Gaits in mammals

1. What they did

When this paper was written, nobody had brought together what was known about how mammals move and related it to their body shape, ancestry and habitat. Anne Innis Dagg set out to do that, using published accounts and her own 13 years of studying mammal movement.

She points out how hard this is. Even for bears, experienced observers disagreed on whether they trot or pace, and small mammals move their legs far too fast to see. Words like “run”, “rack” and “amble” had been used for several different gaits.

To keep the review useful, she:

• Left out domestic animals (bred by people for unnatural traits), young animals, poorly studied families, speeds, abnormal gaits, and gaits used only briefly.
• Built a table (Table 1) of about 90 mammal genera or groups whose gaits were well known, scoring each one’s weight, tail length, slope of back, body shape (leg length relative to trunk length, from side-view photographs), the gaits they use, whether they climb or stand on their hind legs, and their habitat.
• Added her own data from laboratory films of small mammals and from tracks in a field guide (Murie, 1954).
• Defined each gait clearly, with a figure showing which legs support the body through a stride.

2. What they found

The gaits defined

• Walk: the slowest gait, with two, three or four legs on the ground; the left legs repeat the right legs half a stride later.
• Walk using the tail: used by kangaroo rats and kangaroos, which lean on the tail as a fifth leg; pangolins may also use it.
• Running walk: a quick walk, also called an amble. It is the only fast gait of elephants, which cannot trot, gallop or jump.
• Trot: medium speed, diagonal legs together. It needs front and hind legs of similar length and a fairly straight, rigid back. Ring-tailed lemurs, spotted hyaenas, giraffe, apes and monkeys do not trot, and possibly not bears; wildebeest, hartebeest and viverrids do.
• Pace: medium speed, legs on the same side together. Only dogs (long-legged breeds), horses, bears and camels pace. It is less stable than the trot and suits flat ground, such as the camel’s desert. In horses, pacing is governed by a single recessive gene.
• Gallop and bound: the fast gaits of nearly all land mammals. Animals with flexible backs (small squirrels, jumping mice, rabbits and hares, long weasel-like mustelids) only bound; those with rigid backs (hoofed animals, dogs, bears, hyaenas, woodchucks, skunks, wolverines) only gallop. The opossum and platypus can do neither.
• Bipedal hop (ricochet): used by animals with big hind legs and long, heavy or tufted tails, from kangaroos to kangaroo mice, always plant eaters and usually from deserts. Arctic and Greenland hares also hop on their hind legs when closely chased.
• Bipedal walk or run: some primates, and slowly in pangolins, kangaroo rats and jerboas.
• Stott (pronk): all four legs take off and land together. It is found only in herd-living deer, antelope and their relatives (16 species listed).
• Display gaits: gaits that show an animal’s state to others, such as the prance of breeding males, the stiff “warning gait” of disturbed elk and moose, and the swaggering walks of dominant primates.

What shapes an animal’s gaits

• Weight: the heaviest animals are limited. Elephants (up to 7000 kg) cannot gallop, hippos (up to 3000 kg) only trot, and rhinos (up to 2000 kg) can gallop.
• Slope of back: a back sloping down from the head may or may not stop a trot. A humped back in long mustelids goes with no trot.
• Body shape: very long, short-legged animals like otters do not trot. Long-legged hunters and their prey use the common gaits with extra speed.
• Hind legs and bipedalism: two-legged travel evolved in marsupials, rodents and primates. Apart from people, mammals that travel mainly on two legs hop with both legs together.
• Tails: they help with turning, balance and climbing. A kangaroo rat with no tail turned a somersault when trying a high leap.
• Ancestry: body shape alone does not predict gaits. Camels, vicugnas and okapis do not trot although similar-shaped bovids do, and pikas hop even though their front and hind legs are similar in length.
• Habitat: deserts favour hopping and the camel’s pace; forest walks are stable and allow a quick retreat.

Gaits and tracks

The type of track was easy to identify from tracks when an animal was bounding or galloping, but not when it was walking, trotting or doing the running walk. Many species also have distinctive ways of moving that identify them, such as the long high bounds of the white-tailed jackrabbit and the shorter bounds and frequent “spy-hops” of the black-tailed jackrabbit.

Why some animals trot rather than gallop

Large animals such as moose, elk, caribou, eland and waterbuck tend to trot at speed. The trot is less tiring, more stable on mud, snow or rough trails, and lifts the body (and heavy horns or antlers) less than the gallop.

3. What they concluded

• Each species’ gaits depend on its build (weight, back, body shape, legs and tail), but also on its ancestry and its habitat, past as well as present.
• Gaits used a great deal by a group probably appeared early in its evolution, so gaits can hint at how species are related. Young animals of species that hop as adults first move their legs alternately, suggesting alternate leg movement is basic to all four-limbed animals.
• Speed is not always favoured: on Isle Royale, moose that ran from wolves were more likely to be killed than those that did not.
• Ethologists should know how the animals they study move, and should define the gaits they describe. Display gaits, abnormal gaits and the gaits of small mammals are promising fields for future study.

4. How tracking was or was not used

Tracks were a minor source of data. Some gait information was taken from tracks in a field guide (Murie, 1954). The author notes that the gait was easy to read from tracks when the animal was bounding or galloping, but hard when it was walking, trotting or doing the running walk. She also notes that many species have distinctive gaits that identify them, but does not cover those differences.

6. Other technologies, skills or innovations

• Laboratory films of small mammals, which proved hard to use because the animals moved so irregularly.
• A simple body-shape index (shoulder height divided by trunk length, times 10) measured on side-view photographs, to compare relative leg length across species.
• A diagram (Figure 1) of which feet support the body through each gait.