Amphibians
Wallace's Flying Frog
Rhacophorus nigropalmatus · also called Wallace's flying frog, Abah River flying frog, gliding frog
Wallace’s flying frog does something frogs are not built to do: it launches off a branch, spreads its enormous webbed feet like four small parachutes, and glides through the rainforest canopy to another tree, steering as it goes. It is named for the naturalist who first brought it to Western science, and it comes down to earth only to make foam.
Overview
Wallace’s flying frog lives in primary lowland and hill rainforest on the Malay Peninsula, Sumatra, and Borneo. It is almost entirely arboreal and nocturnal, hunting insects high in the trees, and it descends to ground level only to breed at forest pools and animal wallows. It is vivid green with yellow flanks and black webbing.
Built to glide
The frog’s gliding equipment is its fully webbed hands and feet — the webbing runs to the tips of every digit — plus flaps of skin along the limbs. Airborne, it fans all of this out to form airfoils and drag surfaces, descending at a shallow angle and covering 15 metres or more in a single glide. It can bank and turn, flare its feet to brake, and land by gripping a trunk with its large sticky toe pads. It shares its forests with other, unrelated gliders — flying lizards, flying snakes, flying squirrels, colugos — that all evolved the trick separately.
Wallace’s frog
Alfred Russel Wallace, co-discoverer of natural selection, was given one of these frogs by a worker in Borneo in the 1850s and described it in The Malay Archipelago. It was later named in his honour and is one of the animals most associated with him.
Foam nests
To breed, pairs gather on branches or trunks overhanging a pool or a muddy wallow. The female produces a fluid that she and one or more males whip into a firm foam ball with their legs, laying the eggs inside. The foam shields the eggs from drying and predators; after a few days it breaks down and the tadpoles drop into the water below.
Status
Wallace’s flying frog is Least Concern but declining, tied to the fate of Southeast Asia’s lowland rainforest, which is being cleared and fragmented for timber and oil palm. It depends on large, connected canopy and on undisturbed breeding pools.
Wallace's Flying Frog: common questions
Can Wallace's flying frog actually fly?
It cannot fly under power, but it is a true glider, not just a parachutist. When it leaps or falls from a branch it spreads all four limbs wide, fanning out the large webs between its fingers and toes and the skin flaps along its arms and legs. These surfaces generate lift and drag, letting the frog descend at a shallow angle and cover 15 metres or more of horizontal distance from a single launch. It can also bank and turn in the air by adjusting its posture, and it flares its feet to slow down and lines up its big toe pads to stick a landing on a trunk or leaf. It is one of several unrelated Southeast Asian animals — lizards, snakes, squirrels, colugos — that have independently evolved gliding in the same forests.
Why did it evolve gliding?
The likely reasons are efficiency and escape. Moving through a tall rainforest by climbing down one tree and up the next is slow and exposes an animal to ground predators; gliding directly from tree to tree is fast and cheap. Gliding is also a good way to evade a predator such as a tree snake or a bird by simply stepping off into the air. For a frog that spends almost its whole life high in the canopy and only visits the ground to breed, being able to travel and escape three-dimensionally is a major advantage.
How is it connected to Alfred Russel Wallace?
Wallace, the British naturalist who independently arrived at the theory of evolution by natural selection at the same time as Darwin, spent years collecting in the Malay Archipelago in the 1850s. In Borneo a local worker brought him a large green frog with enormous webbed feet that had, he was told, glided down from a tree. Wallace described the animal in his 1869 book The Malay Archipelago, and it was later formally named in his honour. It remains one of the most famous specimens associated with him.
How do the foam nests work?
When it is time to breed, the frogs come down to branches, leaves, or tree trunks overhanging a pool or a muddy wallow — often ones made by pigs or rhinos. A female secretes a fluid that she and the attending male (or several males) whip into a stiff white foam with their hind legs, and the eggs are laid inside it. The foam protects the eggs from drying out and from many predators and keeps them oxygenated. After a few days it liquefies from the inside, and the tadpoles slide out and drop into the water below to finish developing.