conserve ireland

 

Black Rat

 

Common Name Scientific Name Irish Name
Black Rat Rattus rattus Francach dubh
Order Family Group
Rodentia Muridae   Mammal
Black Rat Black Rat Black Rat
Black Rat    

                              
           

                              

Legal Status

None

 

Key Identification Features

The black rat is quite similar to Ireland's only other rat species the brown rat. Black rats are smaller in size with a lighter more slender appearance. Unlike the brown rat they have large almost hairless ears and prominent eyes set in a more slender pointed head, the tail is also thinner and longer usually measuring more than the head and body length combined. They have more whiskers which combined with their larger eyes allow for better exploration of dark environments than the brown rat species. Longer hairs on their backs and sides give them a more shaggy appearance. Despite their name the black rat is variable in colour with isolated populations being grey brown with a pale grey underside while urban populations are totally black. Adults rarely grow to more than 24cm in length excluding the tail with adult males weighing up to 250 grams and females around 200 grams. Track markings will leave less impressions than those of the brown rat with the black rat having a five-toed hind footprint measuring up to 3.8cm in length. Black rats are faster than the brown rat with their track markings leaving less tail drag impressions.

 

Habitat

Black rats are never found in open countryside and are not known to burrow underground, they prefer habitats near buildings especially those which contain cavity walls, false ceilings and wall paneling which provide them with a safe warm place to shelter. Nests are constructed with paper and cloth or other natural materials. In Ireland their main habitats are in port cities like Dublin, Cork and Waterford where they can be found on visiting ships and in the port's warehouses, factories and food processing plants. An individual’s home range will be small in size generally not exceeding 100 metres. Density numbers will vary depending on the time of year and availability of food, black rats defend their entire home ranges unlike the brown rat which will only defend its regularly used pathways. Males are dominant in black rat colonies with dominance been decided by scent information of an individual’s size and maturity. The success of the brown rat species in Ireland has limited the number of habitats that the black rat can occupy as they cannot compete with this larger more adaptable rat species. They are good swimmers when forced to do so but avoid damp environments whenever possible, they are excellent climbers so can occupy habitats such as lofts and attics in urban buildings. The black rat will avoid open areas so habitats tend to remain isolated in port settlements and on offshore islands limiting their range on the mainland.

 

Food and Feeding Habits

Black rats are omnivorous mammals but are known to eat more vegetables and fruit than other rodent species, preferring sugar cane and cereals when available. They are highly adaptable so will eat almost any food source which can be found within their habitats. They are largely nocturnal with their peak activity times between two and three hours after sunset each night although they have been known to forage in daylight hours when food is scarce. They will continually use the same pathways along walls when searching their territories for food. They are more daring than the brown rat climbing and swimming with ease although they are extremely wary of new food or objects which they find in their habitats. The black rat very rarely travels far each night and can live most of its life within a 100 metre radius of the nest if it is located close to a continuous food source.

 

Reproduction and Life Cycle

If the habitat provides suitable food and shelter the black rat can breed throughout the year, in other circumstances the breeding season runs from March to November. The number of breeding males in a colony is usually less than the number of breeding females present so mature males will attempt to gather a harem together and mate with several females. Breeding females can produce up to five litters per season with each litter containing between five and eight pups. Young black rats weigh under 5 grams when born and are hairless, blind and have their ears closed. Weaning is completed after three weeks with the female black rat being capable of becoming pregnant again before the first litter is fully weaned. The gestation period of the first pregnancy is three weeks with longer pregnancies for all following litters. The size of a litter will depend on the maturity and body size of the breeding female with larger mothers producing larger litters. Black rat pups are fully weaned after three weeks with the females ready to begin breeding again after only fifteen weeks. The female black rat is the only one to provide parental care for the pups. The maximum lifespan of a black rat in the wild is likely to be less than 18 months with a 90% mortality rate for pups each year.

 

Current Distribution

Black rats most likely originated in south east Asia and China and have gradually spread worldwide along trade routes and now occupy many areas where human settlements exist. The later arrival of the larger brown rat species to Europe reduced the established population of black rats as they were out competed for food and habitats. They are now largely confined to coastal ports, offshore islands and are found almost exclusively indoors. The black rat is believed to have come to Ireland aboard ships in the Middle Ages but is now one of our rarest mammals. There are currently isolated populations in Countries Down and Tyrone and a large population on Lambay Island off County Dublin.

 

Conservation Issues

Black rats have been described as the world's most destructive mammal after humans when they are found in large numbers as they can be a serious pest of agriculture and can decimate stores of food particularly grain and sugar cane. In Ireland their numbers are low so they do not pose serious problems, however individuals can carry bubonic plague, rabies and typhus and would cause environmental and health problems if a population increase were to occur. The main threat the black rat poses to other animals in Ireland is in raiding the nests of some ground nesting bird species for their eggs. The increase in the numbers of domestic and feral cats in Ireland has helped to prevent the black rat from returning to the high population levels found here in the Middle Ages. Their numbers can be managed better as they are more susceptible to rodenticides than the brown rat species. In some areas they can constitute an important prey item for some species of Irish predatory birds practically owls and hawks. The black rat is not protected under any international, European or Irish legislation.