Taxonomic name: Rattus rattus (Linnaeus, 1758)
Synonyms: Mus alexandrinus Geoffroy, 1803, Mus novaezelandiae Buller, 1870, Mus rattus Linnaeus, 1758, Musculus frugivorus Rafinesque, 1814
Common names: black rat (English), blue rat (English), bush rat (English), European house rat (English), Hausratte (German), roof rat (English), ship rat (English)
Organism type: mammal
A native of the Indian sub-continent, the ship rat (Rattus rattus) has now spread throughout the world. It is widespread in forest and woodlands as well as being able to live in and around buildings. It will feed on and damage almost any edible thing. The ship rat is most frequently identified with catastrophic declines of birds on islands. It is very agile and often frequents tree tops searching for food and nesting there in bunches of leaves and twigs.
A slender rat with large hairless ears, the ship rat (Rattus rattus) may be grey-brown on the back with either a similarly coloured or creamish-white belly, or it may be black all over. The uniformly-coloured tail is always longer than the head and body length combined. Its body weight is usually between 120 and 160 g but it can exceed 200 g.
The work of Yosida (1980) and his co-workers has shown that there are two forms of R. rattus that differ in chromosome number. The more widespread Oceanic form has 38 chromosomes and is the ship rat of Europe, the Mediterranean region, America, Australia and New Zealand. Present indications are that it is the Oceanic form that has reached islands in the South Pacific, but studies are needed to confirm this. The Asian form has probably reached some islands north of the equator, e.g. the Caroline Islands. On the basis of colour variation in rats on Ponape and Koror Islands, described by Johnson (1962) as Rattus rattus mansorius, we suspect that these rats may be the Asian form of R. rattus (SPREP, 2000).
agricultural areas, coastland, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, urban areas
Ship rats can be widespread, utilising most habitat types, but they show a preference for drier habitats. They generally avoid swimming. Ship rats in a New Zealand study (Hooker and Innes, 1995; in Innes, 2001) were mostly arboreal, but were also frequently recorded on the ground. The mean range length for females was 103m, and 194m for males. Another study (Dowding and Murphy, 1994; in Innes, 2001) found that rats generally used 3-4 dens each throughout their range. In the Mediterranean region R. rattus is most common in forests and shrublands up to 1080m in elevation (Martin et al., 2000).
The ship rat has directly caused or contributed to the extinction of many species of wildlife including birds, small mammals, reptiles, invertebrates, and plants, especially on islands. Ship rats are omnivorous and capable of eating a wide range of plant and animal foods. These include native snails, beetles, spiders, moths, stick insects and cicadas and the fruit of many different plants (Innes 1990). They also prey on the eggs and young of forest birds (Innes et al., 1999). In the recovery programme for the endangered Rarotonga flycatcher or kakerori (see Pomarea dimidiata in the IUCN Red List of Threatened Species), Robertson et al. (1994) identified ship rats as the most important predator affecting the breeding success of this bird. Several cases are known where predation on seabirds can be reliably attributed to ship rats. These include sooty terns (see Sterna fuscata in IUCN Red List of Threatened Species) in the Seychelles Islands (Feare, 1979), Bonin petrels (see Pterodroma hypoleuca in IUCN Red List of Threatened Species) in Hawai'i (Grant et al., 1981), Galapagos dark-rumped petrels (see Pterodroma phaeopygia in IUCN Red List of Threatened Species) in the Galapagos Islands (Harris, 1970), and white-tailed tropicbirds (see Phaethon lepturus in IUCN Red List of Threatened Species) in Bermuda (Gross, 1912).
The ship rat is most frequently identified with catastrophic declines of birds on islands. The best documented examples in the Pacific region are Midway Island in the Leeward Islands of Hawai'i (Johnson, 1945; Fisher and Baldwin, 1946), Lord Howe Island (Hindwood, 1940; Recher and Clark, 1974) and Big South Cape Island, New Zealand (Atkinson and Bell, 1973). Atkinson (1977) brought together circumstantial evidence suggesting that ship rats, rather than disease, were responsible for the decline of many species of Hawai'ian native birds during the 19th century.
There are few indications of rat-induced declines in native birds on islands nearer the equator (latitude 15°N to 20°S). This zone coincides with the distribution of native land crabs, animals that also prey on birds and their eggs. The long co-existence between land crabs and some island birds may have resulted in the development of behaviours among the birds that gives them a degree of protection against rats. Atkinson (1985) suggested that this might be the reason why rat-induced catastrophes are less apparent within the equatorial zone, but this hypothesis has never been tested (SPREP, 2000).
Species of weight similar to or smaller than that of rats appear to be the most vulnerable to predation. Impacts also appear to be more severe on smaller islands, where rat densities tend to be higher and do not fluctuate. Constant predation pressure results in a reduction in colony size on these islands (Martin et al., 2000).
Both R. rattus and R. norvegicus transmit the plague bacterium (Yersinia pestis) via fleas in certain areas of the world. There have been a series of recent outbreaks in Madagascar in recent years (Boiser et al. 2002).
Ship rats can be widespread, utilising most habitat types, but they show a preference for drier habitats. They generally avoid swimming.
Native range: The ship rat is native to India, and is now found worldwide. It reached Palestine around 3000 BP (Tchernov, 1986; in Martin et al., 2000). Evidence of rats in the western Mediterranean dates to 2400BP (Vigne and Valladas, 1996; in Martin et al., 2000).
Known introduced range: United Kingdom, France, Italy, Spain, Belgium, Denmark, Malta, Portugal, Indonesia, Tanzania, Tunisia, Madagascar, Mauritius, Seychelles, La Réunion, Caribbean, Canada, USA, Mexico, Chile, Galapagos Islands, Australia, New Zealand, Papua New Guinea, French Southern Territories, British Indian Ocean Territories, Heard and McDonald Islands, US Minor Outlying Islands, and at least 28 groups of Pacific Islands.
Introduction pathways to new locations
Natural dispersal: The recent reinvasions of Rattus rattus onto Motutapere Island (Chappell 2004) and Tawhitinui (Ward 2005) in New Zealand both involved crossings of approximately 500m in calm waters. Both invasions were presumably by swimming and they extend the distance which this species has been suspected of swimming (Russell and Clout, 2005).
Road vehicles (long distance):
Seafreight (container/bulk): Rattus rattus usually stow away in freight carried within the hull, holds and living spaces of ships
Local dispersal methods
Natural dispersal (local): Subadult Rattus rattus are usually forced out of the natal home range but the distance they can travel is not known.
Preventative measures: Research has shown that it can often be difficult to eradicate rats from islands in the early stages of invasion, hence it is better to prevent rodents arriving on islands in the first place. Eliminating a single invading rat can be disproportionately difficult because of atypical behaviour by the rat in the absence of conspecifics, and because bait can be less effective in the absence of competition for food (Russell et al., 2005). Weihong et al. (1999) provide useful information regarding the detection of rodent species using different trapping methods and bait.
Physical: The use of poison baits is the only proven way to remove rodents from large islands. Trapping generally fails to remove all individuals, as trap-shy animals can survive and repopulate the island (DOC, 2004).
Chemical: Rattus rattus can be eradicated from small areas or seasonally controlled using proprietary rat poison products in an appropriate manner. The largest island to date from which ship rats have been eradicated is Barrow Island (23 000 ha, Western Australia) (Morris, 2002).
Second-generation anticoagulant poisons are used widely for ship rat control, but possible consequences of any ongoing control should always be considered. These consequences include primary or secondary poisoning of species we are aiming to protect or other non-target species, secondary poisoning of other vertebrate pests such as cats, and development of resistance to these poisons by ship rats. It is not known whether their tree-climbing habits will make eradication more difficult (SPREP, 2000).
Fisher et al. (2004) suggest that diphacinone especially, and also coumatetralyl and warfarin, should be evaluated in field studies as alternative rodenticides in New Zealand. Brodifacoum, the most widely used rodenticide in New Zealand currently, can acquire persistent residues in non-target wildlife. Mineau et al. (2004) presented a risk assessment of second generation rodenticides at the 2nd National Invasive Rodent Summit. O'Connor and Eason (2000) discusses the variety of baits which are available for use on offshore islands in New Zealand.
An investigation Spurr et al. (2007) was carried out to assess the behavioural response of ship rats to four different bait station types. Yellow plastic pipe, wooden box (‘rat motel’), and
wooden tunnel bait stations were found all suitable for surveillance of ship rats and the first two at least for Norway rats (all were readily entered and had a similar
amount of bait eaten from them).
Biological: Contraceptive methods of control are currently experimental, but the potential for effective control using contraceptive methods is promising. National Wildlife Research Center (USA) scientists are working on several possible formulations that may make effective oral immunisation possible (Nash and Miller, 2004).
Integrated management: Guidelines for the Eradication of Rats From Islands Within the Falklands Group offers guidelines for the eradication of rats from islands, based on the experiences in eradicating rats from the Falklands group. This paper offers guidelines for the eradication of rats from islands, based on the experiences in eradicating rats from the Falklands group.
Ship rats are omnivorous generalists, yet can be very selective feeders. They eat both plant and animal matter all year round.
A Japanese study showed that R. rattus is primarily herbivorous, but can change its food habits when it is thirsty, or when food is in short supply (Yabe, 2004).
A placental mammal with dependent young. Litter size 3-10 (average 5-8), with frequency of litters dependent on season and food supply. The interval between litters may be as little as 27 days.
Rattus rattus: gestation 20-22 days; weaning 21-28 days; sexual maturity 3-4 months; total life may not exceed two years.
This species has been nominated as among 100 of the "World's Worst" invaders
Reviewed by: Dick Veitch, Auckland, New Zealand.
Compiled by: IUCN SSC Invasive Species Specialist Group
Updates with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment
Last Modified: Tuesday, 11 January 2011