Darwin’s Galápagos Mouse

The name darwini seems appropriate

The first rodent native to the Galápagos Islands was collected by the naturalist Charles Darwin on San Cristóbal in 1835: Mus galapagoensis. Seven further species of endemic rodents were subsequently discovered on four other islands of the Galápagos Archipelago. At least four of these species are now extinct, including Darwin’s Galápagos mouse.

In July 1929, the American zoologist Wilfried H. Osgood scientifically described Darwin’s Galápagos mouse as Nesoryzomys darwini  in A new Rodent from the Galapagos Islands. He based his description on three specimens collected by the taxidermist Frank Wonder on January 12, 1929, in Academy Bay in the south of Santa Cruz. Wonder found another animal on January 16 in Conway Bay.

Osgood recognized that Darwin’s Galápagos mouse must already have been rare at the time from the fact that 25 specimens of the Santa Cruz nesoryzomys (Nesoryzomys indefessus) were collected in Academy and Conway Bay at the same time.

All rodent species endemic to the Galápagos Islands belong to one of three genera: the Galápagos rice rats (Nesoryzomys), the ‘giant rice rats’ (Megaoryzomys) or the coastal rice rats (Aegialomys).

Besides Darwin’s Galápagos mouse, the Santa Cruz or Indefatigable Galápagos rat, Aegialomys galapagoensis galapagoensis from the island of San Cristóbal and the Galápagos giant rat are considered extinct today in the Galápagos Archipelago.

At about 22 centimeters in body length (not including the tail), Darwin’s Galápagos mouse was the smallest representative of the Galápagos rice rats. Little is known about the lifestyle of the gray-brown Darwin’s Galápagos mouse. It was probably nocturnal and spent the day in rock crevices under bushes or in caves.

Darwin’s Galápagos mouse – fact sheet

alternative name	Darwin’s Nesoryzomys
scientific name	Nesoryzomys darwini
original range	Santa Cruz (Galápagos Islands)
time of extinction	1930s
causes of extinction	animals introduced to the island

The attempt to rediscover rodents believed extinct

When Darwin visited the Galápagos island of San Salvador (Santiago) in 1835, he encountered black rats (Rattus rattus) there. Early sailors or pirates probably introduced the rodents to the island, according to the biologist Robert Dowler.

In the journal Oryx, Dowler published his study results on the rediscovery of rodents of the genus Nesoryzomys believed extinct in the Galápagos Islands in 2000. In 1997, he conducted several searches for rodent populations on the islands, including on Santa Cruz (also Indefatigable), the former home of Darwin’s Galápagos mouse and the Santa Cruz rice rat. The biologist deliberately searched at two sites away from inhabited areas, because the chances of encountering remaining populations of endemic rodents are greatest there.

The result of the search was sobering. At both sites, Dowler and his team found only black rats and house mice (Mus musculus) that had previously been introduced to the island. In addition, they discovered signs of cats. At the Charles Darwin Research Station on Santa Cruz, they also encountered black rats and house mice, as well as one further introduced rodent species, the brown rat (Rattus norvegicus). The search on the island of Santa Cruz ultimately provided no evidence whatsoever that Darwin’s Galápagos mouse and the Santa Cruz rice rat still survive.

Introduced black rats led to the disappearance of Darwin’s Galápagos mice

Darwin’s Galápagos mouse was last sighted on Santa Cruz in 1930, one year after its first scientific description. The black rat is considered the main cause of the extinction of Darwin’s Galápagos mouse; it probably reached the island in the late 1930s. The IUCN considers it likely that the black rats brought pathogens to which Darwin’s Galápagos mouse was susceptible. Direct competition between the two species may also have caused the disappearance of the endemic species.

Other animal species that reached Santa Cruz and were also documented by Dowler are likely to have contributed to the extinction of Darwin’s Galápagos mouse as well: the house mouse, the brown rat and feral cats.

On the Galápagos island of Fernandina, by contrast, where there are no introduced rats, both endemic rodent species still exist today: the Fernandina Galápagos mouse (Nesoryzomys fernandinae) and the large Nesoryzomys narboroughi. The same is true of the island of Santa Fe, where Dowler also found no introduced rodent species.

During his search for endemic rodents in the Galápagos Archipelago, however, Dowler found something else that gives hope: coexistence between endemic and introduced rodents such as the black rat and the house mouse appears possible. Thus, the introduced black rats found by Darwin on San Salvador in 1835 and the endemic Santiago Galápagos mouse (Nesoryzomys swarthi) have already been living together on one island for more than 180 years.

This means that some rodents of the Galápagos Islands are able to compete with introduced rodents. And who knows, perhaps one day other rodents believed extinct there will be rediscovered.

During their Galápagos Islands expedition in 1997, Robert Dowler and his team rediscovered two rodent species that had long been considered extinct. On San Salvador, they encountered the Santiago Galápagos mouse, which was known only from four specimens collected in 1906. And on the island of Fernandina, Dowler rediscovered the Fernandina Galápagos mouse, which had been described in 1979 on the basis of two owl pellets. 

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