Amphibians
Mossy Frog
Theloderma corticale · also called Vietnamese mossy frog, Tonkin bug-eyed frog
The mossy frog is one of the best camouflaged animals in the world. Its green, black, and rust-brown skin is raised into a dense field of warts, spines, and flaps that mimics not just the colour but the lumpy three-dimensional texture of a moss clump on a wet limestone rock. If that fails, it rolls into a limp ball and plays dead.
Overview
The mossy frog, Theloderma corticale, lives along cold, clear, rocky streams and in the mouths of limestone caves in the mountains of northern Vietnam and adjacent China. It is an ambush insectivore, sitting motionless at the water’s edge or wedged in a crevice until prey comes close, and it spends much of its time part-submerged in cold water.
Moss, and a ball
The camouflage matches its mossy-rock background in colour, pattern, and texture, and the frog holds still for long stretches to make the most of it. Startled, it curls into a ball and goes limp, often dropping into the water — a death-feigning trick that pairs with the camouflage to make a predator lose track of it. Males give a loud, ventriloquial call from rock cavities that is very hard to trace by ear.
Breeding in cavities
Instead of open ponds, mossy frogs lay eggs on the wall of a water-filled hollow — a tree rot-hole, a rock pocket, a small cave — just above the waterline, and the hatched tadpoles drop into the pool below.
Status
Least Concern but decreasing. The frog is fairly widespread in northern Vietnam’s mountains and tolerates some disturbance, but it is collected for the pet trade and pressured by montane-forest loss and pollution of its cave and stream habitats. Several close relatives in the genus Theloderma have smaller ranges and are more threatened.
Mossy Frog: common questions
Why does the mossy frog look like moss?
Its appearance is camouflage taken to an extreme. The mossy frog lives on and among moss-covered limestone rocks and mossy logs beside cold mountain streams, and its skin has evolved to match that background not just in colour but in texture. The green, black, and rusty-brown blotches break up the frog's outline, while a dense covering of raised warts, pointed tubercles, and small skin flaps mimics the three-dimensional roughness of a moss clump, so that even the shadows fall the way they would on real moss. A motionless mossy frog gives a visual hunter almost nothing to lock onto, and the frog reinforces this by staying still for long periods and choosing resting spots where it blends in.
Does the mossy frog really play dead?
Yes. When a mossy frog is grabbed or badly startled it pulls its limbs in against its body, curls up into a rounded lump, and goes limp, often letting go of its perch and dropping, sometimes into the water where it may sink and stay still on the bottom for a while. This death-feigning, or thanatosis, works well in combination with the camouflage. A predator that has briefly seen the frog move may lose the thread when it suddenly becomes an inert, mossy-looking ball, and many predators are less interested in prey that appears to be already dead. After the threat passes the frog uncurls and moves off.
Where do mossy frogs lay their eggs?
Not in open water like most frogs. A male calls from a water-filled hollow, which might be a rot hole in a tree, a pocket in a rock face, or a small cave, all close to a stream and with a pool of standing water in the bottom. The female lays a small clutch of eggs on the wall of the cavity just above the water surface, where they are hidden and protected. When the tadpoles hatch they wriggle or fall down into the water beneath and finish developing there. This use of small, enclosed water bodies rather than ponds is common in the genus Theloderma and is one reason the frogs are so tied to rocky, cave-rich terrain.
Is the mossy frog a good pet?
It is one of the more popular oddity frogs in the hobby, kept for its extraordinary mossy appearance and its relatively bold, out-in-the-open daytime behaviour in a well-set-up tank, and captive-bred animals are now available, which is preferable to wild-caught ones. The catch is that it comes from cold mountain streams and caves, so it needs cool, clean, highly oxygenated water and cool air, ideally kept below the low twenties in Celsius, along with plenty of rock and cork hiding places and a large water area. In a warm living room without a chiller it tends to weaken and become prone to infection, so it is not a beginner species despite often being sold as one.