The scientists’ surroundings were picturesque in daylight but especially hazardous at night: mountain runoff water rushing between steep, forested banks that hindered moonlight from reaching the many rocks and boulders dotting the creek bed. But it wasn’t the beauty of the central China resort area or a possible misstep leading to injury that occupied Albert Feng’s mind. It was a mounting sense of frustration.
Feng and his fellow researchers had used helmet lights and flashlights to navigate the rocks in Tau Hua Creek’s cool, noisy waters for almost a week, recording instruments in hand, listening in vain for the mating calls of a unique species of frog known as Amolops tormotus. They had located the small, tan amphibians whose Chinese name means sunken ear frog among rocks in the creek, and waited patiently night after night in that summer of 2000, hoping for a chorus of what scientists refer to as advertisement calls from the males.
Finally, one night the scientists heard sharp, musical sounds they took to be coming from birds – an assumption, it turned out, the locals had also made when they heard those same calls. As the sounds of the creek’s other creatures began to die down in the latenight hours, the bird-like calls stood out to the bored researchers. Their curiosity aroused, they temporarily abandoned the quarry that brought them halfway around the world and headed to the creek bank where the sounds were coming from.
“The first thing we noticed is these aren’t birds because they are calling from the ground,” Feng said. “They turned out to be frogs. Next thing we know, those are our frogs.”
The team got its recordings, but with them came a surprise. The readouts of the frog calls showed unlimited variations and the frequencies on the high end appeared to extend into the ultrasound range – a discovery that contradicted the biology textbooks. Amphibians weren’t thought to produce ultrasound (frequencies greater than 20kHz), except perhaps as a fluke byproduct of other communication. The results produced a paper, a story in Nature’s online “Science Update” and media attention from NPR to the BBC for the little frogs capable of producing limitless sound patterns and ultrasonic harmonies. What they didn’t show was how far in the ultrasonic range the calls were, or answer the larger question: were the ultrasonic calls just a byproduct of an ordinary mating call or were they used for communication, as is the case with only a few groups of animals in nature.
Fast forward to May of 2002. Feng and his team return to the area, focusing their efforts on detecting ultrasonic calls from the Amolops. This time they brought with them German biologist Hans-Ulrich Schnitzler, a preeminent expert in ultrasound recording of animals in their natural habitat who focuses on bats with ultrasonic echolocation capabilities. The return visit provided a repeat pattern of initial frustration followed by some truly unexpected findings, thanks to Schnitzler’s equipment and adventurous spirit. The results of their research, detailed in a 2004 journal article by the researchers, showed that both songbirds from the area and the Amolops produced ultrasound, with the frogs’ calls extending into the high ultrasonic range.
Then in May of 2005, lugging state-of-theart, PC-based recording equipment created by Schnitzler, Feng and his associates returned to the Huangshan Hot Springs to determine for good whether the little frog with no visible ears could prove the textbooks wrong. What they found should open up a new way of thinking about ultrasonic communication among vertebrates.
A few years before, Cornell biology professor Kraig Adler had directed Feng and his UCLA colleague Peter Narins to Amolops tormotus because it did not have external eardrums like all other frog species; rather, it possessed the unique anatomical feature of an ear canal with recessed eardrums. That fact alone made the Chinese frog worthy of further investigation for a researcher like Feng, whose focus at the Beckman Institute for Advanced Science and Technology is on the neural basis of sound pattern recognition in frogs and bats. Amolops tormotus, or concaveear torrent frog, are located in just two regions of China, both of which feature bodies of water. The rainy season in the frog’s habitats is known to turn meandering creeks into boisterous venues for wildlife – creating such a cacophony, in fact, that Feng and his colleagues often had to shout to one another in order to be heard.
In the spring of 2002, it wasn’t the silence of the Amolops that was frustrating the team, but one of Huangshan’s downpours that prevented a ny serious research work getting done. Feng, the team’s leader, decided to call it a night. But Uli, as the outdoorsy Schnitzler is known, reveled at the idea of some rainsoaked research and set off to record bats in a nearby cave with equipment that detects ultrasound up to 128 kHz. A few minutes later, an animated Schnitzler returned with some astounding news. The Amolops were in full refrain, and their calls were off the ultrasonic charts.
“About 15 minutes later he came back and said ‘come on up, you guys have to come up, these guys are calling like crazy.” Feng said. “And not only that, the call frequencies were extending furthest into the ultrasonic range. It went beyond the capacity of Uli’s equipment. He said ‘I cannot believe that this can happen.’”
The German biologist was shocked, as was Feng.
“Bats can do this, dolphins and whales, among underwater mammals, can do this. But in the vertebrates, it was typically known that they are limited to these small groups of animals that can perform this,” Feng said. “Frogs definitely would not be taken into consideration as a remote possibility. So this came as shocking news to us. We were stunned, in fact.”