Catch me if you can

Now from predators that fly to animals that run after their dinner; a striking feature of this interaction is that the pursued animals often put in frequent turns as they flee, apparently to wrong-foot the predator. But why does this work? Rory Wilson studies how animals move at the University of Swansea...

Rory - Well, some of us have spent a lot of time looking at cheetahs in particular. But actually, it's extremely difficult to get proper information on turns and speeds and so on, by just filming them. So, we were attaching technology to the cheetahs in collars around their necks. And so, when they chase the prey, we could find out things like how rapidly they turn and how many turns they make during chase and so on, and so forth. And then when we recovered the collars, we could take the information off and push it through our computers.

Chris - And what does this show? What do they do?

Rory - Cheetahs will take very small prey and they'll take very large prey. A cheetah itself might weigh 30 or 40 kilograms so as much as a big dog and they'll take prey as small as hares or as big as ostriches sometimes. One of the things that came out was that their hunting performance and the way they hunted depended on the prey they were trying to catch. In essence, the bigger the prey, the less the cheetah had to turn to catch them. So the small prey were jinking this way and that and the bigger prey were just simply trying to run with only a few big turns then.

Chris - Why does turning work as a strategy for a small prey item? Why does that help them to get away?

Rory - If you look at any animal and that includes big humans or you look at big animals like rhinoceroses and you ask them to run, they do run and then they turn as fast as they can, the bigger they are, the wider the turn they will put. A rhinoceros charging along will take a long time whereas something like a rabbit can turn very rapidly. There are power reasons for that and what that essentially means is if you're a rabbit and you're being chased a cheetah, and the cheetah is nearing, getting closer and closer to you, the thing you need to do is turn because you can actually outturn a cheetah.

Chris - When the rabbit turns, the cheetah takes proportionally longer and so therefore, the rabbit gains or puts more distance between the two of them. I presume that the longer the pursuit goes on, the greater the likelihood therefore that the cheetah is going to be tired out before it's actually caught up with the rabbit.

Rory - That's exactly what happens. In other words, you have the first part of a chase where the cheetah runs towards the rabbit or it runs towards the antelope and it gets closer and closer because the cheetah is faster. And then there comes a point where unless that animal then turns, it will get caught by the cheetah and so it tries to do the sharpest turn it can and if it turns at dead right, it will turn really rapidly and the cheetah will overshoot it and then have to come around in a screeching-hard, very power intensive turn. If the rabbit however turns too early then the cheetah says "Aha! It's turned ahead of me and can cut the corner that the rabbit has put in." So, the timing of the turn is really, really critical.

Chris - You've done this on cheetahs. Is this generalisable? If I take a bigger or a smaller animal than a cheetah, would I see the same "law" applying?

Rory - Absolutely and in fact, the physics of it tells us that simply, the bigger animals have less power to turn - relatively less power to turn, so they have these big turning circles. And that means the relative sizes of the predators and the prey are really important. If the predator is bigger than the prey, if you're the prey, the best way to get away is to run like crazy and then just as the predator is close enough, you turn as fast as you can. If you're bigger than the predator then really, there's very little chance that you can get away by turning. You just got to run as fast as you can away, and hope that your size and your power will enable you mitigate some of the problems of the predator.

Chris - What about on the battlefield? If we take the same sorts of things, there are issues with for instance, a missile pursuing an aeroplane or a person pursuing another person. Does the same rule apply, do you think?

Rory - Absolutely. The same rule applies. In some senses, you can see it in rugby. If someone kicks the ball up to the one end of the field and then rushes up, the fallback catches it. And then the big question is, how fast should the person rushing up to the fallback rush up, because the fallback, being stationary, can turn rather more quickly. You have to remember that your ability to turn depends on your speed and your mass. So, the lower your speed, the quicker you can turn. But the bigger your mass, the slower you can turn. So, the fall back catching the ball and being stationary, being charged up by our forward from the other side. If that forward doesn't slow down, it's quite easy for the fallback to sidestep and then run up the field.