The Estonian Fauna stamp series stamp depicts the black rat (Rattus rattus). The other rat species found in Estonia, the brown rat (R. norvegicus), arrived here later and has nearly driven the black rat out of Estonia. Currently, the black rat can be found near Lake Peipus and in south-eastern Estonia, mostly in rural areas, residing predominantly in auxiliary buildings but also attics. The rat gets its name thanks to its dark fur. The black rat is an excellent climber, using its long tail to keep balance. In extreme cold, rats huddle together and can sometimes form a rare phenomenon called the rat king. Rat king means that several rats have their tails entangled in a knot and can no longer break free. In 2005, one such rat king was found in Saru, in southern Estonia. The rat king consisted of 16 rats and is currently visible at the Tartu University Natural History Museum. Black rats are omnivores and commensal with humans, meaning they also spread several dangerous diseases.