bears into brown bears, but not the other way around, was widespread in time and space during the Pleistocene. Of particular note, the bears living on the islands of the Alexander Archipelago of southeast Alaska trace their maternally inherited mitochondrial DNA entirely to polar bears, but over 90% of their nuclear genome to brown bears. This appears to reflect a process in which a population of polar bears was left behind as the species retreated northwards at the end of the last ice age, with male brown bears subsequently introducing genes from the adjacent mainland, but female brown bears being generally unable or unwilling to swim across several kilometres of open ocean to reach the islands (thus the lack of exchange of mitochondrial DNA). Such studies have not been limited to polar bears and brown bears, and it now appears that gene flow between species has been widespread during the evolution of the living species of bears. Naming Since the 2006 discovery placed the hybrid into the spotlight, the media have referred to this animal with several portmanteau names, such as pizzly bear, grolar bear, and polizzly, but there is no consensus on the use of any one of these terms. Canadian wildlife officials have suggested calling the hybrid "nanulak", taken from the Inuit names for polar bear (nanuk) and grizzly bear (aklak). By one convention, the name of the sire comes first in such combinations: the offspring of a male polar bear and a female grizzly would be the suggested nanulak or a "pizzly bear", while the offspring of a male grizzly and a female polar bear would be a "grolar bear" or possibly an aknuk. Captive hybrids Two grizzly–polar hybrid cubs (one female and one male) were born at Osnabrück Zoo in Osnabrück in 2004, and their physical traits are generally intermediate between the polar bear and the grizzly bear. For example, their bodies are smaller than polar bears, but larger than grizzlies, while their heads fall between the broader grizzly head and the leaner polar bear head. They have long necks like polar bears, but small shoulder humps like grizzlies. The soles of their feet are partially covered in hair; polar bears have hair-covered soles, which act as insulation, and grizzlies have hairless soles. Similarly, the hair of the hybrids exhibits a pattern of hollowness, which blends the traits of polar bears and grizzlies. In cross-section, the