A Tiny Frog That Should Be Deaf Hears With Its Mouth

GardinersFrogFar away on the islands of Mahé and Silhouette in the Seychelles in the western Indian Ocean can be found one of the world’s smallest frog species, Gardiner’s frog, just 10 to 11 millimeters in length. These fogs should be deaf, because they lack a middle ear (in humans, that’s the bit with the eardrum). That part of the ear is thought to be a necessary adaptation to life on land, helping sound to make the transition from air to tissue, where it can be translated into nerve impulses that reach the brain. But Gardiner’s frog has evolved a different method for hearing — it uses its mouth, reports a team of French researchers in a new study in PNAS.

Scientists had been somewhat perplexed by Gardiner’s frogs because, despite lacking the necessary equipment for hearing, the amphibians appear to have no problems communicating. The French team began by recording calls from frogs on Silhouette and playing them back over loudspeakers to frogs in their natural forest habitat. When a male heard the call from one of its own species, it called back a response. But the frogs didn’t respond when they heard sounds from other species. They were definitely hearing and well enough to make species distinctions.

Dissecting the frogs might be a classic technique in the classroom, but here it just wasn’t an option; these frogs are far too small to see anything useful with dissection. Instead, the researchers used a technique called X-ray synchrotron holotomography to image the frog’s inner anatomy. Then they used that data to create a computer simulation of the amphibian and determine how the sound was traveling through the frog’s head.

Previous studies had suggested that sound passed through the lungs on its way to the inner ear, where it would be translated into nerve impulses. Another theory was that sound was conducted through the animal’s bones. But the computer simulations revealed that the mouth (or oral cavity as the scientists name it) is actually the ideal anatomical structure for amplifying sound. The cavity resonates sound at a frequency that nearly matches the frequency most commonly found in the frogs’ calls. The researchers also found that the tissue that separates the inner ear from the inner mouth in Gardiner’s frogs is very thin, and there are fewer layers of tissue, which helps the sound to pass through.

The frog’s mouth might also play a role in determining the direction of sound, the researchers suggest. And, they note, this discovery shows that the middle ear isn’t quite so necessary for life on land as had been thought.

Image credit: R. Boistel/CNRS

Frogs Take Up Residence In Manmade Caves In Portugal

frogSerra de Estrela Natural Park encompasses a section of Portugal’s highest mountains. The largest natural conservation area in that country, the park is full of wildlife. There are wall lizards, otters, wild cats, water moles, and even wolves, to name a few species.

Back in the 1950s, before the park’s boundaries were delineated, several artificial caves were created in the area. These were drainage galleries — small, horizontal tunnels built to push a few meters into the hillsides, often with a small stream of water running through.

In May 2010, according to a study from Portuguese researchers published in the Journal of Subterranean Biology, an unexpected resident was found in the galleries: the Iberian brown frog (Rana iberica). These frogs are often found the the mountains, and also live in other moist habitats, such as ponds and soaked fields and humid meadows. But they’d never been found in caves.

The researchers returned to the galleries the next year, every three months at first, then every month from December 2011 to December 2012. Adult frogs were found throughout the year, in day and in night, on the ground, tucked into crevices, swimming in water, and even climbing up the walls (as in the photo). Usually the frogs were living deeper into the galleries, more than five meters from the entrance where the daylight had dimmed.

The frogs mated throughout the year, then laid their eggs within the caves. The tadpoles hatched there, and the frogs grew up there. It wasn’t necessarily an easy life, however. Tadpoles would sometimes eat frog eggs — a first for this species — and tadpoles themselves were occasionally eaten by fire salamander larvae.

This may be just be a case of a species moving into a newly available habitat, but it might instead be a consequence of changes humans have made to the frogs’ home. “Nearby breeding sites may have disappeared or experienced disturbance,” the scientists hypothesize in their paper. But they also note that this region of the world is expected to experience dramatic climate change in the near future, with changes that include increasing aridity and greater fire activity. By taking up residence underground, the frogs may be smartly fleeing the changing landscape above. It could be a hopeful sign that the world’s wildlife will do its best to survive the changes we are making to their planet.

Image courtesy of Gonçalo M. Rosa, via EurekAlert