Female frogs aren't known as great communicators. Although they may squeak when caught by a predator, they spend most of their lives in silence. A new discovery could change this notion: Researchers have found that the female concave-eared torrent frog makes a high-pitched peep to attract nearby males. The call is a genuine siren song: Upon hearing it, males leap toward the source with uncanny accuracy, even in darkness, rivaling the localization abilities of owls, dolphins, and humans.
Male concave-eared torrent frogs (Odorrana tormota) communicate their whereabouts and their availability as mates by means of ultrasonic calls. The high frequency of the sound allows the frogs to be heard over the noisy rapids of their home environment, the Huangshan hot springs in China. Biophysicists led by Jun-Xian Shen of the Chinese Academy of Sciences in Beijing suspected that female frogs were capable of making similar calls when they dissected the females and found that, like the males, they had large larynxes--a trait that separated them from females of other frog species.
The team then caught a number of females and brought them into the laboratory to determine if they could sing like males. Sure enough, the female concave-eared torrent frog had the rare ability to call out like males--however, instead of constantly calling like males do, the females only called just before laying their eggs; after that, they stayed silent.
To see if this siren song helped male frogs find a female, the researchers brought 41 male frogs into a darkened laboratory and played a female's call on a loudspeaker. The males responded by calling back and leaping--sometimes in a single hop--directly to the speaker, evincing as much accuracy at homing in on a noise as is typical for expert species such as humans and dolphins. "The precision with which the males found the loudspeaker is shocking," says co-author Albert Feng, a neuroscientist at the University of Illinois, Urbana-Champaign. "We, frankly, didn't think that the frogs had this kind of ability."
Feng notes that the homing talent is unusual in such small creatures. Normally, animals need to have larger heads with more widely spaced ears in order to locate sounds with accuracy, he says. One possibility is that the frequency of a female's call is so high that the waves bounce off of the listener's head, amplifying the difference in sound level between one ear and the other. This makes the sound that much more intense and easy to detect, the team reports online this week in Nature. Low-frequency waves travel around the listener without this effect.
Darcy Kelley, a biologist at Columbia University, is surprised by the reversal of gender roles. "Females that have a fertility advertisement call--that's pretty unusual," she says.
Senior author Peter Narins, a biologist at the University of California, Los Angeles, believes that the results will increase our understanding of the hearing mechanisms in both frogs and humans. Because frogs are champions at picking out their preferred mate's voice from a chorus of competitors, these animals may help scientists create a more sophisticated hearing aid, for example. "The frog is really a fabulous model of hearing," says Narins. "The more we can learn about hearing and localization in frogs, the more we can learn about human hearing."