A rat king is a rare phenomenon arising in rats. A number of rats become intertwined at the tails, and are joined with blood, dirt, and excrement. Consequently, the animals grow together, joined at the tails, which are often broken. The phenomenon is particularly associated with Germany, where the majority of instances have been located.
Rat kings have always been a subject of fear and superstition, and killed upon discovery. Nevertheless, there were occasional findings of dead and subsequently mummified rat kings. The museum Mauritianum in Altenburg (Thuringia) shows the largest well-known mummified “rat king”, which was found in 1828 in the fire-place of a miller in Buchheim. It consists of 32 rats. Alcohol-preserved rat kings may be viewed in museums in Hamburg, Hameln, Goettingen and Stuttgart. Altogether, the number of well-known finds of rat kings is small. Depending upon source, it varies between 35 and 50 finds.
The earliest report of rat kings comes from 1564. With the displacement of the black rat by the brown rat in the 18th century, the phenomenon has died down. For instance, since the beginning of the 20th century, there have been fewer sightings of rat kings, the most recent find dating from April 10, 1986, and comes from a French discovery in the Vendée.
The “rat king” arises particularly in the black rat (Rattus rattus). The only find involving sawah rats (Rattus rattus brevicaudatus) occurred on March 23, 1918, in Bogor on Java, where a rat king of ten young field rats was found. Similar groupings arise occasionally in other species: in April 1929, a group of young forest mice (Apodemus sylvaticus) was discovered in Holstein, and there continue to exist reports of squirrel kings, of which the Zoological Institute of the University of Hamburg allegedly have a specimen. Rat kings are not to be confused with conjoined twins, which arise in several species. With the rat king, the animals grow together only after birth and are completely separate during birth.
Historically, rat kings were seen as an extremely bad omen, particularly associated with plagues. Such events also occurred, since rat kings arise only if too many rats exist and are according to little place for new buildings. With an increase in the size of the rat population comes an increase of the chance of outbreak of disease, for example the Black Death, which is spread by rat fleas.