Amphibians
Hamilton's Frog
Leiopelma hamiltoni · also called Maud Island frog
For most of the last century, the entire world population of Hamilton’s frog lived in one rock pile on one New Zealand island. It is a small, silent, brown frog from one of the two oldest branches of the frog family tree, and it is still one of the rarest amphibians on Earth.
Overview
Leiopelma hamiltoni is one of just four living species of the New Zealand frog family Leiopelma, an ancient lineage that split from all other frogs over 200 million years ago. It lives in cool, damp native forest with deep rock piles, on a few predator-free islands in the Marlborough Sounds.
No tadpole, almost no voice
The female lays a few large eggs on land, the male guards them, and each hatches into a tailed froglet that rides on the male’s back to finish developing — there is no free-swimming tadpole. The frog has no eardrum and essentially no call.
A tiny population
For decades the whole species lived in a single room-sized rock pile on Stephens Island. It is extremely sedentary, staying under the same rock for years, and may live 30 years or more. Recovery has meant clearing rats from islands and moving frogs to establish new populations.
Status
Vulnerable, confined to a handful of small predator-free sites, its survival entirely dependent on keeping those places free of introduced mammals.
Hamilton's Frog: common questions
Why is Hamilton's frog so rare?
Because New Zealand had no native land mammals except bats, and its frogs evolved with no defences against them, so when rats, stoats, cats, and other mammals arrived with people over the last few centuries they ate the frogs and their eggs and cleared them from the mainland and most islands. Hamilton's frog survived only in the few places those predators never reached or were later removed, which by the mid-1900s came down to essentially one small rock pile. It also recovers very slowly, because it produces few eggs, matures late, and barely disperses, so even a protected population grows only gradually.
How can a frog have no tadpole?
Hamilton's frog and its Leiopelma relatives use a strategy called direct development, common in many frog families but taken further here. The female lays a small number of unusually large, yolk-rich eggs on land, in a moist chamber under a rock or log rather than in water. The male sits with the clutch and keeps it damp, and inside each egg the embryo passes through what would be the tadpole stage, so it hatches as a tiny four-legged froglet with a small tail. The froglet then crawls onto the guarding male's back, where it clings and is carried while it absorbs the last of the tail and yolk. No pond is involved at any point.
What is the difference between Hamilton's frog and Archey's frog?
They are the two rarest of New Zealand's four native frogs, both in the genus Leiopelma, and they look similar, small, brown, warty, and secretive. Archey's frog is the smaller of the two, is more strongly associated with the forest floor of the Coromandel Peninsula and one other North Island area, and is currently ranked as Critically Endangered and near the top of global lists of the most evolutionarily distinct and endangered amphibians. Hamilton's frog is slightly larger, lives on cool islands and sanctuary sites in the Marlborough Sounds, and is ranked Vulnerable, with its situation improving as more predator-free populations are established.
Can you see a Hamilton's frog?
Not easily, and not without a permit. The sites where it lives are strictly protected, access is limited to researchers and conservation staff, and the frogs are nocturnal, cryptic, and buried in rock piles by day. This is deliberate. The species came within a hair of extinction, biosecurity to keep rats off the islands is critical, and disturbance or the accidental introduction of the amphibian chytrid fungus could be catastrophic for such tiny populations. The frog is instead known to the public mainly through New Zealand conservation programmes and captive research colonies.