As New Zealand is isolated from other land masses, it has few of the advanced animal species that evolved elsewhere. Apart from two species of bats, there are no native land animals. Instead, unique and often rare species of plants, birds and insects have developed. Some birds including the kiwi, kakapo and weka thrived on the forest floor, because of the lack of mammals and became flightless. However, with the introduction of European animals such as the possum, these birds have now become very rare.
Many seabirds live on New Zealand's shores including albatrosses, shearwaters, petrels and penguins.
Virtually all New Zealand's native insects are found nowhere else. The world's heaviest insect, the cricket-like giant weta, some of which are about the size of a mouse, is quite harmless. Several species of giant snails and unusual frogs reside in isolated areas. The tuatara, an ancient reptile with a lineage extending back to the age of the dinosaurs, is found only in New Zealand. There are no snakes or poisonous creatures except for the very rare coastal katipo spider.