Echolocation And The Greater Moustached Bat

Kat - You often hear the phrase about being blind as a bat, but one thing bats certainly aren't is deaf. And we're joined now by Ian Russell from the University of Sussex who's working on the rather fabulously-named greater moustached bat. These can whizz through forests and catch their dinner at 40 miles per hour using just the power of their hearing. Ian, tell us about this. How does it work?

Ian - Bats just shout their heads off and then listen for an echo. These bats are rather remarkable in that they have two types of sounds that they produce: constant frequency sound and a so-called frequency modulated sound.

Kat - And what do those kind of sounds sound like? Can you give us an impression?

Ian - I can't!

Chris - Go on, do a bat impression Ian. Please!

Ian - I can do mosquitoes but I'm no damn good at bats! The constant frequency one is a very high pitched pure tone at about 50 or 60 odd kiloHertz.

Kat - So like this [hum] but higher.

Ian - Yes, much higher. And then the frequency modulated one is like this [woo]. It's much lower and in a downward sweep. It sounds a bit like a cow in labour. With a constant frequency one, they can listen to the velocity change as they approach their targets.

Kat - Is that like the Doppler effect?

Ian - Exactly. And because their cochleas are incredibly narrowly tuned, they are tuned to about one thousand times more finely tuned than ours at about the 60 kHz range.

Kat - So it's very high pitched then. They're not hearing bass or anything like that?

Ian - They don't hear anything that we can hear for example. Their ears don't work at low frequencies. They listen to the sounds that they make themselves.

Kat - So we've established that bats make all these weird noises. How come they don't deafen themselves if they have such sensitive hearing?

Ian - Well if you look on the website, you'll see some bats with beautiful faces, or at least I think they're beautiful. Part of the reason they aren't deafened is that their faces are shaped so that the sounds are beamed out of them. But at the same time they're doing that, they're moving their ears backwards. Also, in the middle ear, there are some little bones that conduct the sound from the ear drum to the inner ear. Those little bones are clamped by a little muscle, so the moment they make a sound, these muscles clamp on the transmitting bones and prevent the sounds from getting to the ear.

Kat - I obviously speak on the radio, and I'm fascinated by how when I listen to my voice on the radio, it sounds completely different to what I'm hearing in my head? Why is it that we as humans hear our own sounds so differently?

Ian - I suppose because the bones of the skull tend to filter out particular frequencies. Most of the sound that we hear from our own voices is through something called sound conduction. This is another way of driving the ear. If you have middle ear deafness, which is another form of deafness, you can use a bone conductor, which is a little vibrator that rattles on your skull. This is effectively what we're hearing.

Chris - They did actually try to make a line of headphones that would work a bit like that, but they never really caught on. The idea was, and this was the marketing line, that they would be great for cyclists. They literally sit on your temples and leave your ears free. The idea is that when you're riding along, you don't get distracted or miss things that might alert you to a danger, because you can still hear. However at the same time, you can hear music coming through the bone. I did actually try this and to be honest, Britney Spears being played through it is not something I want to repeat.

Ian - A bit like nails on a blackboard.

Chris - Well not quite that bad! But the technology is useful though, because there was a police force in the States who decided to come up with a similar method for police dogs. A major problem is that in a disaster area, police dogs go long distances from their handlers and they can't hear what the handler is whistling. What they did was build this system that could clamp onto the back of a dog's head and it would re-radiate the sounds of the handler's instructions into the dog's skull and thus into the cochlea, allowing dogs to hear instructions even though they may be miles away.

Ian - Absolutely. It's a good idea.

Kat - That would freak me out if I was a dog. Talking about dogs and bats, why is it that animals hear such a different range of sounds? Are there animals that compare to humans, and what are the super low and super high hearers of the animal world?

Ian - Well let's go for the super low. Those are animals that live down burrows; things like mole rats and golden moles. They're listening for snakes and those animals can literally hear a door open and close. Now I don't mean from the creaking of the door, but from the pressure change in their ears, so their ears are designed for very very low frequencies. Whales, for example, can also hear very low frequencies and they court each other over several thousand miles along the coast of California and also down the East coast as well. So they can communicate over thousands of miles using low frequencies, which transmit a long way in water. As far as high frequencies are concerned, we're pretty bad. We can only hear up to 20 kHz when we're young. Most mammals can hear up to at least 40 or 50 kHz, again because they're trying to pick up communication calls. Mice can hear up to 100 kHz, and that's because the babies can communicate to their mothers without other animals hearing them. So there's an enormous range. The highest frequency animals are whales and sun bats. They can hear at over 200 kHz.