Some foxes have a unique hunting behavior that they only use in winter: snow diving, where they dive face-first into snow without hurting their noses or breaking their heads.
After it snows in cold climates, rodents burrow through the snow, hidden from the prying eyes of hungry predators, particularly foxes. But some red foxes, Vulpes vulpes, and some Arctic foxes, Vulpes lagopus, use a specialised hunting technique known as “snow diving” or, more properly, as “mousing”. They rely on their keen hearing to pinpoint the precise location of a small prey animal, usually a mouse (or a lemming, in the case of an Artic fox) as it moves under the snow, then they jump high into the air and dive nose-first into the snow at speeds of up to 4 metres per second to catch their dinner by surprise.
Although there have been quite a few studies of water birds and animals such as porpoises and dolphins that dive from air into water, snow diving has not been well-studied.
“It’s a very interesting and unique behaviour,” the study’s senior author, biomechanist Sunghwan Jung, a Professor in the Department of Biological and Environmental Engineering and Director of Graduate Studies at Cornell University, said in a statement. Snow diving is a specialized skill requires hundreds of hours of practice before the foxes consistently succeed. Some, apparently, never get the hang of it.
“Not all foxes do it either,” Professor Jung mentioned.
What protects snow diving foxes from hurting themselves on impact? And how does this hunting method compare to other mammals with similar diets? Does the secret to snow diving foxes’ abilities lie in their facial structure?
To investigate these questions, Professor Jung and collaborators scanned and compared the skulls of 13 fox species, as well as those of other hunting mammals, such as lynx and pumas, held in museum collections.
Then Professor Jung and collaborators used a primitive but effective technique to test the biomechanics of these 3D-printed models: they dropped them into either snow or water from a height of 50 centimetres (19.5 inches). Attached to each skull was a sensor that measured the force of the impact, and Professor Jung and collaborators recorded these data into computer models to analyze and compare both.
Professor Jung and collaborators found the fox’s pointy snout shape reduces its risk of injury because the snout gently pushes the snow aside, almost like a fluid.
“The fox’s sharp snout doesn’t significantly compress the snow; it penetrates it without much resistance,” Professor Jung explained. “This kind of elongated shape helps foxes dive into snow safely, so they can focus on hunting.”
But the properties of snow can change. On one hand, snow has fluid-like properties when it’s light and fluffy, but on the other hand, its properties change to more closely resemble a solid when compacted into, say, a snowball. It’s a wonder that snow diving foxes don’t break their snouts or crack their faces.
“This is a very dangerous process, but we haven’t had reports of foxes getting injured,” Professor Jung remarked.
Professor Jung and collaborators compared canid skulls to felid skulls and found that, in contrast to foxes and most other canids, feline skulls have wider and shorter snouts that compress snow upon impact, creating a large and potentially damaging resistance.
Additionally, the blunt structure of feline skulls allows them to produce a stronger bite than foxes, Professor Jung explained, which is useful for them because they are solitary hunters. In contrast, foxes tend to hunt in groups.
“Each member of a pack inflicts incremental damage on their prey by biting with their long snouts, which can kill the prey,” Professor Jung explained.
Another benefit is that, when snow diving, a fox’s long pointy snout reaches its prey sooner than a cat’s flat snout — an advantage because mice are extremely sensitive to movements and sounds in their environment and will swiftly flee to safety.
It’s interesting to note that other studies have found that snow diving foxes show other behaviors to increase their success, such as shaking their heads to better hear to the rustle of small rodents moving under the snow’s surface, so they can use this to pinpoint the depth and location of the sound’s source.
Source:
Jisoo Yuk, Anupam Pandey, Leena Park, William E. Bemis, and Sunghwan Jung (2024). Effect of skull morphology on fox snow diving, Proceedings of the National Academy of Sciences 121(19):e2321179121 | doi:10.1073/pnas.232117912
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