rat

Any of numerous long-tailed rodents (especially of the families Muridae and Cricetidae) larger than mice and usually with scaly, naked tails. The genus Rattus in the family Muridae includes the rats found in human housing.

The brown rat Rattus norvegicus is about 20 cm/8 in long with a tail of almost equal length. It is believed to have originated in central Asia, and is now found worldwide after being transported from Europe by ships. Female brown rats become sexually receptive at the age of 8–12 weeks. If food is plentiful, litters of up to 12 are born every few months. The black rat R. rattus, responsible for the plague, is smaller than the brown rat, but has larger ears and a longer, more pointed snout. It does not interbreed with brown rats. The pack rat or wood rat, genus Neotoma, is common throughout North America and there are seven different species. Their dens, made of partly eaten plants, dung, and miscellaneous objects, are known as middens and can be up to 2 m/6.5 ft across and 20–30 cm/8–12 in high. The rats' crystallized urine preserves the midden, in some cases for up to 40,000 years.

Recent discoveries In 1997 a new species of tree rat was discovered in French Guiana; Isothrix sinnamariensis is 20 cm/8 in long.

In 2003 French scientists cloned laboratory rats for the first time. The technique used for this modified the standard cloning technique (of implanting DNA into an emptied egg) by introducing an additional step where cell division in a harvested rat egg was inhibited prior to the usual procedure. This research was developed to enable the routine alteration of rat genes in order to produce a range of genetically altered rats.

The genome of the brown rat was decoded in 2004. The draft DNA sequence is made up from 2.75 billion pairs of DNA bases and covers around 90% of the rat genetic code.