A Tiny Frog That Should Be Deaf Hears With Its Mouth

GardinersFrogFar away on the islands of MahĂ© 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

To Count Bird Diversity In The Forest, Listen In The Morning

forestkingfisherIf you want to figure out how many different species of birds are in a forest, all you have to do is listen. But there are a few ways you can go about such a study: You and your colleagues can hang out in the forest for 12 hours a day, five days a week. Or you can set up microphones and record the forest sounds for analysis back in the lab, in which case you’ve got a few more options — let a computer listen and identify species, make your students listen to all of it, or listen to only some of the recordings.

But which way is best? The computer isn’t quite talented enough at this point, and though students are cheap, they’re not cheap enough to listen to all of the sound you’d collect. So the options are to count in person or listen to only some of the recordings. But which ones? And are they better than going into the forest yourself?

To figure that out, researchers from Queensland University of Technology in Australia set out to determine how many species of birds were in a eucalyptus forest in southeast Queensland. The study (in press) appears online in Ecological Applications.

They recorded five days of sounds from several sites in the forest and also had two experienced bird surveyors conduct on-site surveys of those areas at dawn, noon, and dusk for five days. Once the recordings were back in the lab, they were sampled in one-minute bites in five different ways (listening to recordings longer than 240 minutes is cost prohibitive and impractical for most researchers): randomly over the 24-hour period, randomly only in the three hours after dawn, randomly only in the three hours before dusk, randomly after dawn and before dusk, and every half hour over the full 24 hours.

Across all the sites in the forest, the researchers found 96 species using acoustic methods and 66 species with the traditional, in-person survey. And when they looked at their samplings of the acoustic data, they were able to find the most species in those three hours right after dawn, almost twice as many as can be found in the same time by the traditional method.

Sampling the recordings isn’t perfect — it doesn’t find all species, especially rare and cryptic ones — but the researchers say that using a computer to help identify the parts of a recording more likely to have a bird call would help avoid getting random samples of only wind. Thus, combining computer analysis with human listeners of these recorded forest sounds could help scientists get a better picture of biodiversity.

Relying solely on the computer to identify so many species isn’t an option for now, the researchers say, because the programs are plagued with false positives and negatives. However, one day they may get good enough that they can detect birds with high accuracy and cut out the need for humans to listen to hours of sound. (But there’s something about that that makes me a little sad.)

Image of a forest kingfisher courtesy of JJ Harrison, via wikimedia commons