The concave-eared torrent frog's unusual ear anatomy lets it hear high-frequency calls, which gives a mating advantage to the littler males that sing soprano.
In the shadow of Huangshan Mountain in southeastern China lives a beige frog with black stripes. The concave-eared torrent frog, or Odorrana tormota, gets its name from its unusual hearing apparatus.
"It has kind of [an] ear canal-like structure like humans, and like most mammals. For most frog species, the eardrums are located on the body's surface, on the lateral part of the body surface. But here the eardrum is invisible because it is embedded deep inside the head, in the skull."
Biologist Albert Feng, from the University of Illinois at Urbana-Champaign.
Back in 2006, Feng and his colleagues discovered that this unusual anatomy allows the frogs to hear ultrasound, which includes frequencies greater than 20 Kilohertz. Biologists had always assumed that ability was restricted to some mammals because it was only known in bats, whales, dolphins, and some rodents. We can’t hear in that range.
The streams that the frogs live in are quite noisy, but most of those sounds are low frequency. By restricting their vocalizations to the higher frequencies, including ultrasound, the frogs are better able to hear each other.
"The higher the frequency, the less that signal is distorted, or at least masked, by the ambient noise."
In many animals, females prefer mating with larger males, usually because that's a sign of health and strength. And bigger males tend to produce lower frequency vocalizations. But with higher frequency calls being more effective in this habitat, Feng wondered if females might actually prefer smaller males.
Over the course of several nights, the researchers captured thirty-five pairs of frogs engaged in what biologists call amplexus. Which means while having sex. The biologists measured the length of each male, as well as the length of those males found nearby who were unlucky enough to be caught and also not having sex. Turns out, size really does matter.
"The females are very unusual in that they really prefer smaller males over larger males."
Next, the researchers brought the frogs into the lab. Each female was placed alongside two males. Here, too, females opted to mate with smaller-bodied males. And an analysis of the mating calls showed that the smaller males did use higher frequency sounds. The study was published in the Journal of Zoology. [F. Zhang et al, Female concave-eared torrent frogs prefer smaller males]
"We are thinking by females preferentially choosing small-sized males, that actually we can see the trend over generations…you can see the gradual evolutionary changes in the trait. Over multiple generations you gradually develop these ultrasonic capabilities."
What isn't clear yet is whether the females are visually assessing the males for their size, or whether they just have an easier time hearing their higher-frequency calls—making them more likely to mate with smaller males. That's exactly what Feng's team is testing now, by broadcasting mating calls to the females in a laboratory—with the lights out.
—Jason G. Goldman
(The above text is a transcript of this podcast)