Danuta Wisniewska, Aarhus University
A number of creatures use echolocation, bouncing sound off structures to catch prey. Bats are one example but they do most of their hunting at close range. So, projecting sounds over long distances isn’t really a priority for them, but it is for toothed whales and their relatives which need to find prey and then close in on it from far away. So, this means they have a problem. To accurately locate something at a distance, you need a tightly focused beam of sound. But as you get closer, you need a wider beam to stop things being able to get away. So, how do they do it? From Aarhus University, Danuta Wisniewska.
Danuta - Toothed whales, sperm whales, dolphins, and porpoises use echolocation to hunt. They produce a sound and then they listen to reflections of those sounds. And when they do that, they direct the sound energy into a very narrow beam so the energy can reach further ranges. However, that is not very suitable for tracking fish or squid at very short ranges because the area covered by that beam will be very small and so prey could easily escape.
Chris - Indeed, because as the whale homes in on the thing it’s trying to catch, because this beam is spreading out from a small point to a broader point at a distance, as it got close then it would be very easy for the prey to just swim out of the beam, wouldn’t it?
Danuta - Yes. It’s like a flashlight with an adjustable beam. When you want to look for something farther away, you will use a narrow beam because at long ranges it will still cover a large area so you’ll find your car or your door. As you get closer, if you want to then find the hole for the key, you would adjust the beam so that it covers a great area because otherwise you would have to scan back and forth to find that small object.
Chris - And is that what the whales actually do? Do they have the ability to adjust the size and, I suppose, the relative rate at which the cone spreads out in accordance with how far or close the prey item is in order to compensate for this effect?
Danuta - Yes. That is what we found in my study.
Chris - So, how did you do it?
Danuta - We recorded porpoises catching fish in front of an array of hydrophones, that is underwater microphones. We threw a fish in front this line of hydrophones and asked animals to catch it, and then we saw that as the animals got closer to the fish they increased the beam.
Chris - Now, obviously, the question is if they have the ability to focus this sound in this way – how do they do it? First of all, where do they make the sound? And second, how can they adjust the way in which the cone of sound spreads out like that?
Danuta - Toothed whales do not produce sounds in the larynx like most mammals would do. Instead, they produce sound in their nasal passages. They have structures called phonic lips. As they push air past those phonic lips they produce sound.
Chris - This would be the blow hole, wouldn’t it?
Danuta - Yes, exactly. The phonic lips sit in the blow hole. They passed air through it as if they were breathing but there’s an air sac above those phonic lips just below the lip of the blow hole. Air is accumulated in that air sac and when they run out of air underwater, they will suck that air back in and reuse it. So, they recycle the same air to produce sound.
Chris - And the sound comes out of this series of phonic lips in the blow hole, how does it then get focused into that cone which you beautifully point out is very similar to shining a flashlight into the dark? How do they do that?
Danuta - The vibrations from the phonic lips is coupled into a fatty structure that is in front of their head, called the melon. That melon makes the bulbous shape of a dolphin head.
Chris - How does it achieve that change in focus of the sound beam then?
Danuta - In principle, there are two ways of doing it. You could either change the frequency of the sound. Another way of doing it is to change the size of the structure that radiates the sound. In this case, that would be the melon.
Chris - And which do the whales resort to? Do they change frequencies or do they adjust the shape in the front of their head to do this?
Danuta - Porpoises don’t change the frequency of their signal significantly. We found that even without changing frequency, they tend to change their beam, meaning that they have to do it by changing the size of the radiating structures.
Chris - But, anatomically, are they endowed to do that? What structures could they employ in order to change the shape of the fat because you’d need to quite literally adjust the shape of the fat structure, wouldn’t you? In order to change the shape of the sound cone that’s coming out of their head.
Danuta - Yes. It’s above the melon and the air sacs around the melon are surrounded by muscles. And these are the same muscles to our facial musculature. They are heavily innervated. Porpoises actually have 4.5 times more neurons in the facial muscles than humans do. And what is special about facial muscles of primates is that we can make all those facial expressions because we have so much control over our muscles. Porpoises with the network of muscles, neurons around their melons should be able to do even more.