How We Hear, Echolocation and Giant Whoopee Cushions

22 October 2006
Presented by Chris Smith, Kat Arney.

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Helping us tune into the science of sound this week is Bob Carlyon, who explains how we hear, how we can concentrate on one voice in a noisy room, and what it sounds like to have a cochlea implant. From the hard of hearing to the most finely tuned ears on the planet, Ian Russell describes how the greater moustached bat catches prey in complete darkness while flying at 40 miles per hour, Trevor Cox turns the sound of breaking wind into a record breaker as he talks about the biggest ever whoopee cushion, and in Kitchen Science, Derek and Dave investigate the science of balance with the help of a humble office chair and some unsuspecting volunteers...

In this episode

Like Bees To a Honey Pot

Researchers in the US have been studying tipsy fruit flies to try and understand what happens to our genes when we go out on the beers. Humans and fruit flies respond to alcohol in very similar ways, so researchers at North Carolina State University decided to study the way that fruit fly genes get switched on and off when the flies are exposed to booze. They found that nearly 600 genes get switched on or off when the flies are given alcohol, and many of these are genes that are also found in humans. They also found another set of genes that are affected when the flies become tolerant to alcohol. So this could help to explain why some humans are more or less susceptible to a night on the beers, and whether some people may be genetically pre-disposed to alcoholism.

Bugs Powered By Radiation

Scientists have discovered a population of bacteria thriving 2.8 kilometres underground, which rely on radiation produced by uranium for survival. The findings make the existence of life elsewhere in the universe much more likely. The discovery, which is published in his week's Science, was made in a South African gold mine near Johannesburg. Hearing about a new water-filled fracture uncovered in the mine, Indiana University Bloomington researcher Lisa Pratt and her colleagues collected samples of the water emanating from the fracture. They used various chemical isotope techniques to date how long the water had been isolated underground, and DNA methods to identify the populations of bacteria it contained. The analysis showed that the water had been trapped underground for between 15 and 25 million years old, meaning that the bacteria it contained must date from at least that time. The DNA tests revealed a host of bacteria but one poulation dominated - a new species closely related to bacteria found at hydrothermal vents referred to as Furmicutes. These organisms are adapted to consume hydrogen and sulphur compounds which are released when minerals are zapped by radiation from decaying uranium, which is also present in the rock. The bi-products and metabolites of the Furmicutes then sustain the other species of bacteria. Previously, scientists had always thought that all forms of life on earth depended, albeit indirectly, on energy from the sun. But these bacteria, which have essentially existed on the energy provided by low-level natural radioactivity for millions of years, prove that's not the case. And at the same time they greatly increase the odds of life evolving to exist in similar conditions elsewhere in the universe...

Good Memory in The Genes

If you're like me, and have a terrible memory, then at last we may have an excuse and can blame it on our genes. Scientists in the US have identified a gene responsible for human memory, after studying more than a hundred people in Switzerland and Arizona. After scanning 500,000 genetic markers in all the individuals, they found that variations in the Kibra gene are associated with memory. People who have a certain version of Kibra have to tax their brains harder to recall the same amount of information than people who have a different version of the gene. The team also found that Kibra is active in a part of the brain called the hippocampus, which is very important for memory. This discovery could lead to new drugs to improve memory, and may help people with memory loss disorders such as Alzheimer's disease.

Tuning in To The Music of Melanoma

US Researchers have developed a test which can detect the spread of melanoma, a form of skin cancer, by listening out for the presence of cancerous cells in the blood. The technique, known as photoacoustic detection, is sensitive enough to pick up just ten cancer cells in a blood sample containing millions of cells. Writing in the journal Optics Letters, John Viator and his colleagues at the University of Missouri-Columbia developed the approach using cultured melanoma cells from a cancer patient. But they also outline how the test could be used as an early warning system to pick up cancer spread in its earliest stages. They suggest collecting blood from potential cancer cases. The red blood cells and plasma would then be discarded, leaving behind just the white blood cells and any melanoma cells. These would then be illuminated with brief (five billionths of a second) bursts of blue light from a laser. Melanoma cells preferentially absorb the laser energy because they contain granules of the brown melanin pigment. This causes them to expand and contract with each flash of the laser, producing ultrasonic shockwaves that can be picked up with a specialised microphone. As other human cells don't contain pigments the same colour as melanin, the melanin "signature" is easy to spot. And the presence of such a melanin signal is highly diagnostic. "The only reason there could be melanin in the human blood is that there would be melanoma cells," explains Viator.

Spiral swirl pattern

Dizziness and Office Chairs

It's absolutely fascinating to hear about how our ears detect and process sound, but our ears also have another really important function, which is helping us to balance and know which way's up. So to have a look at how balance works, we're now off to Hunsbury Park Primary School in Northamptonshire, where Derek and Dave are with Sim, Alex and a rather innocent looking office chair…

To do this experiment, you will need:

Swivel office chair A wide open space with nowhere to hit your head or hurt yourself One person to sit on the chair and another person to do the spinning!

How to do the experiment:

1 - Put the chair in a safe place away from anywhere you might hurt yourself.

2 - Sit on the chair with your ear on your shoulder. Your head should be bent right over.

3 - Stick your legs out and prepare to be spun round and round by your friend. You don't need to go too quickly - just keep going for about 30 seconds.

4 - Stop the chair, stand up immediately and slowly try to walk forwards. What happens?

What's going on?

You should find that you either fall forwards or backwards. This is different to when you spin yourself round on a chair with your head upright, when you tend to wobble off sideways.

The reason you feel dizzy at all is due to fluid moving around in three tubes in your inner ear. These are called the semicircular canals. In normal life (ie: when you're not spinning around on a chair) these help your brain work out which way up your head is pointing and moving. When you move your head, the liquid in the tubes stays still and your head moves round it. Your brain can measure how far it's moved past the liquid and thus how far forwards, backwards, up, down or side-to-side your head has turned.

So why do you feel dizzy? If you spin round and round for a long time, such as on an office chair, the fluid starts moving around within the tubes rather than staying still. When you stop spinning, the fluid carries on moving, making you feel like you're still spinning on the chair. The conflicting information confuses your brain and makes you feel dizzy.

But in our experiment, we found that our unsuspecting victims always fell either forwards or backwards. This is all due to the fact the semicircular canals are made of three different tubes in different orientations: one of them is going round your head in the same orientation as a halo; one goes round your head making a loop from ear to ear; and the third goes round as if through your nose and chin and back round the back of your head. This last tube helps detect how far your head is moving backwards and forwards. However, if you turn your head on its side, that tube is now in the position the first tube would normally be in. Spinning the chair makes the fluid swirl in that tube. When you stand upright, the tube with the spinning fluid is now in its normal position, and makes you feel as if you're spinning forwards.

- How We Hear And Cochlea Implants

The Naked Scientists spoke to Dr Bob Carlyon, MRC Cognition and Brain Sciences Unit

How We Hear And Cochlea Implants
with Dr Bob Carlyon, MRC Cognition and Brain Sciences Unit

Chris - We all take our ears for granted, I think. How do they actually work?

Bob - Basically, sound is vibration in the air and it is picked up by the floppy bit on the side of your head. This is called the pinna. Sound is then transmitted to the inner ear, inside of which there's a membrane which is thin and stiff at one end and wobbly at the other end. The thin, stiff bit vibrates most to high frequency sounds and the low, wide, wobbly bit vibrates most to the low frequency sounds.

Chris - And that's the bit called the cochlea, isn't it?

Bob - Yes, that's a membrane called the basilar membrane inside the cochlea. There are an array of receptor cells along the length of that, which pick up the vibrations and transmit it along neurofibres to the brain.

Chris - So at different points along the cochlea, you're literally vibrating some bits more than others and this is creating electrical signals in the nerves that the brain can understand.

Bob - That's absolutely right, yes.

Chris - If I stand in a noisy room, I've got sound bombarding me from all angles and in both ears. But you then say something from the other side of the room and I can focus in on just the sound of your voice. So how the hell do I do that?

Bob - That's right. It's particularly impressive that we can do it because my voice and other people's voices are at least physically quite similar. So the brain uses two or three different tricks. One of these is that you've obviously got two ears. For example, my voice might be louder at one ear than the other, and we can use those differences. But even if you're listening to something on a mono radio and there are lots of people arguing on the same programme, you can still separate out the sounds. What the brain uses is that different frequency components of the same person's voice will tend to start and stop together at the same time, and they will also share a common pitch, and the brain can use a pattern recognition method to group those things together.

Chris - Can your ears literally tune into certain sounds then?

Bob - The ears are pretty good at tuning into individual frequencies, but the problem is that my voice contains lots of frequencies. It's got the high frequency hiss of the fricatives, and the low frequency parts produced by vibration around my nasal cavity and my voice, and what the brain has to do is group together all those little bits together and ignore other frequencies that might belong to someone else. That's the cunning bit.

Chris - So once it's been converted into electrical nerve signals, where do those signals go to get interpreted?

Bob - There's lots of processing in different neural pathways all the way up the auditory system. So in vision, the retina does a bit of the work and it goes straight up to the visual cortex without much in between. But in the auditory system, there are lots of nuclei in the brain stem; one called the cochlea nucleus, and there's another one called the inferior colliculus. By the time it gets up to the auditory cortex on the surface of your brain, quite a lot of processing has already taken place.

Chris - I was reading a wonderful piece of research the other day about earworms; songs that go round and round in your head and you can't get rid of them. What's going on there?

Bob - It's an interesting question. There's a certain type of musical hallucination which people hear. For example, some people can force themselves to imagine sounds, which we all do, but some people just get songs stuck in their heads and they can't get rid of them. Some people have done brain imaging work to identify the area of the cortex, although not primary areas of the cortex, but secondary areas. One was dubbed the 'football's coming home' part of the brain because the particular person who was in the scanner showed activation of this area when the particular song came on.

Chris - And are we any closer of getting rid of it, because it's damn aggravating!

Bob - Not for songs that keep going round in your head. I think you're best off going down to the pub and acting like a fruit fly! There's another irritating sound that you get, which is called tinnitis. It's the bells that you get and the whistles and hums, and it can be very very debilitating.

Chris - Is that the same phenomenon?

Bob - No, not really. I think tinnitis is basically the brain interpreting activity that is going on in the periphery of the auditory system. Sometimes tinnitis occurs as the result of or following some hearing loss or an event in the area.

Chris - Sometimes if someone loses something, such as part of a limb, then the person who's lost the limb feels as though they can still feel it. They can also experience phantom pain. One suggestion that I did hear from somebody is that when you have long term exposure to lots of loud sounds, it damages certain parts of the cochlea that would turn certain frequencies into signal sent to the brain. As a result of damage or losing certain parts of the cochlea, in the same way that phantom pain hurts, the tinnitis is the equivalent in the auditory system.

Bob - I think that one way of looking at tinnitis is the brain responding to spontaneous activity firing the auditory nerve fibres, which we all get, as some kind of threat signal. There's some interesting research showing that in a third of the cases when people get tinnitis, it actually follows a stressful event in their life. So in some cases there isn't really anything particularly that has gone wrong with their ear. But it may just be that their brain has started interpreting this signal as being something as a threat. This makes it sound more threatening to them, and they become more stressed.

Chris - Talking about stress signals, can you explain to me why it is that when somebody puts their fingers down a blackboard, it's so unpleasant?

Bob - There was an interesting paper on that. I think if you ask most scientists what frequency components of that sound would be the most irritating, you'd say it was the high frequency components of the sound. What these scientists did was to filter the sound in different frequency regions and then present these transformed versions of the sound to people to find out which bits were spine chilling. The paper was called 'Psychophysics of a chilling sound' and the surprising finding was that it's the low frequency components of the sound that are responsible for that horrible shiver up your spine.

Chris - But why do we get it? One theory that I read was that when an animal is subjected to horrendous events such as a lion biting into the back of an antelope that's trying to run away, it makes a very high pitched sound. This carries very far, and lots of other animals hear it. This alerts you and galvanises you to run away; it's a danger signal. Is that perhaps what's behind this?

Bob - Well it might be, but then there are lots of other dangerous sounds that you might hear. I could play you the sound of a lion in your ear and it wouldn't make your spine tingle in that way, or your toes curl.

Chris - No, but the sound of the animal in distress would. The sound of an animal squealing is similar to the fingers down a blackboard.

Bob - Yes but what we need to know is why it is that some of those sounds that are distressing make you feel like that and others make you think, poor animal. I think there are some low frequency modulations in the sound, and that may be activating some brain structures, but I don't think anybody's really looked at it in any great detail.

Chris - Let's look at when hearing goes wrong, because people obviously do go deaf and hearing becomes less acute. Is that because the cochlea is losing nerve fibres or cells that do that conversion process?

Bob - It's usually the receptor cells that die off. There are usually two types of cell that act on the basilar membrane. One of them acts purely as receptors and the other acts as a mini amplifier, if you like. They kick energy back into the sound and make that bit of the basilar membrane a bit more picky or selective about the frequencies it likes. Often those things are the first to go.

Chris - So when we lose them and we want to restore them using this cochlea implant technology, how does that work? What does that do?

Bob - I'd just like to say that the standard treatment for people with hearing loss is still a conventional hearing aid, and cochlea implants are really for people in whom the receptor cells have completely died off or are doing rather poorly. But a cochlea implant looks a bit like a normal hearing aid. It's worn behind the ear and there's a microphone attached to it. There's a little radio frequency transmitter that's worn on the surface of the head just above the ear, and that transmits energy to a little receiver, which is implanted inside the person's head. This then sends electrical impulses to electrodes inserted inside the inner ear. Basically, high frequency sounds go to electrodes which are located on that bit of the membrane that would normally encode high frequency sounds in normal hearing.

Chris - How good is it?

Bob - It's pretty good if you're listening in quite surroundings. Speaking in quiet face-to-face or on the telephone, many patients do extremely well. The problems occur first of all when there's more than one person talking at a time. The other situation is when they're listening to music or listening to singing.

Chris - You've given us a sample. Let's have a listen to what a piece of music would sound like if you're listening to it with a cochlea implant. [sound] That doesn't sound like they'd enjoy that concert very much.

Bob - Not very much. And what's more, they can't hear what the person is singing either.

Chris - Shall I actually play the normal one now?

Bob - Yes [sound].

Chris - A bit of Ella Fitzgerald there. Now if I play the first version, it's amazing because you can almost hear what you should be hearing. [sound] But why is it so bad, the rendition? Why are they not experiencing the wonderful sound that most of us are?

Bob - I think the reason is that you can hear the words that are being said, and really when cochlea implants were developed, that was the main aim because people needed to speak and understand what people are saying to them. But they weren't really designed with pitch perception in mind, so basically the way in which pitch is encoded in cochlea implants is quite different from the way that it's encoded in normal hearing.

Chris - Can it be improved?

Bob - It can possibly be improved, yes. There are certainly small incremental improvements being made all the time. One of the things we're looking at is whether there's any sea change that can be made. In other words, we're looking for the possibility of a more radical way in which sound is encoded.

- Echolocation And The Greater Moustached Bat

The Naked Scientists spoke to Professor Ian Russell, University of Sussex

Echolocation And The Greater Moustached Bat
with Professor Ian Russell, University of Sussex

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.

- Ultrasonic Frogs

The Naked Scientists spoke to Professor Albert Feng, University of Illinois

Ultrasonic Frogs
with Professor Albert Feng, University of Illinois

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.

Shopping bag

- Sex Obsessed Prairie Dogs and Shopaholics

The Naked Scientists spoke to Chelsea Wald and Bob Hirshon from AAAS, the science society

Sex Obsessed Prairie Dogs and Shopaholics
with Chelsea Wald and Bob Hirshon from AAAS, the science society

Kat - Now it's time for us to go Stateside, where Bob and Chelsea reveal why males obsessed with sex could be digging themselves an early grave - but that's only if you're a prairie dog - and how males of our own species have been exposed as closet shopaholics.

Bob - This week for the Naked Scientists, we have new research that shows that a single-minded preoccupation with sex can be fatal-especially if you're a male prairie dog. But first, for all the male humans out there, Chelsea's here to tell us that men may be more vulnerable than scientists have thought to a condition often ascribed to women.

Chelsea - Despite popular stereotypes, men are about as likely as women to be compulsive shoppers. This according to a new study by psychiatrist Lorrin Koran and his colleagues at the Stanford University School of Medicine. The researchers screened over twenty-five hundred randomly selected adults for compulsive shopping patterns, like irresistible urges to buy things they never use.

Lorrin - One man had fifty cameras that he never took a picture with. Or another man had two thousand wrenches.

Chelsea - Koran says that until now, all the information on gender came from treatment studies, which actively recruited volunteers who thought they had a shopping problem. 80 to 90 percent of these volunteers were women.

Lorrin - And so it was thought that men weren't particularly affected by this problem. But now it looks like men are about equally affected, and just don't come for help. Which is what we see also in men with major depression.

Chelsea - Technically, compulsive shopping currently falls under the heading of miscellaneous impulse control disorders. But between the explosive growth of internet shopping and the ever-broadening selection of consumer goods, Koran says the problem may soon warrant its own clinical definition. And while unchecked compulsive shopping can wreck credit ratings, family finances, and even marriages, Koran says treatment is effective. He hopes the findings will encourage more people, especially men, to seek help.

Bob - Thanks, Chelsea. These Utah prairie dogs are barking out a warning that they've spotted a hungry fox. You'd think that the fox's best hope for dinner now is a prairie dog who's too old or sick to get away. But University of Maryland behavioural ecologist John Hoogland found that during prairie dog mating season, the easy targets are healthy males.

John - And the bottom line is, because of this obsession with sex, these males were highly vulnerable to predation. So during that seventeen days of the mating season, we saw ten males get taken by either a red fox or a northern goshawk.

Bob - That's out of just twenty-six prairie dogs that were caught all year. He says saving endangered prairie dogs may require keeping predators away when the males are preoccupied.

Chelsea - Thanks, Bob, and good luck to males out there of all species. Next week learn how early European explorers in the Americas purchased gold with shoelace tags. Until next time, I'm Chelsea Wald.

Bob - And I'm Bob Hirshon, for AAAS, The Science Society. Back to you, Naked Scientists…

Kat - Thanks guys, and they'll be back next week. But as always, you can always hear more from Bob and Chelsea by going to their website: www.scienceupdate.com.

- Why hasn't the earth's core cooled down yet?

The Earth's core is basically molten and the Earth is in the region of four billion or so years old. How come it hasn't cooled down over ...

Why hasn't the earth's core cooled down yet?

We have tried to tap into that energy. Incredibly, Icelandic farmers can even grow bananas (albeit in small quantities), and the reason is that they use geothermal energy from the hot Earth's interior. Iceland has a lot of hot magma near the surface, and so they use that heat to do all sorts of things. There are also other places around the world where they use this heat in the hot Earth around them to heat water and power things. But the ultimate question of where does that heat come from, is that the heat has, to a certain extent, always been there. The Earth is a huge body, and as a result, it has a huge amount of energy trapped under its surface, but it has cooled down. When the Earth was first formed, it was essentially a blob of molten material in space. Since that time it has cooled a lot, but because we're quite a big planet, we haven't lost all our heat yet. Then there's a second contribution. In the early days of the Earth when it was still molten, all the heavy and dense elements sunk deeper into the Earth's crust than the lighter ones. The heavy, dense things included radioactive elements like uranium, which continue to decay today, giving out heat as they do so. This so-called "radiogenic heat" accounts for about 90% of the planet's heat production.

- How do bats distinguish their own sonar?

If all these bats are whizzing about and making all these noises, how do they not fly into each other? How do they know who's noise is wh...

How do bats distinguish their own sonar?

They're all tuned to different frequencies, but only very slightly different frequencies. Some of them will be tuned to 61.45 kHz, others will be tuned to 61.44 kHz and so forth. Because their ears are so narrowly tuned, they can pick up these very narrow echoes that come back. So they're very specifically only hearing their own noises coming back.

- Why don't aeroplanes have some sort of sonar?

If bats can do this, why aren't aeroplanes equipped with the same sort of things?

Why don't aeroplanes have some sort of sonar?

Aeroplanes are, but just not at those frequencies. They use other types of techniques such as radio waves for finding direction, landing and other components. Bats are not equipped with radio receivers, but they are equipped with sound receivers and they use those instead. Submarines use sonar, which is the same sort of thing. They borrowed that from bats and dolphins.

- Can light and sound sensitivity be related?

I have very sensitive hearing and I don't like sudden loud noises or repetitive sounds. I am also very sensitive to sun light and bright ...

Can light and sound sensitivity be related?

Some people have more sensitive hearing than others and it's called hyperacusis. I'm very interested in that but I don't know the basis for it. I don't know whether it's a central basis or a peripheral basis. It would be fairly straight forward to check that out. I guess the other interesting question is whether people sensitive to some types of sound are more likely to find visual stimuli aversive as well.

- Does sound travel further in cold weather than warm?

Is it true that sound travels further in cold weather than in warm weather and how does this work?

Does sound travel further in cold weather than warm?

The speed of sound does depend on temperature and it is slower at colder temperatures, so it can sound different. Deep down, sound is the vibration of molecules and if you've got a lower temperature and you meet them, they just vibrate a bit slower.

- Why do you hear a swoosh as you drive past things?

Why is it that when you're in a car and going past a bunch of other parked cars all in a long row, you hear a swoosh for every other car ...

Why do you hear a swoosh as you drive past things?

My best guess would be that it's quite a good guess! I have no idea. It would be some sort of turbulence effect. We're used to turbulence making noise, which is how things like recorders work. But you also get that whooshing sound from cars when you go past, or if you stand on a platform and a high speed train is going past.

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