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The rat king with the greatest number of rats in it was found in a miller's chimney at Buchheim, Germany, in May 1828. It comprised of 32 individual rats. A rat king typically occurs when young rats living close to each other get their tails entangled and encrusted with dirt. This knot then tightens when they pull away from the group. Being trapped, they cannot feed themselves and eventually die.
This rat formation is called a 'rat king' or roi de rats in French, and a number of examples have been documented in Europe during the past centuries.
The earliest recorded was on the 13 July 1748, when German miller Johann Heinrich Jager found 18 rats all inextricably linked to one another by their tails, intertwined in a large, intricate knot.
Main photo: Museum Mauritianum in Altenburg, Germany via Wikipedia Creative Commons