The Adaptive Significance of Tetrapod Gait Selection

1. What they did

Over many years Milton Hildebrand developed a way to record, analyse and compare how four-legged animals move, using film of animals in motion. In this paper, first given at a symposium in 1978, he uses that method to explain why animals choose the gaits they do, with emphasis on mammals.

• Symmetrical gaits (walk, pace, trot, single-foot): the feet of each pair strike evenly spaced in time. He describes each gait with two numbers, plotted on a “gait graph”: how long each foot stays on the ground, and how soon a forefoot follows the hind foot on the same side.
• Asymmetrical gaits (bound, half bound, gallops): the feet of a pair strike unevenly. He uses the lead (the gap between the two feet of a pair), the time both hind feet are on the ground, and the “midtime lag” between the hind and front pairs.
• For each gait he asks what it offers in stability, economy of effort, avoiding interference (a hind foot hitting a forefoot), speed and manoeuvrability. He then checks his predictions against more than 1,000 gait records for 156 genera.

2. What they found

Walking gaits: stability matters most

• At a slow walk, animals avoid gaits where the two feet on the same side support the body together, because the body tends to roll.
• Most walk in lateral sequence (a hind foot is followed by the forefoot on the same side). This gives larger, better placed triangles of three supporting feet than diagonal sequence, and lets the spine undulate, which lengthens the step.
• Tippy animals such as turtles are expected to use these lateral-sequence walks. Animals known to walk in diagonal sequence (primates, giant armadillo, aardvark and kinkajou) are ones whose stability comes from large feet or a wide stance.
• The real records matched the predicted favourable and unfavourable areas of the gait graph.

Running symmetrical gaits: economy and interference

• Running animals use four symmetrical gaits: the pace, the trot and the two single-foots.
• The single-foots give the fastest travel before the animal has to leave the ground altogether, and spread the load evenly across the four legs.
• In the trot and pace two legs work together, absorbing impact and giving thrust, but with longer moments in the air.
• Most tetrapods trot if they run at all. Short-legged animals (most rodents and carnivores), stocky ones (bovids, rhinoceros, hippopotamus) and those with legs splayed to the side (lizards) have no other running symmetrical gait.
• Long legs bring a risk of kicking themselves. The dachshund trots without trouble; the German shepherd must pace or “crab” (place the hind feet to one side of the forefeet); the greyhound commonly paces. Camelids pace, and the horse paces slightly faster than it trots.
• The elephant uses the lateral-sequence single-foot, which is smooth and avoids moments with no feet down. Some small antelopes (duiker, muntjak) use the diagonal-sequence single-foot, for reasons that are not clear.

Bounds and half bounds: small, agile leapers

• The forelead tends to be longer than the hind lead, because the hind feet push and the forefeet support.
• The bound and half bound are used by small to medium, agile animals that move in a series of leaps over rough ground, through vegetation, or to accelerate and dodge: slender marsupials, insectivores, rodents, hyraxes, rabbits, many carnivores and smaller antelopes. The longest jumpers (squirrels, jumping mouse, impala) favour the bound; the half bound suits leaps of moderate length.
• Apes sometimes use the crutch walk because of their long arms.

Suspensions: when all feet are off the ground

• An extended suspension (legs stretched fore and aft) is part of leaping. It is commonly half the stride and can reach 80% in pronking deer, Patagonian cavy and jumping mouse.
• A gathered suspension (feet bunched under the body) is rarely as much as a quarter of the stride and saves effort. Large animals running long distances in the open (horse, rhinoceros, camelids, larger bovids) usually use it.
• The fastest runners (hounds, cheetah, pronghorn, antelope, rabbit) use both suspensions to get a long enough stride.

Transverse versus rotary gallop

• In the transverse gallop the front and hind pairs lead on the same side; in the rotary gallop they lead on opposite sides.
• The transverse gallop is more stable, more economical, and the only gallop used at slow speeds. It also lets the horse and guanaco avoid kicking themselves.
• At top speed, with two suspensions, these advantages vanish. Rotary gallopers are usually medium-sized sprinters with less endurance but a little more speed; long-legged ones (giraffe, antelopes, cheetah) must swing the hind legs wider than the forelegs.
• The author suspects the rotary gallop offers subtle advantages of timing, balance and “feel” at speed, and may help with quick changes of lead and dodging. Stability and manoeuvrability tend to work against each other.

Other points

• His gait formula method and the “walk-pattern” method used by others deal with the same data and are interchangeable, not in conflict.
• Variation from stride to stride can be noise (a cold turtle on uneven ground) or remarkable fine control (a cheetah matching every swerve of a blackbuck at 100 km/h).

3. What they concluded

• Animals choose gaits that suit their body build, speed and terrain. At slow speeds stability matters most; at faster speeds, economy of effort and avoiding interference between the legs become important too.
• Small, agile animals use the bound, half bound and the extended suspension; large animals running long distances in the open use the shorter, more economical gathered suspension; the fastest runners use both.
• The transverse gallop gives stability and economy at moderate speed. At high speed those advantages no longer apply, and the rotary gallop may give better manoeuvrability.
• Gait analysis may have limited use in neurophysiology today, but it will be needed to understand how the best runners control their movement.