The flying frogs of Sabah
By Shavez Cheema and Martin Parry, 1StopBorneoWildlife
Reading Alfred Russel Wallace’s descriptions of his travels in ‘The Malay Archipelago’, tells us much about the man: he had courage, often undertaking dangerous sea journeys where he was at the mercy of prevailing winds and currents, and relying on the skills of local seamen. He was also very observant and, although he first came to S.E. Asia as a collector, mainly of beetles, birds and butterflies, in his classic book we can read detailed accounts of the fauna, flora and human societies of the region.
He never visited the land now called Sabah but, in the period 1854-1862, he journeyed and collected in both Sarawak and more widely in parts of present day Indonesia and Papua New Guinea. His time is Sarawak is also celebrated in Kuching’s Sarawak State Museum, which should be re-opening soon.
In Chapter 4 of his book, he describes ‘One of the most curious and interesting reptiles which I met with in Borneo...’ This was a tree frog brought to him by a Chinese forest worker, who described how he ‘had seen it come down, in a slanting direction, from a high tree, as if it flew.’
Wallace was meticulous in his observations, a role model for any 21st century ‘budding scientist’, describing how the frog’s toes were webbed, especially the hind legs, the fore legs were bordered by a membrane and how it had large discs on its toes. He wrote: ‘This is, I believe, the first instance known of a “flying frog”.’ He suggested how this species had, over time, modified structures that most other frogs still used to swim and crawl, in order to ‘fly’ from tree to tree in the rainforest. Wallace was a close collaborator of Charles Darwin and later both men would publish their own arguments in support of ‘evolution by natural selection’.
For both men, the evidence supporting their theory of evolution came from careful observations of animals and plants, in Wallace’s case much of this was in S.E. Asia. He sent the frog specimen back to UK, where it was classified as Wallace’s Flying Frog and today it seems obvious that the behaviour of this frog, and the structural modifications recorded by Wallace, enable it to adapt to the challenges of living in the rainforests of S.E. Asia. For Wallace, it would have provided part of the evidence that enabled him to publish his views on how natural selection leads to the origin of species.
Today, it is widely known that Borneo has ‘flying snakes’, ‘flying geckos’, ‘flying squirrels’ as well as ‘flying frogs’. ‘Flying frogs’ show a worldwide distribution in rainforests and in Sabah, we are lucky to have four species: Wallace’s Flying Frog, the Borneo Flying Frog, the Harlequin Tree Frog and the Jade Tree Frog that are known to ‘fly’ but there may be more. In fact, these animals are not active flyers like insects, birds and bats; they don’t possess proper wings with attached muscles to make them beat; instead, they use modifications of their skin to enable them to be excellent gliders, or long distance jumpers, within the rainforest. Flying frogs such as these four are tree frogs that have adapted to life in the canopy by evolving the features first described by Wallace. They should more accurately be called ‘gliding frogs’ since they usually glide or parachute downwards from a high point on a tree to a lower point on another tree or the ground. This doesn’t involve the frog using up energy: the hard work comes when the frog has to climb upwards again before its next glide. The furthest distance, gliding or parachuting, so far recorded is 50 feet, an impressive distance for a small frog.
Wallace’s Flying Frog at Tawau Hills Park (2021)
The picture above was taken in Tawau Hills Park in 2021 and shows an adult Wallace’s Flying Frog with its adaptations for gliding: on its hind legs it has deep, long, fully webbed toes with dilated discs at their extremities and the smaller fore limbs are bordered by a membrane and also possess toes ending in discs.
In his book Wallace includes a black and white sketch that is remarkably similar to this photograph. He also describes the animal’s ‘deep, shining green back and limbs’ and its ‘yellow toe webs with black rays’. He estimated that frog’s normal body length is 4 inches and that the webs of its 4 feet provided an additional 12 square inches when fully expanded: they act as sails during gliding and the foot pads provide adhesion for clinging to vegetation, landing and taking off.
Tawau Hills Park is a ‘frog hotspot’ and 1StopBorneoWildlife has recorded 25 species of tree frogs in the Park, including all four of Sabah’s flying frogs. It has meant years of patience to understand their ecology. Like many birds, their presence can be detected by their calls or croaking but that doesn’t mean that they can easily be seen. Like many birds, they keep to the rainforest canopy for much of the time but, unlike most birds, they are only active at night but with experience they can be tracked by their characteristic calls, although this requires rainforest night walks with flashlights.
Borneo Flying Frog at Tawau Hills Park (2020)
The Borneo Flying Frog is a similar species to Wallace’s but is smaller and perhaps more colourful, with blue streaks in its toe webbings. This is a rarer frog and endemic to (found only in) Borneo.
Spiny Wart Frog in Tawau Hills Park (2021)
In contrast to the previous two species, the Spiny Wart Frog shown above is not as beautiful but is still an elusive tree frog. In 2021, it was photographed for the first time at Tawau by 1StopBorneoWildlife and this proved to be only the 5th record of this frog in Borneo. The picture shows how well camouflaged it is on its background of a tree trunk, which may explain why it is only rarely recorded.
The fourth frog is the Harlequin Tree Frog, which is another brightly coloured tree frog known to glide. The picture below shows its webbed feet with suction pads at their ends but also illustrates how it got its name: a harlequin is an old theatrical character in Europe who always appears dressed in coloured, chequered clothes, just like this frog’s legs. Although not commonly named as a ‘flying frog’, this species seems to have some of the physical adaptations that might enable it to ‘glide’.
Harlequin Tree Frog in Tawau Hills Park (2020)
Three of the four frogs shown here have strikingly bright colours which should make them more obvious during their gliding or parachuting. Perhaps their colours have evolved to surprise a potential predator, with even the toe webbings likely to attract attention and add to a predator’s confusion? But there might be another reason: the colours could act as warnings that say: ‘Don’t try to eat me, I am unpleasant to eat!’, which is a defensive strategy also seen in some large rainforest butterflies; it is tempting to suggest that this is one of the reasons for the splendour of the male Rajah Brookes Birdwing.
However, colour is more commonly used by many animals as a means of communication between individuals of the same species, typically by a male to attract a female. In the case of the Rajah Brooke’s Birdwing, for example, the male’s bright colours and obvious flight is thought to help him attract the less colourful females. In the case of tree frogs, what is the function of their skin colour? Is it camouflage, sexual attraction or even as a defence against predators? This is not an easy question to answer since we should never forget that the human eye may detect different colours and patterns compared to eyes of insects and frogs.
Tree frog breeding
Since they are small, live in the canopy and are active only at night, there is often little known about the breeding behaviour of these frogs. However, like all amphibians, they must lay their eggs in water and so, in Tawau Hills and elsewhere, often the best time to observe them is when they descend from the canopy to breed. Some, like the Harlequin Tree Frog, are known to place their eggs in a foam that is deposited on leaves above ponds so that when they hatch, the tadpoles drop into the pond below to complete their life cycle.
Wallace’s and the Borneo Flying Frogs, prefer to breed in muddy water, often in wallows of wild pigs or old, abandoned rhinoceros pools. Both of these species, are, therefore, likely to be found near these wallows when the best time to observe them is after heavy rainfall.
A Bearded Pig Wallow in Tawau Hills Park (2020)
In Tawau Hills Park the 1StopBorneoWildlife team have been able to identify some of these rainforest pools and wallows near which tree frogs are concentrated. On rainy nights, by calling up into the canopy they have been able to ‘call’ the frogs down to the water where identification and photography is much easier.
It is clear that in the rainforest, even unlikely species are interconnected: in this case, these frogs are reliant on bearded pigs that are powerful enough to maintain their wallows, so necessary for frog breeding. It follows, of course, that any event in the rainforest, for example uncontrolled pig hunting locally or the more widespread African swine ‘flu (ASF) can also have serious effects on other species such as these frogs. ASF might impact Sabah’s bearded pigs in two ways: by killing animals as it spreads through the wild pig population or when the culling of pigs is carried out in order to protect nearby domestic herds.
Flying frogs are also threatened in other ways, for example from the poaching that supplies the worrying, illegal trade of amphibians and snakes from S.E. Asia to Europe so as to supply countries such as Germany, The Netherlands and Belgium. It is important to educate people of the importance of the need to conserve our rainforests and the biodiversity they support. Fellow Malaysians should be encouraged to visit forests such as those within Tawau Hills Park to marvel at their flora and fauna but, even then, they must be aware that unnecessary disturbance, even by excessive flash photography or straying from a forest path, can upset species such as our flying frogs. They are sensitive to habitat and climate changes, making them important indicators of the condition of our forests. The WWF has described frogs as the ‘forests’ barometers’: we must keep observing them and fight to protect their populations!
All pictures in this article were taken in Tawau Hills Park by Shavez Cheema of 1StopBorneoWildlife.
For anyone interested in further information on tree frogs, we recommend the following reading:
A Field Guide to The Frogs of Borneo (2017) Third Edition by Robert F. Inger, Robert B. Stuebing, T. Ulmar Grafe and J. Maximilian Dehling.
ISBN 9789838121767