Authors: John Marchant
Last Updated: May 29th, 2012
Formerly widespread and abundant in GB, the ship rat is currently restricted to the Shiant Islands and to a small section of the Thames Estuary.
The large size and long, thin, almost hairless tail distinguish rats from other grey-brown rodents. Water voles are smaller and darker brown, with shorter ears, a furry tail and a blunter snout. The ship rat is slimmer than the brown rat, with relatively longer ears: its uniformly coloured tail is always longer than the head and body length combined.
The ship rat has spread around the world and is a major pest species in many countries.
Ship rats were formerly found in a very wide range of habitat types in GB, typically in association with man. They now occur mainly in seaports, with an isolated population in the Hebrides.
The original distribution of the ship rat was probably India, from where it spread westward to Egypt by the 4th century BC and subsequently to Europe.
Remains have been found in Roman settlements in GB dating from as early as the 3rd century BC.
Ship rats can thrive on ships at sea and come ashore in ports or through shipwrecks. The first arrivals are believed to have been with the Romans and the Vikings. Shipborne arrivals continue in GB ports.
The ship rat is highly invasive and is regarded as a serious pest in many countries, especially in warmer climates where it appears better able to co-exist with or outcompetes the brown rat. In New Zealand, where there are no native land mammals, the ship rat has become a widespread arboreal predator in native forests. Having been widespread in GB, it was eventually displaced almost completely by the brown rat.
Ship rats have spread around the world through their readiness to travel in ships. They are poorer swimmers than brown rats and are less likely to disperse naturally to and from offshore islands.
Ship rats breed from March to November, each female producing up to five litters a year, each averaging about seven young. Females mature sexually after just 12–16 weeks.
Ship rats fall prey to a variety of avian and mammalian predators, including owls, domestic cats and dogs, and foxes.
Ship rats occupy warehouses and other buildings in seaport areas and, being better climbers, are more likely than brown rats to be found in roof spaces. Following shipwrecks, they have survived well on seabird islands.
Strong populations exist in the Shiants and, until they were recently extirpated, on Lundy Island. Ship rats survive in some seaport areas, such as at Tilbury.
Ship rats have contributed to many extinctions or near-extinctions of island endemic species and particularly of seabird colonies. In GB they present a serious threat to seabirds on the Shiant Islands.
Ship rats can spread a number of diseases to humans through their bites and droppings, including leptospirosis (Weil's disease). Inside buildings, rats can cause considerable nuisance through fouling surfaces and water tanks, gnawing timbers and wiring, and through their noise and aggression.
Ship rats previously had major economic impacts through their consuming and contamination of foodstuffs in storage and through damage to property. The species is now very rare in GB, however, and its impacts are much reduced.
There is little that can be done to prevent rats arriving in GB, but it is important to protect important sites, such as seabird islands, as much as possible.
Trapping is widely used as a method of control, although rats can be notably shy of traps.
Anticoagulant rodenticides are extensively used to control rat populations. The use of stronger poisons poses threats to non-target wildlife.
Ship rats have no legal protection in GB. Whether this species should now receive a level of protection due to its poor status is a highly contentious issue. Ship rat is listed under Schedule 9 of the Wildlife and Countryside Act 1981. It is thus an offence to release or to allow the escape of this species into the wild.
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