LSM1303 Animal Behaviour Student Blog

We always think of frogs as being amphibians whose existence is restricted only to land and water. This is largely true, given the fact that frogs have evolved over time to develop specific physical characteristics – lungs for breathing on land, webbed feet for swimming – that maximize their ability to survive in both these environments.

However, for some frogs, being masters of both land and water just isn’t enough… read on to find out more…

  Wallace's Flying Frog. Photograph by Tim Laman

The ‘frog-ject’ of interest…
The particular frog being examined in this blog post would be Wallace’s Flying Frog, also known by its scientific name as Rhacophorus nigropalmatus. Named after the biologist, A R Wallace, who was the first person to collect the species for official identification, the frog has an overall length of between 90 and 100 mm. Its large eyes, tympanum and limbs are particularly distinct, and the large size and unique structure of its limbs will be the central discussion point in this blog post.


What’s so interesting about this?
Simple! These frogs can fly!!! Unlike the majority of its fellow cousins, Wallace’s Flying Frogs spend almost the whole of their lives living in trees, and only come down to the jungle surface to mate and spawn.

“When threatened or in search of prey, they will leap from a branch and splay their four webbed feet. The membranes between their toes and loose skin flaps on their sides catch the air as they fall, helping them to glide, sometimes 50 feet (15 meters) or more, to a neighboring tree branch or even all the way to the ground. They also have oversized toe pads to help them land softly and stick to tree trunks.”

Swooping in for the kill: Wallace's Flying Frog. Photograph by Tim Laman

     "Look out! Here I come!!!" Photograph by Tim Laman

This behavior is particularly interesting because:

• It illustrates how the environment can influence the ways in which animals develop and evolve: The dense jungle floor can be a dangerous place for frogs as predators can easily camouflage and ambush unsuspecting frogs. By eliminating the need to travel on the jungle floor with their ‘flying’ ability, Wallace’s Flying Frogs are able to increase their chances of survival as exposure to land-based predators is significantly minimized. Furthermore, this ability to ‘fly’ also increases their chances of capturing their prey. As highlighted earlier, Wallace’s Flying Frogs spend most of their lives living in trees. The sheer height of the trees provides these frogs with a significant ‘high ground advantage’ – from an elevated position, these frogs are far more able to spot their prey, and swoop down on their prey using the most direct route without warning, thereby increasing their chances of grabbing a meal.

• It demonstrates the power of evolution: This ability to ‘fly’ represents a departure from how one would normally expect a frog to behave. Originally beginning their existence as water-borne creatures, frogs gradually evolved over thousands of years to conquer the land. This evolution of the frog species to include the ability to fly highlights the fact that evolution knows no boundaries – it is a process steeped in continuity, where unpredictable but necessary results often occur to enhance and prolong the existence of animal species, where only the fittest survive.

A few additional points…
How then does Wallace’s Flying Frog fly? Wallace’s Flying Frog doesn’t fly per se. Rather, it glides through the air by extending the thin membranes and skin flaps extended over their feet and toes. Linking this with the Lift Equation below, where F_L represents the lift force experienced by the lifting surface and A represents the surface area of the lifting surface, the amount of lift force generated by the lifting surface is directly proportional to the surface area of the lifting surface. By extending its membranes and skin flaps, Wallace’s Flying Frog is actually maximizing the surface area of its ‘flying surface’, thereby maximizing the amount of lift that can be generated, hence allowing it to glide over considerable distances.

F_L = C_L  (1/2) pv² A

While videos that show Wallace's Flying Frog in action are few and far between, the workings behind its flying technique can be understood after you view the following video of how the Flying Squirrel glides.

As seen in the video, the Flying Squirrel extends its legs outwards, stretching the flap of skin linking these legs together, providing a substantial lifting surface that enables it to glide from point to point. (Recall the lift equation I mentioned earlier...) It can be observed that both these animals utilize the same principle of flight to glide over distances - an interesting observation, considering how different these two animals are: one's an amphibian, one's a mammal. This then leads to another interesting question: Were they related to each other in pre-historic times but evolved separately to be so different yet so alike? Or is this shared technique just a pure coincidence, with each animal owing it to the shared circumstances of escape and predation?

References:

The Lift Equation. National Aeronautics and Space Administration, Glenn Research Center. 15 June 2007.
http://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html

Wallace's Flying Frog. National Geographic Society. 18 March 2008.
http://animals.nationalgeographic.com/animals/amphibians/wallaces-flying-frog.html?nav=FEATURES