Santiago Galapagos mouse  (Nesoryzomys swarthi)

Classified as Vulnerable (VU) on the IUCN Red List (1).

First described in 1906 from the island of Santiago in the Galapagos, the Santiago Galapagos mouse was believed extinct until its rediscovery in 1997 (2) (3) (4). Previously, it was assumed that this and other endemic ‘rice rats’ in the Galapagos could not coexist with the introduced black rats which infest the islands (3), making this species’ rediscovery all the more remarkable. One of the few native land mammals to inhabit the Galapagos Islands, the Santiago Galapagos mouse has harsh, brownish fur, lighter on the underparts, and a long, sparsely-haired tail (3). It is the largest of the Galapagos rice rat species, and is sexually dimorphic, the male being larger than the female (4).

The Santiago Galapagos mouse is endemic to Santiago Island, in the Galapagos Islands, where it appears to be restricted to a tiny, 14 kilometre stretch of the north-central coast (1) (4) (5).

The Santiago Galapagos mouse is confined to lowland areas of dry forest and arid thorn scrub, occurring in areas with a high density of the endemic cactus Opuntia galapageia (1) (2) (5) (6).

Little is known about the Santiago Galapagos mouse. It is believed to be nocturnal and terrestrial, living in burrows or rock crevices beneath bushes (1) (7), and may feed on grasses, fruits and seeds (3), and possibly some insects. Breeding occurs in the wet season, with pregnancy peaking in April, and the mating system appears to be promiscuous or polygynous. The young grow and develop slowly, and do not breed in the first year. Lifespan may be over two years (8). It is thought that the species’ relatively high survival and low reproductive rate may be adaptations to the unpredictable and ephemeral resources found in the Galapagos arid zone (4) (8).

The endemic rice rats have lost more species than any other group of vertebrates in the Galapagos, a decline believed to have been caused by introduced black rats (Rattus rattus), mice and feral cats (2) (3) (4) (5). The Santiago Galapagos mouse is the only one of the remaining species to occur alongside the black rat, which is known to have been present on Santiago since at least 1835, when Charles Darwin collected it during his visit to the island (2) (5). However, the coexistence of the two species is not stable. The black rat is larger, more aggressive and more dominant than the Santiago Galapagos mouse, with studies showing it displaces the smaller endemic species at food sources and has a negative impact on its population (4) (5).

Although two new populations of the Santiago Galapagos mouse were found in 2005 (1) (6), the species has been lost from parts of its historical range (5), and its small, highly restricted population apparently owes its continued survival to areas of Opuntia cacti (4). These cacti provide a rich source of food, water and nesting sites, particularly in the dry season when alternative resources are hard to find. However, the black rat appears not to feed on Opuntia, giving the Santiago Galapagos mouse a vital refuge from competition (4) (5) (9). In addition, the particularly arid local climate at this site allows the more drought-resistant Santiago Galapagos mouse to survive while the black rat population crashes during years of drought (4) (10). The biggest threat to the Santiago Galapagos mouse now comes from climate change, with a predicted increase in rainfall that could lead to increased vegetation and food supply, allowing black rat populations to explode, while at the same time decimating the Opuntia vegetation on which the Santiago Galapagos mouse now depends (4) (10).

The rediscovery of the Santiago Galapagos mouse has raised hopes of finding populations of other rice rat species previously thought to be extinct (2). Further searches for this species on Santiago Island have also been recommended (6). Projects are currently underway to monitor and study the Santiago Galapagos mouse, and an action plan for the species has been proposed (11), as well as the possibility of establishing a captive population (2). The Charles Darwin Foundation and visiting scientists have been working with the Galapagos National Park Service to perform experimental black rat removal on Santiago, and black rat eradication has already begun on surrounding islets, which may be used as possible refuges for the Santiago Galapagos mouse (3) (11). Eradication of feral cats also needs to be undertaken (3), and the house mouse (Mus musculus) monitored to ensure the population of this introduced species does not increase in the absence of the black rat (10).

View information on this species at the UNEP World Conservation Monitoring Centre.

To find out more about rice rats, and about conservation in the Galapagos Islands, see:

• Charles Darwin Foundation:
  http://www.darwinfoundation.org/

• Galapagos Conservation Trust:
  http://www.gct.org/