A rat king is formed when a number of
rats become tied together by their tails. Most rat kings consist of a
tangled
mass of between 5 – 10 rats, although rat kings comprised of up to 32 rats have been reliably
recorded.
"There is no doubt that rat kings exist: sixty or so have been reported in Europe since 1564 and about 40 (most of them found alive) have been authenticated , the latest in 1963. Rat kings have frequently been preserved, painted, and photographed." (1)
Rat kings are thought to be formed when the tails of young rats become glued together by feces and urine while in the nest. The knots thus formed are then pulled tighter when the rats attempt to extricate themselves. Sometimes rat kings perish due to their inability to forage effectively. However, they are sometimes also fed by other family members.
Attempts to induce rat kings in the laboratory by tying the tails of live rats together have been unsuccessful to the extent that the results look nothing like real kings. However, gluing the tails of rats together has been more successful. Although initially not knotted, the rats themselves struggle until a true knot is formed.
Zoologists have yet to prove exactly how rat kings are formed in nature. "It is possible that the tails become entangled when the rats huddle together facing outward for warmth and security, urine and feces from those in the upper circle falling onto the entwined mass of tails.... Other possibilities are that the tails might become entangled while the males are wildly fighting for females, or during mass grooming, or in the nest shortly after birth, or after the tails of a number of rats come into contact with some sticky substance." (1)
The most persuasive theory that I have been able to find to account for the origin of the name "rat king" is that at one time it was believed that the conjoined rats were actually a single organism.
Apparently, there have also been a few verified squirrel kings and several unverified mice kings.
Sources:
- (1) More Cunning than Man: a social history of rats and men; Robert Hendrickson; Dorset Press, New York, quoted at http://www.panix.com/~sjl/wonder/lewis/ratking/rat_intro.htm
- Articles from Fortean Times magazine referred to from memory. Let's be honest, this isn't the sort of thing you forget in a hurry.