Let's be honest, I like rats. I don't love them, but the horrified reactions I've witnessed to the sight of them do puzzle me a little. I am not, however, trying to use geocaching to change anyone's minds about unpopular things. The second in this series was intended to be a butterfly, and that one will be created eventually! In the meantime, here is some hopefully unbiased information about arguably the most successful mammal on the planet, only humans excepted.
Rats are associated with many negative connotations, such as "dirty rat" or "drowned rat", and are frequently portrayed as villians in popular culture, take Ratigan in "The Great Mouse Detective" for example. Occasionally though they turn up as heroes, such as "Ratatouille".
The Old World rat and mice family has more species in it than any other mammal family. It includes all the British species of mice and rats, plus gerbils, spiny mice and Australian native mice. The brown rat (Rattus norvegicus), is one of the best known and most common rat. It is also referred to as common rat, street rat, sewer rat, Hanover rat, Norway rat, brown Norway rat, Norwegian rat, or wharf rat. They have prominent pointed muzzle, eyes and ears, greyish-brown fur and a long, almost naked, tail, about as long as the head-and-body. They are one of the largest muroids, with a common body length of 20-25 cm (8-10 in), and the tail adding 18-15 cm (7-10 in); the male weighs on average 350 g (12 oz) and the female 250 g (9 oz). This is up to twice the size of the black rat.
Thought to have originated in northern China, this rodent is the dominant rat in Europe and much of North America. As a species, brown rats have spread and established themselves along routes of human migration and now live almost everywhere humans are. They may have been present in Europe as early as 1553. As they spread from Asia, the brown rats generally displaced the black rats in areas where humans lived. In addition to being larger and more aggressive, the change from wooden structures and thatched roofs to bricked and tiled buildings favoured the burrowing brown rats over the arboreal black rats. Brown rats also eat a wider variety of foods, and are more resistant to weather extremes. In the absence of humans, brown rats prefer damp environments, such as river banks. However, the great majority are now linked to man-made environments, such as sewage systems. In addition to sewers, rats are very comfortable living in alleyways and residential buildings, as there is usually a large and continuous food source in those areas. The only brown rat-free zones in the world are the Arctic, the Antarctic, some especially isolated islands, and the province of Alberta in Canada, and certain conservation areas in New Zealand.
Selective breeding of Rattus norvegicus has produced the laboratory rat as well as pet rats. Both have evolved from the unusual of the species caught by rat catchers working in the nineteenth century. Caught rats were used in the sport of rat baiting, and unusually coloured ones began to be rescued and bred. Laboratory rats are descended from albino of the species, while the fancy rats kept as pets have developed great variety of colouring, coat pattern, style of coat and more recently variations in body size and structure. While the species is the same, the different living conditions affect their life span, diet and likelihood of carrying or suffering from different diseases.
Brown rats have acute hearing, are sensitive to ultrasound (which they can also produce), and possess a very highly developed olfactory sense. They perceive colors rather like a human with red-green colour blindness, and their colour saturation may be quite faint. Their blue perception, however, also has UV receptors, allowing them to see ultraviolet lights that some species cannot. Brown rats also produce communicative noises capable of being heard by humans. The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet.
The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders. Brown rats dig well, and often excavate extensive burrow systems. Burrows provide rats with shelter and food storage, as well as safe, thermo-regulated nest sites. The brown rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet. This makes them a real pest and it is estimated they are responsible for depleting a fifth of the global food supply every year. Rats impact on our wildlife too, preying on insects, small mammals, amphibians, reptiles and more. Ground-nesting birds make particularly easy targets. On the other hand, they provide a source of food for our predators. Foxes, stoats and barn owls for example will readily hunt and eat rats. And through their natural foraging techniques, rats act as seed dispersers. Their burrows also tend to aerate the soil, improving its overall condition. These scavengers also deal with the mounting piles of waste we leave behind, doing a vital job of reducing scraps and waste.
The brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and litters can number up to 14, although seven is common. They reach sexual maturity in about five weeks. The maximum life span is up to three years, although in the wild most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecies conflict as major causes.
Brown rats may carry a number of pathogens, which do affect humans. The accusation that they carried bubonic plague and caused the Black Death is unfair though. This was transmitted by rat fleas carried by black rats and still found on certain ground squirrels and wood rats. It is also commonly believed that keeping pet rats can lead to disease transmission. However, domesticated rats are physiologically and psychologically different from their wild relatives, and, when acquired from reliable sources, they pose no more of a health risk than other common pets.