All four species (Archey's frog, Hamilton's frog, Hochstetter's frog, and the Maud Island frog) are unique in that they have kept features which have evolved out in other more "modern" frogs. These features include the absence of a tadpole stage - New Zealand adult frogs lay eggs and the young frogs develop inside the egg sac, and then after hatching, the young climb onto their father's back to complete the last weeks of development. New Zealand frogs also do not have webbing between their toes (Hochstetter's frog is only partially webbed), and do not have an external eardrum or a croak. They make a faint squeaking noise instead.
Archey's frog is found on the Coromandel Peninsula and near Te Kuiti. Hochstetter's frog covers an extensive part of the North Island, from the East Cape to Whangarei. Both of these populations are believed to be in the thousands.
Hamilton's frog is found only in a small 600-square-metre area on Stephens Island in Cook Strait, and numbers only about 200, but it is believed to live for more than 23 years. It is one of the most endangered frogs in the world.
The Maud Island frog was thought to be the same as Hamilton's frog, but is now considered a separate species. Its numbers are about 19,000.
The New Zealand frogs live in the forests under damp rocks, rotting logs and plants, rather than swamps or ponds. They need to keep their skin moist but can stand higher levels of dehydration than other frogs. Their main defensive weapon, apart from their protective colouring, is their ability to "freeze" in the presence of a possible predator. They also secrete a substance from their glands which gives them an unpleasant taste.
The New Zealand frogs are smaller than common frogs, with Archey's frog the smallest at 31 mm for males, and 37 mm for females. They are all nocturnal. The tailed frog of North America is the only other species of frog in the world which is as primitive as the New Zealand frogs.
Three species of frog have been introduced to New Zealand. They are bigger than the native frogs, go through a tadpole stage, and croak. New Zealand originally had seven species of native frog, but three have become extinct since the arrival of humans and animal pests, such as rats and ferrets..
Frogs numbers are declining throughout the world. Because frogs absorb many things through their skin, they are sensitive to disease, pollution and other environmental changes. Attempts to protect the most endangered of the frogs, Hamilton's frog, include the establishment of other populations of the frog on other islands. This is done so that if disease or some other incident, such as fire, wipes out one population, there will be survivors elsewhere.