Polar bears are unable to meet the criteria to be classified as marine mammals.
 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)
 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

