Moles are busy creatures. When not digging their tunnels, eastern moles (Scalopus aquaticus) are living, foraging and traversing through their labyrinthine territories. In a single day, a mole may walk 400 m through pitch-dark, cramped, narrow tunnels; that's a lot of (under) ground to cover. Moles do this in their own particular style – with their palms facing outwards. This unusual forelimb posture is probably an adaptation to a digging lifestyle; equipped with an extra ‘false’ thumb on their spade-like hands, moles push soil away from their body and against the tunnel walls. But how does an animal specialised for digging deal with the demands of walking such long distances?

Yi-Fen Lin at the University of Massachusetts (UMass) Amherst and colleagues at UMass Lowell and the University of California Merced, USA, set out to record high-precision measurements of how moles walk. The team studied three eastern moles, a species that lives close to Lin's lab. To measure how the moles’ skeletons move, the scientists turned to a technique known as X-ray Reconstruction of Moving Morphology, or XROMM for short. XROMM combines X-ray videos of the moving moles with 3D models of their bones and then splices the data together using Hollywood-standard animation software to reconstruct the moles’ motion. For the experiment, the moles walked back and forth through a purpose-built tunnel, positioned in front of the X-ray cameras. When the motion was reconstructed from the X-ray videos, the resulting 3D animations were beautiful and precise.

Analysing the movies, Lin found that moles power-walk in a most unusual manner, probably a by-product of their digging tendencies. Whereas all other quadrupeds place their feet on the ground behind their shoulder joint, moles break the rules by holding their feet in front. With each step, they load their weight fleetingly onto their thumbs and false thumbs, way out in front of their body. This instance of weight bearing was so brief that the moles’ walking gait almost approached some definitions of running and was more like human power-walking, where people walk as fast as possible without actually breaking into a run. This might seem like an awkward way to get about, but considering that moles walk with shovels permanently attached to their forearms, awkward seems normal.

The XROMM animations also show that moles point their improbably shaped upper-arms to the ceiling, not downwards like other quadrupeds – effectively staging the action high above the shoulder joint instead of below it. As each foot lifts off the ground, the limb moves forward in line with the body, then retracts back. This is a very different motion from that seen in digging, where the moles shovel earth either side by twisting their arms outwards. This walking configuration means that moles fold their limbs into a nice compact package, perfect for squeezing down narrow tunnels.

Lin's moles show us how demanding life can be, sometimes right beneath our feet. Whilst it's incredible that moles have evolved such effective digging modifications at the ends of their limbs, it's even more astonishing that they still perform all of their other daily activities while burdened with these apparently cumbersome appendages. Sometimes a strange lifestyle leads to a unique way of getting around – no other known animal walks quite like a mole.

Y. F.
E. R.
How moles walk; it's all thumbs. 
Biol. Lett.
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