The concave-eared torrent frog.
Wouldn’t it be nice if we could hear each other over the low-frequency roar of jetliners and subway trains? For some rodents, bats, and marine mammals, environmental noise doesn’t normally pose a problem, as they can communicate at ultrasonic frequencies (greater than 20 kHz, just above our maximum hearing range). There are also a couple of amphibians that exhibit this trait, but in an odd twist, researchers have now learned that female concave-eared torrent frogs are deaf to the ultrasonic components of the males’ calls.
The concave-eared frog is a tree-loving native of the Huangshan Mountains in China. In choosing this woodsy area, the nocturnal amphibians must put up with one minor annoyance: streams that produce constant ambient noise. In 2006, Jun-Xian Shen, a biophysicist at the Chinese Academy of Science in Beijing, and his research team discovered that the frogs get around this sonic clutter by adding ultrasonic frequencies to their normal calls (pdf). The frogs were the first non-mammalian vertebrate found to do this, and scientists have since learned that Borneo’s hole-in-the-head frogs (yes, that’s the actual name) also chirp in ultrasonic frequencies. After finding these ultrasonic noises, researchers wanted to know what they were saying with these super-high-pitched croaks.
In a darkened room, Shen and his colleagues played prerecorded male calls to the females. When the females heard normal calls, they would hop towards the loudspeakers and sometimes chirp back. However, when the researchers played only the ultrasonic components of the male calls, it elicited no response from the females.
Could the female frogs have simply been ignoring the high-pitch chirps? The researchers tested this by measuring the auditory brain responses of the females during call playback. They found that lower frequency sounds registered a distinct response in the female frogs’ brains, but frequencies above 16 kHz did not.
So while the female frogs can call out at ultrasonic frequencies, they can’t actually hear those very chirps. What gives?
Like most other frogs, female concave-eared frogs have thick eardrums close to the surface of their skin (they have no ear canal). Males of the species, however, have extremely thin eardrums sunken into their ears (hence their name), which allows them to hear high pitches. As far as scientists know, this specific sexual dimorphism is present in these frogs and no other species. Shen believes that females may have evolved different ears because they generally live in “rocky cracks, trees, or muddy caves with less ambient background noise,” and only leave for short periods to reproduce. Simply put: they don’t need to hear ultrasonic frequencies.
That could be the reason. Or maybe they just got sick of listening to the males’ pathetic pick-up lines.
Image: Wikimedia Commons/Albert Feng