Brown Rat (Rattus norvegicus)

The Brown rat is one of the most common mammals on the planet. It is also known as the common rat, sewer rat, Norway rat (although this is misnomer as the rat didn't evolve in Norway), wharf rat and the Hanover rat.   It is one of the largest rats commonly found reaching 25cm in length of body and the same length again in tail, average male weights in at 550g with the smaller females being around 350g, although some individuals have been recorded reaching over 1000g.  Stories do persistently occur of sightings of much bigger rats even the size of domestic cats but these are either misidentifications or best left to the pages of horror novels.

They are intelligent problem solvers with excellent swimming ability (better than black rats) acute hearing (some ultrasound) and can see colours much like humans but with some colour-blindness although they can see into the ultra violet spectrum which some animals cannot. They are not good climbers. They can use ultra sound chirps to communicate in addition to normal vocalizations

If the conditions are right a brown rat can breed throughout the year producing up to 5 litters in a single year each litter can be up to 14 pups although the average is 7, a single pair can multiply to over 200 rats in a single year making eradication difficult. The death rate is very high and although they can live for up to 3 years most only survive a year with infant mortality very high due to predators.   They are a social animal with a clear hierarchical structure which is established by play fighting. In times of food shortage rats lower down in this order die first.

Brown rats can carry various diseases, including Weil’s disease, and viral hemorrhagic fever (VHF) and Q fever which it is thought 50% of the wild UK rat population carry; this is an unpleasant infection and can result in pneumonia.  Brown rats often carry the parasite that causes toxoplasmosis this is normally passed to humans by domestic cats who have fed on the rats. Interestingly there is some evidence that this parasite may have evolved to make the rat more vulnerable to cat predation to increase its transmission rate.

Prey/ Diet

Not surprisingly they are omnivores and will eat almost anything with a preference to cereals and when available eggs and cheese. They are incredibly adaptable in hunting strategies some US populations have been observed catching small fish,  some in Europe have been known to dive for shellfish and a colony on the island of Norderoog off the German coast have been know to kill birds even ducks. At times of over population they are also thought to be cannibalistic.


The Brown rat is thought to have originally evolved in China but can now be found nearly everywhere on the planet (with the exception of Antarctica) making it one of the most adaptable creatures on the planet only surpassed by humans, in fact for the most part where there are humans , there are Brown rats. The Brown rat has certainly been spread across the globe by humans often by ship in earlier centuries; it may have arrived in Europe in the 16th century and reaching America around a hundred years later. The industrial revolution favoured the Brown rat as houses became brick and sewer systems developed the amount of habitat increased whereas the  climbing black rat was more at home in thatched roofs.

Brown rats are adept burrowers normally making a lair next to a building so it can be used to support walls and roof, older lairs can have many levels of tunnels often with a secondary entrance. The bulk of the digging is completed by young males and females with older rats not taking part. These nests provide safety from predators but also areas to store food and help to regulate temperature for any pups. The rat’s first response to a threat is to retreat to these burrows; they will normally stay within 20meters of the burrow.

Brown rats are normally associated with human populations and they often outnumber the human population in urban areas but not by as much as some stories put forward. In the UK milder winters have meant a rise in the rat population with an estimated 81 million rats. Other factors in the UK have also increased rat populations, the move to fortnightly rubbish bin collection in many areas and the reduction of the invasive mink population (a major rat predator) as the native otter population increases have all contributed. When humans aren’t present brown rats like damp areas such as river banks as the soft banks can be easily dug and the rats are good swimmers.

Brown rats are also kept as pets in many countries with many pet shops in the UK and US now selling them. These ‘fancy rats’ have now been developed in a variety of breeds of different colours, coats and even sizes, even with their own rat associations establishing standard breeds and promoting events