Frogs in the trees

In making the transition from the swamp to the tree-tops, different groups of frogs have evolved in remarkably similar ways.

Several unrelated groups of frogs have made the enormous leap from life in the swamp to dwelling in the trees. In order to make the transition, these groups have evolved into strikingly similar body shapes and behaviors. The result is that “tree frogs” all over the world and from a number of different families of frogs look and behave spectacularly similarly.

Millions of years of convergent evolution has honed an array of unrelated frogs into the tree-dwelling creatures that we see today. In Australia and Europe, tree frogs belong to a group of frogs known as the “true tree frogs” (family Hylidae). In Southeast Asia, their unrelated doppelgangers are “flying frogs” (family Rhacophoridae). In Central and South America, live the small, nearly transparent “glass frogs” (family Centrolenidae), while in Africa, “reed frogs” (family Hyperoliidae) are a common arboreal frog, and some Madagascan matellid frogs (family Mantellidae) have also opted for life in the trees. In some places, even the most unexpected group of frogs- toads (family Bufonidae)-have ditched the ground and quite happily live up above.

Although quite unrelated, many of these frogs look so similar that even a frog biologist such as myself has to examine them very closely to determine to which group of frogs they belong. Take the Australian White-lipped Tree Frog (Litoria infrafrenata, family Hylidae) and the Asian Feae’s Tree Frog (Rhacophorus feae, family Rhacophoridae), for example. Who would think that millions of years of evolution separates them?

So what challenges did frogs have to overcome in their ascent to the trees? Well, perhaps the most pressing problem in adapting to arboreality was water loss. Most aquatic frogs lose water from their skin like crazy. That’s fine when you live in or very near to water, but it’s not going to serve you well sitting up a tree all day evaporating. As a solution to this problem, many tree frogs secrete waxes or lipids from their skin, to serve as water-proofing. In fact, many tree frogs are so ‘waterproof’ that they are as resistant to water loss as reptiles!

The other big challenge was the need to climb, a feat for which aquatic and terrestrial frogs are ill-adapted. This need was solved in tree frogs by having expanded pads on their fingers and toes, thereby increasing their adherence to surfaces. One group of tree frogs (some flying frogs of the genus Rhacophorus) has taken arboreal life to the extreme. Not happy with simply climbing up and down trees, they use their enormous, webbed hands and feet to glide, or parachute down from the canopy home!

A tree frog also tends to look like a tree frog in ways other than simply having toe pads- they're typically relatively slender, have large heads and eyes, long limbs and long toes. This is quite the opposite of their burrowing relatives, which tend to be rotund, have small heads and eyes and short limbs. Clearly, one body shape suits the trees, another a life underground!

Some tree frogs have become so adapted for living in the trees that they have lost their last tie to the ground- the need to lay eggs in streams or ponds. Although most species still do come down from the trees to lay their eggs in waterbodies, others deposit eggs on leaves overhanging water (where they drop into as tadpoles). Still others opt for laying their eggs within the trees themselves, using water-filled holes in the tree trunk, or water-filled plants (eg. bromeliads) on tree branches.

Overall, in adapting for life in the trees, a number of unrelated groups of frogs have independently overcome the challenges of aboreality in remarkably similar ways. The result is that tree frogs around the world all look and behave like "tree frogs"- convergent evolution at its best!

Dr Jodi Rowley
Amphibian biologist & coordinator, AMRI

This post is part of a Reptile and Amphibian Blogging Network (RAmBlN) online event called #CrawliesConverge. We are writing on convergent evolution in reptiles and amphibians. Find our event schedule here. Follow on Twitter or Facebook.