Groups of rats can become bound together by their tails.
A “rat king” is a group of rats whose tails are intertwined and stuck together. Sometimes the tail hairs are simply entangled, and at other times sticky substances like sap or gum may be the culprits. As many as 32 rats have been found joined together in this way.
Rat kings are very rare, and they have sometimes been believed to be mythical. There are several well attested modern finds, however, and similar occurrences have been discovered among other small rodents, such as squirrels.