How do "paratrooper" frogs fly?

Some frogs in the jungles of East Asia take the leap to the extreme. Nicknamed paratroopers, they jump from the top of the trees and glide through the canopy to escape from enemies, reaching twenty meters in a single jump.

Lacking wings like birds or bats, they use the interdigital membranes as a gliding surface to slow down their descent. They also have feet disproportionate for their size, skin flaps along the extremities and pads on the fingers that cushion the landing.

David Hillis, an evolutionary biologist at the University of Texas, and his collaborators at the Chengdu Institute of Biology in China collected several specimens of Rhacophorus reinwardtiia lime-green parachuting frog with black and yellow membranes, native to the southern Chinese rain forest, to unravel the genetic basis for these unique adaptations.

The team compiled the genome of this tree frog and compared it to other nearby anurans that are unable to glide. In a study published in Proceedings of the National Academy of Sciences USA, noted the location of 455 modified genes. ‘Many of the genes we found are linked to various aspects of limb, foot and webbed toe development. They all agree with the great morphological adaptation of flying frogs to glide”, explains Hillis. The researchers found that some elongated the limbs and improved the grip of the digital suckers during the climb. By studying limb development in tadpoles of each species, they also discovered the network of genes that is likely responsible for the extraordinary growth.

In order to see these differences in action, they conducted a series of flight tests under controlled conditions. They placed individuals of each species on perches and recorded jumps and glides. Before, they placed soft sponges underneath in case something went wrong in the air. This proved providential for the non-gliding frogs, which plummeted on top of them. The skydiving frogs, on the other hand, spread their fingers and gently glided to land.

According to biomechanics expert Mimi Koehl of the University of California at Berkeley, who has studied the biodynamics of gliding frogs but is outside the work, the wide interdigital webs slow down the fall while helping to control flight through the tops of the trees. The frogs dodge trees by moving their hind legs like rudders as they descend toward pools, where they congregate before mating and laying. “If they couldn’t maneuver through the thicket, they wouldn’t be on time for the appointment,” explains Koehl.

The authors say that a thorough understanding of these anurans’ adaptations may offer clues to how other animals, from squirrels to flying lizards, took to the air.

Koehl and his collaborators have been using gliding frogs as models of dinosaur flight. Flying dinosaurs had “feathered tails and feathers on their hind limbs. That is to say, the same anatomical design as frogs,” says the expert.

Jack Tamisiea

Reference: “Genomic adaptations for arboreal locomotion in Asian flying treefrogs”; Wei Wu et al. in PNAS, vol. 119, e2116342119, March 2022.

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