Analysis of Posture and Gait Selection During Locomotion in the Striped Skunk (Mephitis mephitis)

1. What they did

Cats and dogs, the carnivores studied most, are built for running. Most mammals are not, so the authors studied the striped skunk, a cat-sized carnivore that moves fairly slowly and holds its body in a more general way, to learn what the original (primitive) pattern of carnivore locomotion might look like.

• Footfalls and gaits. They filmed four captive adult skunks and six domestic ferrets at 64 frames per second while the animals walked and ran past a ruled grid, then drew gait diagrams showing when each foot was on and off the ground.
• Posture inside the body. They made X-ray films (cineradiography) at 200 frames per second of three skunks walking in a clear box over a treadmill at a slow (0.28 m/sec) and a fast (0.72 m/sec) walk. Skeletons were then mounted in the filmed postures so joint angles could be measured.
• Comparison. They plotted skunk, ferret and cat walking gaits on Hildebrand’s gait graph, alongside about 1,000 plots for 156 genera of four-legged animals.

2. What they found

Walking

• Skunks walk far more than they use any other gait. Walking speeds filmed ranged from 0.16 to 0.92 m/sec.
• Even on a clear surface, skunks keep changing how many feet support them. As they walk faster, the diagonal pairs of feet move more in step and the periods on three feet get shorter.
• Their walk is almost always a lateral sequence, single-foot gait: each hind foot is followed by the front foot on the same side, and the four footfalls are evenly spaced in time. Only a few diagonal couplet patterns were seen.
• Walking cats used the lateral couplet and single-foot gaits about equally. Ferrets used a wide range of walking gaits, which the authors link to their very flexible spine and short legs.

Running

• When speeding up, skunks seldom trot. They move straight from a walk into a slow transverse gallop. Trots were seen only now and then, as an animal slowed from a gallop to a walk.
• One skunk filmed galloping at about 1.6, 1.85 and 2.56 m/sec used transverse gallops every time. The faster it went, the more the hind legs moved together. Only at the fastest speed was there a brief moment with all four feet off the ground, with the legs gathered under the body.
• No rotary gallop and no stretched-out (extended) suspension were seen. The authors note the laboratory may have kept the skunks from reaching top speed.
• Ferrets also preferred slow transverse gallops, but unlike skunks they showed extended suspension at speed.

Hindlimb and pelvis

• When the foot touches down, the thigh bone (femur) angles out from the body at about 25 to 40 degrees, compared with 10 to 15 degrees in cats and dogs.
• Just after the foot lands the femur swings in towards the body (to about 15 degrees), then out again late in the stance. The pelvis shifts and rotates from side to side, about 20 degrees on average. These features give a wide, stable stance.
• Unlike the mammals thought to be primitive, the skunk always holds the lower end of the femur below the hip socket.

3. What they concluded

• The skunk’s preferred walk and its slow transverse gallop both give maximum stability and are considered primitive gaits; the skunk avoids the less stable trot.
• More specialized runners (cats, dogs and even ferrets) use a much wider range of running gaits.
• Several features of the skunk’s hindlimb posture match the pattern thought to be primitive for mammals, as also seen in raccoons and opossums.
• The low position of the femur may mean either that the primitive condition has been misread, or that it was an adaptation in carnivores that walk about on the surface, while carnivores with a raised femur may be adapted to chasing rodents through burrows.
• Overall, in gait selection and posture, the striped skunk represents the primitive condition for the order Carnivora.

6. Other technologies, skills or innovations

• High-speed 16 mm cinematography (64 frames per second) against a ruled grid of 10 cm squares, bowed into a semicircle to reduce distortion.
• Cineradiography (X-ray film) at 200 frames per second over a variable-speed treadmill, viewed on a television monitor at the same time.
• Hildebrand’s gait diagrams and gait graph, a standard way to compare footfall timing across species.