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Why wet dogs are a Mars rover’s best friend

A BBC Natural History camera team reveals how slow motion filming doesn’t just create memorable sequences, it can uncover hidden secrets with unexpected benefits.

A Golden Labrador shaking itself dry may sound like a mundane topic for a film. But couple it with the magic of ultra-slow motion cameras and you have something that has not only piqued the interests of nature lovers and the scientific community, but may also help design future Mars rovers.

The idea for the film came about when the BBC’s Earth Unplugged team read a study by Andrew Dickerson of the Georgia Institute of Technology. Fur loses its insulation and mammals can face hypothermia pretty quickly, so it’s vital to get dry as quickly as possible. The mechanical engineering student made quite a splash when he published his work showing how furry mammals, from mice to brown bears, shake their bodies when wet – the frequency being directly related to their size.

A bear, for instance, needs to shake from side-to-side four times per second. The loose skin on its large frame whips around its body with enough force to shed the water quite easily. A mouse on the other hand has such a small frame, it needs to shake proportionally faster to generate the same forces to break the surface tension of water on its fur – up to 33 times per second, pulling forces of up to 70G. All of which suggests that animals tune the frequency of their shakes to maximise the efficiency of getting dry.

Having already shot a few “slo mo” films, it was the perfect study for the Unplugged team to try to go one step deeper – using camera tech to extend and develop the field of knowledge. We’ll let the team tell you what happened next:

"As good science depends on repeatability of data we chose some species that Dickerson had already tested, as well as adding a few of their own into the mix – a colleague’s pet rat called Coco, a pygmy hedgehog, and Mogwai the golden lab.

Dickerson had filmed his soggy animals at around 500-1,000 frames per second (fps). But using a camera normally found only at major sporting events or on high-end Natural History documentaries, we cranked up the frame rate to around 2,500 fps – around 100 times faster than a standard camera.

The first guinea pig was, well, the hedgehog.

It didn’t work. It looked spectacular surrounded by falling water droplets but ultimately, even in slow motion, it looked like a wet hedgehog. It wasn’t able to shake off much water at all. But the result lent itself quite nicely to a theory, which states that animals from places such as North Africa or Australia that get wet, but maybe not that cold, simply don’t need to shake so efficiently.

Then it was the turn of the rat and the dog – both are found across much of the world and in wet and cold environments. In theory, they should have evolved to dry off as quickly as possible.

And it proved to be the case. Coco was quite at home facing up to the watering can, relishing the shower before contorting herself around 15 times a second to dry herself. Though slower, Mogwai still managed to shake around five times per second, shifting so much water that he immediately soaked the retreating crew and camera equipment – perhaps hardly surprising following Dickerson’s findings that dogs can get rid of 70% of the water on their fur in just four seconds.

All of which is interesting, and indeed beautiful when seen in slo-mo. But shaking dogs may also have a practical application.

Dickerson has used his insights to create a “wet dog simulator”; a rotating device that looks at the speed wet brushes need to spin to rid themselves of water. He hopes that it could become the basis of self-cleaning robot – a potentially useful design for Mars rovers which often find their solar panels and sensors covered in Martian dust.

So, perhaps in the not too distant future, Mars rovers will shake their circuits in a similar way to a dog emerging from a pond. If they do, we’ll make sure Earth Unplugged gets one on film before its long journey to the Red Planet."

Watch more slo-mo movies over at BBC’s Earth Unplugged on YouTube.

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