Key Identification Features
Similar to the house mouse the brown rat population is variable in size and colour depending on the habitat type in which they live, generally they are a grey to brown colouration with their undersides having a more pale grey tint, often individuals will have a white patch on their chests. Adults can grow up to 28cm in length with males weighing up the half a kilogram, females are lighter weighing 400 grams. The tail is long, sparsely haired and scaly containing up to two hundred rings but it is shorter than the combined length of the head and body. The head is carried lower than the back giving the brown rat it’s distinctive hump backed appearance when moving. Hind foot tracks measure up to 4.5cm in length and leave a five-toed star like footprint. They are a vocal species of rodent emitting a range of piping calls, shrieks and whistles. They also use ultrasound to communicate especially between a mother rat and her young pups in the 20 – 100 kHz range. They have smaller eyes and ears than the black rat but use long whiskers and an excellent sense of smell to locate food sources in dark environments. The brown rat is extremely agile and can run quickly over short distances to avoid threats. Strong bones and hind leg muscles allow the animal to jump high and long to gain access to new areas while the long flexible tail provides extra balance.
The brown rat is the most common rodent species found in Ireland, it is highly adaptable to most habitat types preferring to remain in areas which are in close proximity to human settlements as they provide these opportunistic animals with an abundant source of food and shelter. This species of rat can be found living in commercial and industrial buildings, on landfills, in sewer systems, along inland waterways such as urban canals, on rural farms and along Ireland’s coasts. Brown rats occupying urban habitats usually nest in cellars or on the ground floor although they are excellent climbers and have been found occupying the top levels of grain stores. A home range of an urban dwelling individual may be quite small and generally will not exceed 100m2. Natural habitats that are adjacent to human settlements are also exploited, especially in the warm summer months, these can include hedgerows and fields containing crops. Brown rats living in open areas dig burrows with nearby food storage chambers linked by underground tunnel systems. Home ranges of individuals found in more rural habitats will be larger than those in urban areas with nightly feeding trips of up to 4km along hedgerows often undertaken. Rats will share loose colonies which can be composed of related individuals, a dominant pair or one dominant male with a harem of females. The brown rat species can tolerate much damper environments than other rodents with only exposed mountainous areas and isolated uninhabited islands being unsuitable habitats.
Food and Feeding Habits
Brown rats are omnivorous and will eat almost anything although they prefer starchy and protein rich foods, such as seeds and cereals. Their diet includes scavenged meat, fish, weeds, vegetables, nuts and fruits or any scraps which can be found within human settlements. Populations that live in more rural locations will feed on silage, root crops, seeds and have been known to hunt for earthworms, shore crabs, snails and the eggs of any ground nesting bird species they might find. They are typically nocturnal feeders but will scavenge during the daytime if their area contains night time predators like owls and foxes. Juvenile brown rats may also be forced to feed during the day if there is too much competition from more dominant adult rats at night. For nocturnal feeding the peak times are one hour after sunset and one hour before sunrise. The brown rat will move along habitually used pathways in its territory using their sense of touch in a process known as thigmotaxis which involves using muscle memory to move the same way every time. Rural populations of brown rats will travel up to 4km per night along regular pathways in search of food which is then stored in a hoard to be consumed later. This species will resort to cannibalism if food sources become scarce.
Reproduction and Life Cycle
If the weather is mild and there is an abundant food supply the brown rat can breed throughout the year otherwise births will take place between March and November. Scent information is important during this time to establish the breeding condition of individuals. Males will become more aggressive during the mating season with the use of ultrasound when fighting other males and during copulation. Breeding females can produce up to five litters per year with on average each litter containing between one and ten pups. The size of each litter will depend on the size of the breeding female with larger mothers giving birth to larger litters. Pregnancy lasts for just over three weeks with pups born naked and blind generally not weighing more than 6 grams. Weaning begins almost immediately after birth and lasts for three weeks with parental care given by the female only. Female brown rats can become pregnant again before the first litter is fully weaned. Typically the smaller the litter size than the faster the pups will grow due to the availability of more milk. Young brown rats reach full sexual maturity after only three months allowing for rapid increases in population numbers if suitable conditions are provided, however the mortality rate of some rural populations can reach as high as 90% in their first year. Brown rats can live for up to 18 months in Ireland.
Along with the house mouse the brown rat is considered to be the most widespread terrestrial mammal in Ireland. They are thought to have originated in southeastern Asia and spread with human settlements to Europe and Africa. They arrived in Ireland by the 18th century most likely aboard ships from Britain and mainland Europe, out competing the closely related black rat species they are now found throughout the country except at high altitudes and on some of the more remote western islands. Brown rat population densities vary greatly depending on numerous factors including the time of year, the habitat they occupy and the severity of the weather. The population will usually reach its height by autumn and early winter with fewer rats present in spring due to lower birth rates at that time of year. A single pair of breeding adults can, if conditions are favourable produce a colony of up to eight hundred rats after just one year.
Brown rats are numerous and provide an important source of food for larger predatory animals such as foxes, stoats and birds of prey. In most parts of Ireland the brown rat has become the main food source of the barn owl. To humans the brown rat is Ireland’s most serious pest species which can pose considerable public health and environmental problems. The brown rat species can harbor a number of disease carrying organisms such as mites and fleas which can pass on Q and hantaan fever to humans. The brown rat also spreads weil’s disease from a bacteria in their urine called leptospirosis which is dangerous to humans, if a large population of rats occupy a watercourse the water quality can become effected over time posing risks to any people using the same water source.
Each adult brown rat has 4 incisor teeth which grow continually throughout their lives and so need to be filled down which is usually done by gnawing on hard objects which can lead to internal wall damage and may be the cause of electrical fires in some buildings. The brown rat has so far beaten all attempts at large scale control over its numbers by becoming resistant to poisons and regularly avoiding food traps. Like other rodent species found in Ireland the brown rat is not protected under any legislation.