Chris - But it's not just bats that can hear ultrasound. Recently, researchers discovered a population of frogs known as concave-eared torrent frogs, which live in China's Huangshan hot springs. These animals communicate with each other using ultrasound, and that's to prevent their calls being drowned out by the sound of nearby running water. From the University of Illinois, here's Albert Feng.
Albert - The discovery centres around resolving this mystery about this unusual frog in China. In this paper, we describe that these frogs are able to produce, but more importantly actually to communicate with, ultrasound. This is sound at very high frequency that we humans do not hear very well.
Chris - This is not something that frogs are normally known for doing are they?
Albert - No. The wonderful thing is that this now makes so much sense that in order to hear ultrasound, the eardrum has to be very thin. Additionally, the middle ear ossicles, namely the bones that transmit the sounds, have to be of low mass in order to transmit high frequency sound, which is exactly what the frogs do.
Chris - Why do these frogs need ultrasound? What do they do with it?
Albert - That's a very good point. Ultrasound is a way for them to get around this masking problem by very intense background noise from the running water. This is very similar to why bats use ultrasound for echolocation.
Chris - How did you actually discover that they use ultrasound?
Albert - When we first recorded their communication signals in 2000. We were using an audible tape recorder and audible microphone and at that time we noticed that there was energy seemingly at the very high end of the frequency range that we can record reliably. So three years later, our colleagues in Germany brought along a very sophisticated device, and to our surprise, their signals have energy in the range of over 120 kHz.
Chris - So when one frog makes this noise, how do the others respond?
Albert - They follow. These frogs typically form what we call a chorus and so when one frog calls, this induces other male frogs to call also to form this chorus. Chorus sound is believed to be more effective in attracting females.
Chris - It's a bit like wolves howling isn't it. So it's entirely a male mating call.
Albert - Yes, very much so.
Chris - And how do the females respond? Have you recorded from females?
Albert - No, unfortunately we haven't encountered too many females at all. Just one to be truthful.
Chris - So it's debatable as to whether it works then!
Albert - We don't know actually how it works, but we saw a whole bunch of nest eggs from this frog species, so they obviously do hear it.
Chris - So how do you know that the frogs are genuinely responding the ultrasound if they also make audible sounds? How have you dissected those two effects apart?
Albert - So we have to utilise the special filtering mechanism so we can present either the audible components of the call or the ultrasound component of the call.
Chris - And they respond equivalently to each?
Albert - Yes indeed.
Chris - And how do you know how that's actually being transmitted to the nervous system? Is it sensation through bone or is this being conducted through these modified ears?
Albert - It is clearly conduction through the ear, because we have found an experiment to block the ear canal. When we do that, the auditory response completely vanishes.
Chris - And is this unique amongst frogs?
Albert - As far as we can tell it is unique. The concave ear frogs are very rare.
Chris - So what are you actually looking at now? Where will you take the research next?
Albert - We would really like to find out what the females hear and how do they respond to each of these components.