There are around 1,300 known species of insects in Iceland, which is low compared with other countries (over one million species have been described worldwide). The only native land mammal when humans arrived was the Arctic fox,[57] which came to the island at the end of the ice age, walking over the frozen sea. On rare occasions, bats have been carried to the island with the winds, but they are not able to breed there. Polar bears occasionally come over from Greenland, but they are just visitors, and no Icelandic populations exist.[58] There are no native or free-living reptiles or amphibians on the island