction. The limbs are more densely haired, and additional tactile hairs occur around the forefeet, known as carpal vibrissae. Such hairs can help detect touch, currents or prey nearby. These tactile hairs are absent in Colomys. For an animal hunting in turbid or moving water, such sensory hairs are a major advantage.

The skull also shows unusual traits. Osgood had already pointed to the distinctive shape of the snout and eye region. Later studies confirmed that Nilopegamys also differs clearly from Colomys in skull structure and dentition. Notable features include a relatively large brain, about 1.5 times larger than in other murid rodents of this size, and an enlarged foramen magnum, the opening through which the spinal cord exits the skull. Such traits may be linked to special locomotor and sensory adaptations.

The key point, however, is the difference between wading and swimming. Several African forest rodents use shallow streams. Representatives of the genera Colomys, Malacomys and Deomys are often described as “waders”: they move mainly by wading through shallow water, search for food on submerged vegetation or run along the banks. They are adapted to wet habitats, but are not strong swimmers.

In the Ethiopian amphibious rat, specialization seems to have gone further. Kerbis Peterhans and Patterson regarded it as a semiaquatic hunter that not only moved along and through shallow water, but probably could also swim actively. In 2017, Leonid A. Lavrenchenko and Afework Bekele even called it the only swimming rodent in Africa.

Later assessments qualify this picture. David Happold (2013) points out that although the broad hind feet clearly indicate swimming ability in the Ethiopian amphibious rat, it still lacks typical traits of highly aquatic mammals. These include adaptations such as pronounced webbing or a laterally flattened paddle tail. It was therefore probably not a fully aquatic animal, but a semiaquatic specialist: mobile on land, active along banks, but much better equipped for water than other African mice and rats.

Regarding the diet of the Ethiopian amphibious rat, there is only indirect evidence. According to later accounts by Louis Agassiz Fuertes and Osgood, the stomach contents of the holotype clearly smelled “fishy”. This was interpreted as evidence that small fish may have been at least part of its diet (Kerbis Peterhans & Patterson, 1995). Aquatic insects, larvae, crustaceans or other invertebrates occurring in highland streams are also conceivable.

The Ethiopian amphibious rat was therefore not a purely aquatic animal, but neither was it an ordinary forest stream mouse. It was probably a semiaquatic predator that used the mountain streams of the Ethiopian highlands. The powerful limbs and large hind feet indicate that it not only searched for food by wading, but was also able to actively pursue prey in the water. This combination makes the species exceptional among African rodents.

Colomys—a close relative

The Ethiopian amphibious rat was therefore not a subspecies of the African forest stream mouse. But placing it close to Colomys was probably not entirely wrong, because more recent studies show that N. plumbeus is an independent species in its own genus, but probably belongs among the close relatives of the African forest stream mice.

For a long time, it remained unclear exactly where Nilopegamys belonged within the Old World mice and rats (Murinae). The species was known from only a single specimen, and genetic material was unavailable for a long time. Earlier systematists therefore had to rely mainly on external traits, skull structure and comparisons with other African rodents. Nilopegamys was repeatedly placed near Colomys, but initially without the certainty provided by modern molecular genetic analyses.

As early as 2005, Guy Graham Musser and Michael D. Carleton grouped Colomys, Nilopegamys and Zelotomys in a so-called “Colomys division”. They therefore did not view the Ethiopian amphibious rat as a completely isolated special form, but as part of a small group of African mice with similar anatomical traits. Later studies also pointed in this direction: Pierre-Henri Fabre et al. (2015) proposed Nilopegamys as a possible member of the African tribe Praomyini, and Christiane Denys et al. (2017) cautiously listed the genus as a very rare form that shares several traits with this related group.

The Praomyini are a species-rich, exclusively African group within the Murinae. They include numerous mice and rats that inhabit very different habitats. This diversity makes the placement of Nilopegamys interesting: within an already morphologically diverse African radiation, the Ethiopian amphibious rat appears to be a particularly specialized lineage—adapted to small watercourses, moist banks and probably an at least partly swimming lifestyle.

Molecular genetic studies, including those summarized by Violaine Nicolas et al. (2021), support this placement. They indicate that Nilopegamys does indeed belong to the Praomyini and is closely related to Colomys. A phylogenetic study by Thomas C. Giarla et al. (2021) went further: according to it, Nilopegamys forms the sister group of Colomys. The two lineages may have separated about 1.8 to 3 million years ago, at the transition from the Pliocene to the Pleistocene.

The Ethiopian amphibious rat is therefore not merely an Ethiopian variant of Colomys goslingi, but an independent lineage. At the same time, it is evolutionarily close to Colomys. Its similarities to the forest stream mice are therefore probably not just coincidence, but reflect shared ancestry and similar ecological adaptations.

It is currently impossible to say with certainty whether the semiaquatic way of life was already present in the common ancestor of Colomys and Nilopegamys, or whether it developed further independently in both lineages. What is clear is that within the African Praomyini, forms using wet habitats, stream banks and watercourses evolved several times. The Ethiopian amphibious rat appears to be one of the most specialized representatives of this development. At the same time, the evidence remains thin: it is essentially based on a single known specimen, so statements about its evolutionary history are necessarily difficult.

Why were there so few “amphibious rats” in Africa?

The Ethiopian amphibious rat shows that African rodents, too, were able to adapt to life in water. Nevertheless, it remained an exception. While several strongly aquatic rodents evolved in South and Central America, comparable forms are rare in Africa. Most African species that use small streams tend to wade in shallow water rather than live as specialized swimmers (Kerbis Peterhans & Patterson, 1995).

A possible reason lies in the different ecological conditions. In the Neotropics, certain niches on rivers and streams have long been less occupied by other mammals. There, rodents were able to penetrate these habitats more extensively and develop into specialized aquatic inhabitants. In Africa, however, they probably competed with other groups of animals that were already looking for food in or near the water—such as otter shrews and other aquatic insectivores. This left less room for rodents to specialize in this direction (Kerbis Peterhans & Patterson, 1995).

Against this background, the Ethiopian amphibious rat appears all the more extraordinary. The species did not live in the moist lowland forests of Central Africa, but in the Ethiopian highlands. Suitable waters there are often small, locally limited and strongly dependent on the seasons. Permanently flowing mountain streams could therefore form special refuges: cool, isolated and ecologically stable enough to allow close adaptation to this habitat (Kerbis Peterhans & Patterson, 1995; Lavrenchenko & Bekele, 2017).

The landscape itself also favored isolation. The Ethiopian highlands are shaped by large differences in elevation, deeply incised valleys, gorges and separated plateaus. More than half of Ethiopia consists of highlands; as a result, many habitats are spatially separated from one another. The Great African Rift Valley in particular separated the highlands early into a western and an eastern plateau, creating conditions for local evolution and speciation (Lavrenchenko & Bekele, 2017).

Even within the highlands, river valleys and gorges can act as barriers. Danila S. Kostin et al. (2020) showed, for example, that the Blue Nile Gorge, a gorge in the Ethiopian highlands, can genetically separate populations. Such barriers make exchange between neighboring animal groups more difficult and can contribute to isolated lineages developing independently over a long period of time.

There were also repeated climatic fluctuations. During humid phases, forests and suitable habitats could expand; during dry phases, they contracted again. This repeatedly created isolated refuges in which specialized species could persist and continue evolving. Lavrenchenko and Bekele (2017) see this combination of geographic isolation, elevational differences and climatic fluctuations as an important reason for the high proportion of endemic mammals in Ethiopia.

River systems may also have played an important role for the ancestors of the Ethiopian amphibious rat. Giarla et al. (2021) suggest that they may have reached the Ethiopian highlands from more southerly areas—first along the White Nile and later via the Blue Nile system. Rivers would therefore not only have been barriers, but also routes of spread. However, later climatic changes, especially during the Middle and Late Pleistocene, may have disrupted this exchange again and increased the isolation of individual populations.

The Ethiopian amphibious rat was therefore probably the result of several developments: a rare ecological niche in permanent mountain streams, a landscape full of natural barriers and a long history of climatic change. This very combination could explain why such an unusually specialized semiaquatic rodent evolved in the Ethiopian highlands (Kerbis Peterhans & Patterson, 1995; Lavrenchenko & Bekele, 2017; Giarla et al., 2021).

Ethiopia as a hotspot of mammal endemism

One thing is certain: the Ethiopian amphibious rat lived in an extraordinary landscape. The Ethiopian highlands are among the most ecologically distinctive regions in Africa. A large part of the country lies above 2,000 meters elevation; in addition, Ethiopia accounts for around 80% of the African land area higher than 3,000 meters (Kostin et al., 2020). These high altitudes do not form a uniform landscape, but rather a mosaic of plateaus, mountains, deep valleys and isolated habitats.

Such highland regions often function like islands in the landscape. Species that evolve there are often restricted to small areas. Kerbis Peterhans and Patterson (1995) emphasize that Ethiopia harbors a particularly distinctive flora and fauna, including several isolated rodent species. The Ethiopian short-headed rats (Stenocephalomys) are closely tied to these altitudes. In this context, the Ethiopian amphibious rat appears not as a random exception, but as part of a specialized highland fauna.

Lavrenchenko and Bekele (2017) show how distinctive this mammal fauna is in their overview of Ethiopia’s mammals. They listed 311 mammal species for the country, including 55 endemic species. The level of endemism is 17.7%, higher than in other species-rich African countries such as Kenya, Tanzania or the Congo. Particularly striking is the diversity of small mammals: Several rodents have been reevaluated, raised to species rank or formally described for the first time. Other forms were considered probably new to science at the time, but had not yet been formally described.

This connection is particularly important for the Ethiopian amphibious rat. Ethiopia is not only rich in endemic mammals, but also a region in which new small mammals are still being discovered or taxonomically reclassified even today. Many of these species are distributed over small areas, are difficult to detect and have hardly been researched ecologically. Lavrenchenko and Bekele therefore warn that unknown or poorly studied species could disappear before their distribution, lifestyle or need for protection is even understood.

Nilopegamys plumbeus fits exactly into this pattern. The researchers place the species among mammals with an extremely limited range—highland specialists for which a single destroyed stream, a changed valley or a disappearing wetland can determine survival. The fact that the Ethiopian amphibious rat is only known from a single specimen and has never been observed alive makes it an example of this gap in knowledge.

The Ethiopian amphibious rat could therefore be one of the species that disappeared before its way of life was even understood. Its case raises a larger question: How many highly specialized small mammals of the Ethiopian highlands could be lost—or already lost—before they are properly recorded by science?

Search expeditions into a destroyed habitat

After the discovery of the only known specimen in 1927, decades passed without any targeted search for additional Ethiopian amphibious rats. The species remained a museum animal: described, taxonomically controversial, later recognized as independent again, but not detected again in the wild.

Only Kerbis Peterhans and Patterson (1995), after reassessing the species, made clear how urgently a targeted search was needed. Especially because N. plumbeus was so strongly adapted to water and probably had only a very small range, it was necessary to determine whether surviving populations still existed in the Ethiopian highlands. This call remained unanswered for a long time.

New evidence came only with the work of Danila S. Kostin and her team (2020). In 2012 and 2018, the researchers conducted extensive field studies on and around Choke Mountain, also called Mount Choqa. This area also includes the region of the Ethiopian amphibious rat’s type locality. In total, the team collected 195 rodents that could be genetically assigned to nine species. After comparing with the available literature, this species list was largely in line with what would be expected for the region—with two exceptions: the Ethiopian amphibious rat and the Cheesman’s lamellar-toothed rat (Otomys cheesmani), which was last documented in 1968, were missing.

The 2018 fieldwork was especially important: Kostin et al. specifically sought out the type locality of the Ethiopian amphibious rat. The area was inaccessible by vehicles; the researchers had to use horses to get there. This alone shows how remote the original location is. But even this isolation had obviously not been able to protect the habitat.

Instead of a “small mountain stream” near the source of the Little Abbai, as Osgood had described it, the scientists found a greatly altered environment. The original stream habitat had disappeared or was so severely degraded that it seemed unsuitable for a semiaquatic rodent. Kostin et al. therefore reached a sobering conclusion: the Ethiopian amphibious rat has probably disappeared from the area; it may even be extinct worldwide. Their judgment for Cheesman’s lamellar-toothed rat is similarly negative; its habitats have also been destroyed by human use.

Dando (2021) points out that the habitat at the type locality is now severely degraded. According to a personal communication from Leonid Lavrenchenko, the original site has now largely been converted into pastureland. The IUCN also states that the habitat was already severely degraded at the time the holotype was collected in the 1920s; however, what this historical assessment is based on remains unclear.

As Lavrenchenko and Bekele (2017) had already shown, small mammals in the Ethiopian highlands—such as Nilopegamys plumbeus—often have extremely limited ranges, making them especially vulnerable to habitat change. Even small-scale interventions can then have serious consequences—especially when species are tied to rare, permanent stream systems.

Agriculture has shaped Ethiopia for thousands of years, but population growth, the expansion of arable land, overgrazing and the conversion of natural vegetation have greatly altered many habitats. Lavrenchenko and Bekele point out that large areas of forests and wooded landscapes have been cleared for settlements and agriculture; the dense forest that once covered around 40% of the country’s area has shrunk to less than 4%. In highlands, where wet habitats are often small and isolated from one another, such changes can have extremely serious consequences.

For the Ethiopian amphibious rat, the loss of its stream habitat may have been crucial. An animal adapted to cool, clear mountain streams, vegetated banks, and constant access to water probably could not simply move into any other habitat. If the small stream at the type locality disappeared or its banks were altered by grazing and agricultural use, the species may have lost the very habitat to which it was attached.

Local knowledge of the species also does not appear to have been passed down. Branden Holmes (2021) points out that people in Ethiopia would probably have encountered N. plumbeus if it still existed in significant numbers. But its distinctiveness long remained in the background: For decades, the Ethiopian amphibious rat was listed only as a subspecies of Colomys goslingi. If greater attention had been paid to its independence earlier, the targeted search for surviving populations might have started earlier—and perhaps protective measures would have been possible in time.

So the previous search for the Ethiopian amphibious rat ends with a blank space. Almost a century later, researchers found no suitable habitat where an unusually specialized rodent was caught in 1927. What remains is a single museum specimen.

The Ethiopian amphibious rat—lost or extinct?

The Ethiopian amphibious rat was discovered almost 100 years ago—and has never been recorded again since. One might think that a species would automatically be considered extinct after such a long time, but scientists are cautious about such judgments. It is often not possible to clearly determine when the last individual of a species actually disappeared.

Between “still present” and “extinct” there is therefore a gray area: lost species. This refers to species that have not been observed for decades or even centuries, but whose continued existence cannot be ruled out with certainty. Some of them are actually extinct. Others may survive in hard-to-reach areas, in very small remnant populations, or in habitats that have rarely been studied.

The Ethiopian amphibious rat also falls into this category. On the IUCN Red List, it is listed as “Critically Endangered (Possibly Extinct)”—that is, as critically endangered and possibly already extinct (Dando, 2021). This classification means that its disappearance is probable, but has not yet been conclusively proven.

There are good reasons for this cautious classification. Species that have not been seen for decades are constantly being rediscovered. A premature declaration of extinction can be problematic, because it could end searches and protection measures even though individual animals still exist. Especially for small, nocturnal or difficult-to-detect mammals, a lack of detection is not automatically proof of extinction.

Nevertheless, the outlook for the Ethiopian amphibious rat is bleak. Several recent papers describe the species as extremely rare or possibly already extinct. Lavrenchenko and Bekele (2017) write that at least one species from their Ethiopian mammal list—Nilopegamys plumbeus—may now be extinct. Kostin et al. (2020), Nicolas et al. (2021) and Giarla et al. (2021) also treat the species as possibly disappeared or at least extremely rare.

The reason is not only that no other specimen has been found since 1927. What is crucial is that the known habitat at the type locality has since changed significantly. Where Osgood once discovered the animal on a small stream, later researchers no longer found suitable conditions for a semi-aquatic rodent. If the Ethiopian amphibious rat actually had a very small range, the loss of this stream system may have been enough to push it to the brink of extinction—or beyond (Kostin et al., 2020; Dando, 2021).

The case fits well with what Thomas E. Martin and his team described in 2022 for so-called “lost species”. In their global analysis, they showed that lost species are particularly common in the tropics, often in regions with high biodiversity, strong endemism and hard-to-reach habitats. Around 92.5% of these species occur exclusively in the tropics. Many live in areas that are insufficiently studied scientifically and at the same time are under great pressure from deforestation, agriculture or settlement.

It is also notable that a significant proportion of lost species are tied to mountain regions. According to Martin et al., about 31% of “lost species” are highland endemics whose ranges are small and fragmented. This combination is strongly reminiscent of N. plumbeus: a species from a remote highland area, probably with a very small range and close ties to a specialized habitat. Such species can easily disappear unnoticed—or simply be overlooked.

This makes the Ethiopian amphibious rat one of those species where two possibilities exist side by side: Either it has actually already died out unnoticed—or it is still surviving in a remote stream system in the Ethiopian highlands, so rare and local that it has so far escaped any search.

Its possible extinction is especially serious because Nilopegamys is a monotypic genus. With it, not just a single species would disappear, but an entire evolutionary lineage. Lavrenchenko and Bekele (2017) emphasize that monotypic genera have special conservation priority because their loss wipes out a disproportionate amount of evolutionary diversity.

As long as no new record is found, the Ethiopian amphibious rat remains in an uncertain intermediate state. It is not officially extinct, but no longer documented alive either. Its status as “possibly extinct” is more than a formal category. It describes a scientific and conservation dilemma: too little is known to close the case definitively, but perhaps enough is already known to suspect that the search may have begun too late.

Africa’s lost rodents

The Ethiopian amphibious rat is not the only African rodent about which science has little more than an old museum specimen, a location and many unanswered questions. Especially when it comes to small mammals, the line between “rare”, “overlooked” and “extinct” is often difficult to draw. Many species live in hiding, are nocturnal, difficult to distinguish from one another and have never been specifically studied over long periods of time.

Rodents are by far the most species-rich order of mammals in Africa. According to Ara Monadjem et al. (2015), the continent has more than 460 rodent species; they make up almost half of all African mammal species. They inhabit deserts, savannahs, rainforests, swamps and alpine grasslands. Only a few of them—among them Colomys and Nilopegamy—use water at least partially as a habitat.

Despite this diversity, African rodents are still poorly researched in many regions. Giarla et al. (2021) emphasize that the actual species diversity is probably underestimated. Many lineages show high species-level endemism and distinct phylogeographic structure within individual species. In other words, what appears to be a widespread species may, upon closer examination, turn out to be a complex of several regionally limited lineages.

In addition, many rodents are difficult to distinguish from one another externally. Small differences in skull, teeth, fur coloration or genetics can determine whether it is a known or previously unrecognized species. That is why rodents—along with bats—are among the groups of mammals in which new species are often described or old species boundaries are reassessed.

This is exactly what makes assessing the risk of extinction so difficult. Mice and rats are often considered resilient and adaptable. Some species actually benefit from human-modified landscapes, but this is by no means the case for all. Particularly at risk are highly specialized species with very small distribution areas, such as forms that only occur on a single mountain, in a specific highland area or on a small island.

History shows that rodents can also disappear for good. This became very clear on islands. The Maclear rat (Rattus macleari) of Christmas Island disappeared after the introduction of black rats and their parasites. The Bramble Cay mosaic-tailed rat (Melomys rubicola) is considered the first mammal whose extinction was officially linked to the consequences of human-caused climate change. Both cases show: rodents are not automatically “survivors” when their habitat is small, isolated and suddenly significantly changed. Such extinction processes are often more difficult to detect on the African mainland because many species are rarely recorded and historical data are missing. However, that does not mean they are any less at risk.

Together with comparatively limited research interest and ongoing habitat loss, this means that many African rodents remain mysterious to this day. Some may have already disappeared without their extinction even being noticed (Holmes, 2021).

Several African rodents have not been documented for decades, or have been documented only extremely rarely. These include the groove-toothed forest mouse (Leimacomys buettneri) from Togo, known only from 1890, the giant thicket rat (Grammomys gigas) from Kenya, described based on two specimens from 1910, the velvet climbing mouse (Dendroprionomys rousseloti) from the Congo, which has not been detected since 1966, as well as the Kahuzi climbing mouse (Dendromus kahuziensis), which is only known from two specimens collected in the Congo in 1967 and 1972. Cheesman’s lamellar-toothed rat (Otomys cheesmani) from Ethiopia also belongs in this series.

At the same time, individual rediscoveries show that long periods without records do not automatically mean extinction. The Mount Oku forest mouse (Hylomyscus grandis) from Cameroon was for a long time only known from the type specimen from 1967, but was detected again in 2013. The Ethiopian striped mouse (Mus imberbis) was photographed alive in November 2012 for the first time since 1980, possibly because previous search methods were unsuitable. Such examples show how important targeted searches and appropriate survey methods are for small mammals that are difficult to detect.

The threats resemble those facing many other animal groups: habitat loss, deforestation, agriculture, slash-and-burn clearing, settlement expansion and overgrazing. Small rodents often rely on dense vegetation, which offers them cover from predators. If this vegetation is destroyed by livestock, fire or conversion to arable land, the small mammals will also disappear. Even protected areas do not always help if they were set up primarily for large mammals and little consideration is given to the habitats of small species.

Climate change adds further pressure. Highland species are especially at risk because suitable climatic conditions shift ever farther upward as temperatures rise. On a mountain, this room to retreat is limited. At some point the habitat ends at the summit. In the long term, this can become a trap for species that only inhabit small, cool highland areas.

The fact that new small mammals are still being discovered in Ethiopia even today is also shown by the first description, published only in 2025, of Crocidura stanleyi, a tiny highland shrew. Although it is not a rodent, it fits the same context as an example: small mammals can remain unnoticed for a long time, even in comparatively well-known regions. The species weighs only about three grams and was recognized as new only after years of research.

---

Sources

• Dando, T. (2021). Nilopegamys plumbeus. The IUCN Red List of Threatened Species 2021: e.T40766A22431448. https://dx.doi.org/10.2305/IUCN.UK.2021-3.RLTS.T40766A22431448.en
• Denys, C., Lecompte, E., Dubois, A., & Taylor, P. J. (2017). Diagnoses and contents of new African and Eurasian Murinae (Rodentia, Muridae) tribes. Dumerilia, 7, 82–90.
• Dieterlen, F. (1983). Zur Systematik, Verbreitung und Ökologie von Colomys goslingi Thomas & Wroughton, 1907 (Muridae; Rodentia). Bonner Zoologische Beiträge, 34, 73–106.
• Giarla, T. C., Demos, T., Monadjem, A., et al. (2021). Integrative taxonomy and phylogeography of Colomys and Nilopegamys (Rodentia: Murinae), semi-aquatic mice of Africa, with descriptions of two new species. Zoological Journal of the Linnean Society 192(1), 206–235. https://doi.org/10.1093/zoolinnean/zlaa108
• Happold, D.C.D. (2013). Genus Nilopegamys: Ethiopian water rat. In D.C.D. Happold (Ed.), Mammals of Africa: Vol. III. Rodents, hares and rabbits (pp. 508–509). Bloomsbury Publishing.
• Hayman, R.W. (1966). On the affinities of Nilopegamys plumbeus  Osgood. Annales Musée Royal de l’Afrique Centrale, Tervuren, Belgium, ser. 8 (Sciences Zoologiques), 144, 29–38.
• Holmes, B. (2021). What’s Lost and What Remains: The Sixth Extinction in 100 Accounts (eBook). Self-published.
• Kerbis Peterhans, J.C., & Patterson, B.D. (1995). The Ethiopian water mouse Nilopegamys  Osgood, with comments on semi-aquatic adaptations in African Muridae. Zoological Journal of the Linnean Society, 113, 329–349. https://doi.org/10.1006/zjls.1995.0012
• Kostin, D.S., Martynov, A.A., Komarova, V.A., et al. (2020). Rodents of Choke Mountain and surrounding areas (Ethiopia): The Blue Nile gorge as a strong biogeographic barrier. Journal of Vertebrate Biology 69(2), 1–12. https://doi.org/10.25225/jvb.20016
• Lavrenchenko, A.L., & Bekele, A. (2017). Diversity and conservation of Ethiopian mammals: What have we learned in 30 years? Ethiopian Journal of Biological Sciences, 16(Suppl.), 1–20.
• Martin, T., Bennett, G., Fairbairn, A. J., & Mooers, A. Ø. (2022). “Lost” taxa and their conservation implications. Animal Conservation 26(1), pp. 14–24. https://doi.org/10.1111/acv.12788
• Monadjem, A., Taylor, P. J., Denys, C., & Cotterill, F. P. (2015). Rodents of sub-Saharan Africa: a biogeographic and taxonomic synthesis. Walter de Gruyter GmbH & Co KG.
• Nicolas, V., Mikula, O., Lavrachenko, L.A., et al. (2021). Phylogenomics of African radiation of Praomyini (Muridae: Murinae) rodents: First fully resolved phylogeny, evolutionary history and delimitation of extant genera. Molecular Phylogenetics and Evolution 163(1), 107263. https://doi.org/10.1016/j.ympev.2021.107263
• Osgood, W.H. (1928): A new genus of aquatic rodents from Abyssinia. Field Museum of Natural History, Zoological Series, 12(15), 185–189.