Winter is the perfect time of year for gazing at icy evergreen trees, drinking warm beverages by a roaring fire, and dropping 3-D printed fox skulls into the snow.
At least, that last activity was true for scientists at Cornell University in northern New York.
They recently studied how the pointy shape of a fox's snout makes it particularly adept at diving headfirst into the snow. Their findings, published in the spring, intrigued other researchers who immediately set out to replicate the work.
Both Arctic and red foxes hunt in the snow by using their sensitive hearing to locate the precise location of tunneling rodents. The foxes then take a great leap into the air and dive nose-first, often plunging so deep into the whiteness that only their tail and hind legs stick out.
"It is like they're just drilling a hole to their target," notes Deby Dixon, a wildlife photographer who works in Yellowstone National Park and has seen this unique hunting behavior many times. "I've watched wolves jump for voles and stuff like that. I've never seen them dive into the snow like the foxes do."
While previous research had looked at foxes' success rates for this type of hunting, a team led by Sunghwan Jung of Cornell University decided to investigate how the the foxes manage to penetrate through the snow without hurting their fuzzy little faces.
Jung notes that snow is a "complex but interesting" material that exhibits both solid and fluid properties. Like a solid, snow can be compressed and form shapes like snowballs. But if you sweep your hands through snow, it moves around your fingers like a fluid.
"Snow has two different behaviors," says Jung, "depending on how you deal with the snow."
It seemed certain that foxes would want snow to act like a fluid as they dove, because compressing snow into a solid would make things hard for them, since videos show that they penetrate the snow rather quickly.
"We estimate the diving speed is around two to four meters per second," says Jung, which is around 8 miles per hour.
The researchers used a 3-D printer to recreate animal skulls from museum collections: both foxes as well as animals with flatter faces, like bobcats. Then they dropped the skulls into the snow in a controlled way, taking measurements with force sensors.
The results show that a long, slightly curved snout does indeed reduce impact forces.
"Since they have a very sharp snout, they can shear the snow," says Jung. "The snow behaves like a fluid, and they can reach deep into the snow."
All of this fascinated other scientists who study skull shape.
Another group did some more tests of fox skulls, adding a rotating motion to the plunge.
"Further experiments allowing the possibility of skull rotation, leg activity, and perhaps mouth opening are mandatory to get a more realistic biomechanical characterization of fox mousing," those researchers noted.
Rachel Roston, a comparative anatomist with Seattle Children's Research Institute, says that normally, the skulls of mammals are thought of as mostly housing and protecting sensory organs like the eyes and nose.
"I think anytime we see the skull and the head sort of being used for a different purpose, it catches our attention," she says. "I had never really thought about foxes jumping into snow before."
She notes that some mammals burrow through soil, or swim through the water, but snow creates a very different kind of challenge that's intriguing to think about.
"It's a very different behavior than we tend to think of mammals doing," she says. "We don't know a whole ton about how snow moves in this context."
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