Amphibians
Darwin's Frog
Rhinoderma darwinii · also called southern Darwin's frog, Darwin's frog
Darwin’s frog is a tiny frog from the cool southern beech forests of Chile with two claims on our attention: it is one of the best leaf mimics of any vertebrate, and it has one of the strangest ways of raising young in the animal kingdom — the male broods his tadpoles inside his own throat.
Overview
Darwin’s frog lives on the floor of temperate Nothofagus forest and in mossy forest bogs in central and southern Chile, with a toehold in adjacent Argentina. It is small — barely the length of a thumbnail — diurnal, and slow-moving, picking small invertebrates out of the leaf litter. Its whole strategy is not to be seen.
The leaf disguise
The frog is shaped like a dead leaf. Its back is brown or green, its snout is drawn out into a pointed, leaf-tip flap, and low ridges on its skin suggest a leaf’s veins. A resting Darwin’s frog among fallen leaves is genuinely hard to pick out. If a predator does disturb it in the open, the frog falls onto its back or goes limp, showing its dark, mottled belly, and stays still — reading, once again, as a scrap of leaf rather than food. If it is near water it may drop in and float motionless.
The father’s vocal sac
Darwin’s frog is a mouth-brooder. The female lays 3 to 30 large eggs in moist leaf litter, and the male stands guard over them for about three weeks. As the tadpoles start to move inside the eggs, the male takes them into his mouth and passes them into his vocal sac, the elastic pouch under his chin. There the tadpoles grow for six to eight weeks, fed by their yolk reserves and by the sac lining, while the male continues to move, feed, and call around them. When development is finished, the froglets crawl forward and the father releases them from his mouth, fully formed. No other frog raises its young in exactly this way.
Two species, one nearly gone
There are two Darwin’s frogs. The northern species (Rhinoderma rufum) has not been recorded since the 1980s and is considered Critically Endangered and possibly extinct. The southern species covered here is Endangered and declining.
Threats and conservation
The southern Darwin’s frog is losing its native forest to logging and to plantations of introduced pine and eucalyptus, which do not support it. The amphibian-killing fungus chytrid has been found in its populations and is linked to sudden local disappearances. Conservation efforts include protected forest reserves, captive “assurance” colonies in Chilean and international zoos, and disease monitoring, all aimed at keeping the world’s only father-brooding frog from following its northern cousin.
Darwin's Frog: common questions
How does Darwin's frog carry its babies in its mouth?
The female lays a small clutch of large eggs on damp ground, and the male guards them for about three weeks. When the eggs are ready to hatch and the tadpoles begin to wriggle, the male gathers them into his mouth and slides them down into his vocal sac — the stretchy throat pouch he normally uses to call. The tadpoles develop there for six to eight weeks, nourished by their yolk and by secretions from the sac wall, until they are complete little frogs. The father then opens his mouth and they hop out. He is not eating them; the vocal sac is acting as a portable nursery.
Charles Darwin discovered this frog?
Yes. Darwin collected the frog in 1834 during the voyage of HMS Beagle, in the forests of Chile, and it was formally named after him in 1841. The extraordinary mouth-brooding behaviour was not described until later in the nineteenth century, and for a long time some naturalists doubted it was real.
Why is Darwin's frog endangered?
There are two species. The northern Darwin's frog has not been seen since the 1980s and is presumed extinct. The southern Darwin's frog survives but is declining because its temperate forest habitat is being logged and converted to pine and eucalyptus plantations, and because the amphibian fungal disease chytridiomycosis has reached its populations — studies link local disappearances of the frog directly to chytrid outbreaks. Its small size, low reproductive output, and patchy distribution leave little margin.
How does the leaf disguise work?
The frog's body is shaped and coloured like a fallen leaf, right down to a triangular fleshy point on the snout that mimics a leaf tip, and skin folds that suggest veins. It relies on staying motionless. Caught in the open, it uses a second trick — it throws itself onto its back or lets itself go limp, exposing its boldly blotched underside, and drifts or lies still so that a predator sees only leaf litter rather than a frog.