Some frogs have evolved ultrasonic mating calls so they can be heard above the background rumble of the fast-flowing streams they depend on, say researchers.

Biologist Dr Sandra Goutte of Sorbonne University in Paris and her and colleagues studied the calls of a group of "torrent frogs" in Borneo, Indonesia, Malaysia, China and Cambodia.

They discovered the frogs all had higher pitched calls than most other frogs in the world, and a few species even had ultrasonic calls.

"You can see the frog calling but you cannot hear it," said Dr Goutte, who carried out the research for her PhD research.

"The call of torrent frogs has most probably been constrained by the environment they live in — which is the torrents — that are really noisy."

Male torrent frogs generally put out mating calls while sitting in vegetation next to fast-flowing streams. Females lay their eggs on rocks and then the tadpoles thrive in the oxygen-rich waters nearby.

The problem is falling water makes a low pitched rumble of about 2 kilohertz that would mask the pitch of most frog mating calls, which are generally under 5 kilohertz.

Looking for love in a torrent

The mating calls of this torrent frog (Odorrana graminea) from China are partly ultrasonic, which means humans can only hear a part of the call. ( Sandra Gouttte )

Sorry, this audio has expired Listen to the call of the large odorous frog (Odorrana gaminea) Credit: Sandra Goutte

Dr Goutte and colleagues measured the call pitch of 70 species of torrent frogs, that range in size from 2 to 15 centimetres in body length.

They found that, on average, most of the frogs had calls that ranged between 4 and 10 kilohertz.

A few species had calls that consisted of frequency above 20 kilohertz, which is in the ultrasonic range, above the human range of hearing.

For example, the hole-in-the-head frog (Huia cavitympanum), which is found in Borneo, has purely ultrasonic calls.

"As a result we don't hear anything, but the frogs do," said Dr Goutte.

While the large odorous frog (Odorrana graminea), a species found in China, had partially ultrasonic calls.

"We hear only a part of the call," said Dr Goutte.

Co-author Dr Jodi Rowley of the Australian Museum Research Institute said the calls of the large odorous frog vary in frequency from very low to extremely high — up to 44 kilohertz.

"They're much more like bird songs than most frog songs in their complexity and frequency modulation," she said.

"There's only a few other frogs known to call ultrasonically and they are all torrent dwelling."