Male (top) and female Odorrana tormota frogs .

Credit: Jun-Xian Shen

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OXFORD: Male concave-eared torrent frogs can communicate in the ultrasonic range in a way their female counterparts cannot hear or respond to, according to a new study.

This finding gives a new perspective on the evolution of communication, implying that the use of high-frequency ultrasonic calls can progress differently between sexes of the same species.

"This is the first demonstrated amphibian (and even bird or mammal) where the males of the species have ultrasonic hearing capacity but the females are unresponsive to ultrasonic components of males' mating calls," said Jun-Xian Shen, professor at the Institute of Biophysics at the Chinese Academy of Sciences and lead author of the paper published in Nature Communications.

Ultrasonic messages

In 2006, Shen and his colleagues discovered that male concave-eared torrent frogs (Odorrana tormota), found in Huangshan Hot Springs, China, produce a wide range of melodic, bird-like calls, which make use of both normal sound and ultrasonic frequencies.

Prior to this finding, only mammals such as bats, dolphins, whales, porpoises and some rodents were known to use ultrasonic frequencies (20 kHz and above) for echolocation or communication.

Amphibians, reptiles and birds are typically able to hear a much narrower range of sounds, usually below 12 kHz, and the discovery that male O. tormota frogs make ultrasonic calls and respond to each other showed that ultrasonic communication has evolved in the frog lineage independently from mammals.

Are females just ignoring the males?

Now the researchers have shown that the use of ultrasound by O. tormota is a sexually dimorphic trait, with stark differences between genders. While males can communicate with each other using ultrasonic calls, females are unable to hear them.

First, the team captured females from the wild and played them recorded male calls in a darkened room. When the females heard normal calls, they usually turned towards the speaker and frequently jumped towards the sound source or called back.

However, when the females were played only the ultrasonic components of a male's call, they neither moved towards the sound nor vocally responded.

To see whether the females were ignoring the ultrasonic calls or genuinely could not hear them, the scientists next examined the responses of the female auditory midbrain to different sounds.

When played lower frequency sounds, a peak of activity in the midbrain could be seen, showing the female had perceived the call. However, ultrasonic sounds did not produce any activity and the team found the female midbrain did not respond to sounds above 16 kHz.