Galápagos Giant Rat

Its size was due to island gigantism

A living Galápagos giant rat was never sighted or collected. Jochen Niethammer described the mouse-like rodent from the group of New World rats and mice in 1964 on the basis of three bone fragments found in a cave on Santa Cruz. Later, further fossil remains were discovered, all of them on the Galápagos islands of Santa Cruz and Isabela.

With a head-body length of 20 centimeters, the Galápagos giant rat was one of the larger representatives of the New World rats and mice. In The Relationships of Megaoryzomys curioi, an extinct Cricetine Rodent from the Galápagos Islands, Ecuador (1982), the paleontologist David W. Steadman and the paleobiologist Clayton E. Ray suggest that its size was due to the phenomenon of island gigantism.

For small mammals, the so-called island rule states that their body size increases over generations (island gigantism). The body size of large mammals, by contrast, decreases over time (island dwarfism). For example, about ten million years ago a giant hedgehog (Deinogalerix) 60 centimeters long and weighing ten kilograms lived on the former island of Gargano.

The New World rats and mice native to the American continent are a group that includes around 500 species. Besides the Galápagos giant rat, other New World rats and mice have become extinct in modern times. These include the Fernando de Noronha rat, the Barbados giant rice rat, the Barbuda giant rice rat, the Martinique giant rice rat, the Saint Lucia giant rice rat, Darwin’s Galápagos mouse and the Santa Cruz rice rat.

Galápagos giant rat – fact sheet

scientific name	Megaoryzomys curioi
original range	Santa Cruz, Isabela (Galápagos Islands)
time of extinction	early 20th century
causes of extinction	animals introduced to the island

Introduced species exterminated the rodent

The International Union for Conservation of Nature IUCN lists the Galápagos giant rat as extinct, because it is known exclusively from subfossil material. On the basis of the bones found, scientists were able to determine that the rodent species must have existed into modern times.

Invasive species such as domestic cats, feral dogs, goats, pigs and rats (Rattus rattus) introduced to the islands were most likely responsible for the extinction of the Galápagos giant rats.

Exactly when the Galápagos giant rat became extinct, however, can only be guessed. Most researchers assume that the species existed until the beginning of the 20th century, because human settlement of Santa Cruz began at that time.

The Santa Cruz nesoryzomys (Nesoryzomys indefessus), which was also once endemic to Santa Cruz, and Darwin’s Galápagos mouse survived until about 1935 or 1945. But Steadman and Ray assume that the Galápagos giant rat became extinct somewhat earlier, which can be attributed, for example, to the animals’ size and to their lack of fear of mammals, which had not existed on the island until then.

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