A deep dive into one of the enduring mysteries of life: how dogs can spray so much water from their fur after you give them a bath.
Dog gets wet. Dog shakes. Water comes flying off fur.
"Engineers are interested in new kinds of ideas and any type of animal that is a champion of something," Hu told me. "Dogs are good at getting dry. Any time an animal is really good at something, there is an idea there that can be used."
First presented at a conference in 2010, their work on how mammals shake was just published in the Journal of the Royal Society Interface and it is fascinating.
Let me give you the dog-park conversation-making factlet up top: A dog can shake roughly 70 percent of the water from its fur in four seconds. Nearly three quarters of the moisture in the time it took you to read that last paragraph. Pretty amazing stuff.
But that champion efficacy raises more questions than it answers.
First, why does it work so well? How long does it take your socks to dry a comparable amount if you get them wet? How are they generating all that force? Second, many mammals are capable of the shake. Is how your dog does the same way that a mouse or a lion does? Third, why do animals do the shake at all? What's the evolutionary advantage that it confers?
Let's look at the actual mechanism. A dog's backbone can't really whip all the way around. In fact, Hu told me, it can can only move around 30 degrees in either direction. If you imagine a clock face with the backbone at 12 o'clock, the backbone is making it to the 11 and 1 marks.
But think about a dog's skin. You know how loose it is? I had previously thought the main purpose of loose dog skin was so that they would look funny on UpsideDownDogs.com. But it turns out there is another more important reason. Because the skin is loose, it can whip around farther and faster than the backbone can. The skin, to which the fur is attached, travels at three times the speed of the backbone, which, according to the math, generates nine times as much force on the water droplets, helping fling them off. That's the magic of the mammal shake.
The chart below shows the backbone with the black dotted line. It's moving back and forth but not a huge amount. The skin, on the other hand, denoted here by the blue line, is moving a huge amount. It's going 90 degrees in either direction, or to keep with the clock-face visual, it's swinging from 3 o'clock to 9 o'clock.
So, get this, the process that dogs use is common to many mammals, even if some, like kangaroos and elephants, don't really need to use it for a variety of reasons. And the researchers found something astounding: the animals tuned how quickly they shook to their size. That is to say, the bigger animals shook slower while the smaller ones shook really quickly. That's because they need to exert a certain amount of force on the water droplets to shake them off. For the little guys, that means moving really quickly: a mouse has to shake 30 times per second, a rat 18 times per second, and a cat nine times per second. (Remember the labrador retriever was at about 4 times per second.)
"The largest animal is 10,000 times heavier than the smallest animal," Hu told me, "but the forces on the drops are basically constant across all these mammals."
Given the prevalence of this shaking mechanism across so many different kinds of mammals, we have to ask, what's the big advantage this little trick confers?
Here's Hu's hypothesis. Imagine you are an ancestor of a modern dog or lion or goat. It's winter and you fall into a cold stream. It is imperative that you dry off because water destroys the insulation of your fur. Assuming there isn't a warm sun to do some of the evaporating, you've got to do it yourself. If you couldn't shake, you'd have to use body heat to warm the air and do the evaporation. Hu's team calculated how much energy that would take and it is substantial.
"A wet 60-pound dog, with one pound of water in its fur, would use 20 percent of its daily caloric intake simply to air-dry," the team wrote in their most recent paper. "It is thus a matter of life or death that terrestrial animals remain dry in cold weather."
The shake, by contrast, is a highly energy-efficient way of getting mostly dry. "My biologist hat," said Hu, who has dual appointments in biology and engineering, "says as soon as you evolved hair because it traps warm air between the hairs, you have to evolve a way to keep the hair dry."
But he attempted to prove that hypothesis with his engineering hat on. That's the promise of biomechanics, Hu's field. It lets you run experiments that test the physics that underpin living creatures. "We mostly use [engineering's] analytical tools to look at synthetic things," he said. "But lately, part of the grand challenge of engineering is to understand biology and make our robots and vehicles not more living, but more like living things."
No humanmade robot is designed with the loose skin of a labrador. But why not? "Take this idea of self-drying and self-cleaning [machines]. There is a lot of literature like that because autonomous robots are going to have to deal with this on their own." Perhaps the Mars rovers of the future should shake the dust off themselves.
"If we relook at these ancient mechanisms," Hu concluded, "we can build robots better."
And while we're at it, we can explore the enduring mysteries of life, too. Like why that damn ungrateful dog soaked you better than a hose after you gave it a bath. Now if only they could figure out that wet dog smell.
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