Aerodynamic amphibians

It became obvious to me that some frogs had truly “taken to the trees” during my PhD research in the rainforests of northeastern Australia. Radio-tracking frogs, I found most frogs more or less where you’d expect- in the river, on the forest floor, or on vegetation not too far off the ground. Green-eyed tree frogs (Litoria serrata), however, amazed me by spending weeks hanging out in the rainforest canopy- about 20 m off the ground. For days at a time, they stayed high up in the trees, seemingly pretty happy up there with the birds.

Female Black-webbed Flying Frog (Rhacophorus kio)

However, it wasn’t until I began spending time in the forests of Southeast Asia that I encountered the frogs that have taken arboreal life to the extreme. So much so, that they’re known as “flying frogs”. One rainy night in the Cambodian forest about six years ago, I had the privilege of having dozens of frogs fly out of the trees around me and into the pond I was wading in. All around me I could hear the frogs land in the water “Plop”, “Plop”, “Plop!”, and shining my light into the sky I saw frogs, arms and legs outstretched, gliding gracefully into the water.

Unrelated to the tree frogs of Australia, the Americas and Europe (family Hylidae), the frogs leaping from the trees (family Rhacophoridae) were the pinnacle of arboreal adaptation in frogs! For me, it was love at first sight! “Flying” frogs are not actually capable of powered flight (they don’t flap!), but they are capable of travelling impressive horizontal distances as they descend from the forest canopy to breed in the ponds below, escape predators, or look for food.

The best “fliers”, or more correctly, “gliders”, have enlarged, extensively webbed hands and feet, lateral skin flaps on the arms and legs, and reduced weight per snout-vent length. These adaptations allow them to glide great distances and manoeuvre themselves mid-air. When gliding, they hold their front and hind limbs lateral to their bodies and spread their fingers and toes. One species of flying frog, Denny’s Flying Frog (Rhacophorus dennysi), is even estimated to be about one-third as maneuverable in the air as a bird of prey, the Laggar Falcon (Falco jugger)!

Flying frogs (L-R): Large arm flaps in the Spinybottom Flying Frog (Rhachophorus exechopygus); an extreme flier, Helen’s Flying Frog (Rhacophorus helenae); and the fully webbed foot of the Annam Flying Frog (Rhacophorus annamensis).

While there are some amazing adaptations to arboreal living, such as waterproof skin secretions to prevent dehydration, and the ability to lay eggs in the trees (often hanging eggs in foam nests on leaves, or laying eggs in tree-holes), the ability to “fly” wins my vote as the most impressive. The most extreme flying frogs include those from Southeast Asia; Reinwardti’s Flying Frog (Rhacophorus reinwardtii), Wallace’s Flying Frog (Rhacophorus nigropalmatus), and the relatively recently discovered Helen’s Flying Frog (Rhacophorus helenae).

Flying frogs are all so well-adapted to living in trees that they can’t do without them: forest loss throughout the region threatens their continued survival.

More information:

Emerson, S. B., & Koehl, M. A. R. (1990). The interaction of behavioral and morphological change in the evolution of a novel locomotor type:” flying” frogs. Evolution, 1931-1946.

Emerson, S. B., Travis, J., & Koehl, M. A. R. (1990). Functional complexes and additivity in performance: a test case with” flying” frogs. Evolution, 2153-2157.

McCay, M. G. (2001). Aerodynamic stability and maneuverability of the gliding frog Polypedates dennysi. The Journal of Experimental Biology, 204, 2817-2826.

This blog is part of the Reptile & Amphibian Blogging Network) event, “Herps Adapt”.