The locomotor kinematics and ground reaction forces of walking giraffes

1. What they did

Giraffes are the tallest living animal and the heaviest ruminant, with very long legs and neck. Earlier writers have called their walk a pace, a rack or an ambling walk, which suggests something unusual, but this had never been tested. The researchers set out to measure how giraffes actually walk.

• Animals. Three adult reticulated giraffes at Whipsnade Zoo in the United Kingdom (one male and two females, 750 to 800 kg).
• Equipment. They buried a row of 10 force plates (6.0 m by 0.9 m) 5 cm under the sand of the outdoor enclosure, so the giraffes could walk normally over it. Two GoPro cameras filmed from 5 m away at 120 frames per second. Markers could not be stuck to the giraffes, so the researchers tracked points on the video by hand: each hoof’s coronary band, a point behind the ear, the withers and the lower back.
• Trials. Keepers led the giraffes back and forth along the fence, carrying food ahead of them, and walked at different speeds to encourage different giraffe speeds. Recording took 1 hour a day for 6 days, in two sessions 3 months apart.
• Analysis. They kept only strides at steady speed: 75 strides, about 5% of all those recorded. They measured stride length, stride and stance duration, duty factor (the share of a stride each foot is on the ground), limb phase (the timing of a front foot relative to the hind foot on the same side), the forces on the ground, and neck movement. They also modelled the neck as a weighted rod to see how its swing affects the body, and compared the giraffes with predictions made for other four-legged mammals.

2. What they found

The gait

• The giraffes chose a narrow range of walking speeds, from 0.74 to 1.3 m per second (mean 0.98 m per second), even though the keepers tried to make them walk faster or slower.
• Mean duty factor was 0.70 and mean limb phase 0.14. In gait terms this is a 70:14 symmetrical gait, a lateral sequence walk: a typical walk for four-legged animals.
• At no point did limb phase fall low enough to count as a pace (below 6.25%).
• Giraffes walked faster mainly by taking longer strides. Stride frequency rose only slightly, stance time shortened, and swing time stayed the same.
• Duty factor was higher in the forelimbs than in the hindlimbs (1.07 times).

Forces on the ground

• The vertical force curves had the M shape (two peaks) seen in other walking animals, usually with a higher second peak in the forelimbs. Two forelimb curve shapes differed between giraffes, and some hindlimb steps showed a third peak in mid-stance, even within the same giraffe.
• Peak vertical force was 1.9 times greater in the forelimbs and did not change with speed.
• The forelimbs carried about 65% of body weight and the hindlimbs 35%, similar to most other four-legged mammals despite the giraffe’s heavy, long neck.
• The forelimbs did most of the braking; pushing forward was shared between front and hind legs. Sideways forces were very small (0.7% of the total).
• The three giraffes did not differ in stride length or peak force, and the older female’s history of overgrown hoofs did not appear to affect her gait.

The neck

• The neck swung up and down twice in every stride, with its greatest upward (dorsal) extension early in each front foot’s stance and its greatest downward flexion in mid-stance.
• The neck moved through a larger angle at faster speeds.
• The forward and backward movements of the neck were largely out of step with those of the body (a phase of 23%), so the neck partly cancels the body’s speeding up and slowing down. The up and down movements were in between in and out of step (15%). Measurements from the video agreed well with the model (22% and 17%).

Comparison with other mammals

Stride length and duty factor were broadly predicted by the equations for other four-legged mammals, with prediction errors of about 9% to 27%, and as low as 5% for stride length within the equations’ confidence ranges.

3. What they concluded

• Despite popular accounts, giraffes do not pace when they walk. They use a lateral sequence walk, like most other four-legged mammals.
• Giraffes speed up mainly by lengthening their stride. Their narrow range of stride frequencies may be close to the natural swing frequency of their heavy limbs, since swinging limbs faster costs more effort.
• Peak forces may stay level as speed rises because giraffes’ long, slender tendons absorb some of the load.
• The neck acts somewhat like a jockey in the modern “Martini glass” riding style: by moving out of step with the body, it reduces the horizontal speeding up and slowing down the legs must provide. Vertically the energy saving is modest, and the neck’s inertia may limit how fast giraffes can walk before changing gait.
• The giraffe’s walk is not as unusual as often stated. Models of other four-legged animals can be used to form testable ideas about giraffes, though not simply copied.
• Limitations: the narrow speed range (partly chosen by the giraffes), noise from the sand over the force plates, feeding the giraffes to lead them (which may have affected neck movement), and front and hind leg forces coming from separate strides. The authors note that the sand makes the results more relevant to giraffes in natural settings than to those kept on hard floors.

6. Other technologies, skills or innovations

• Ten three-axis force plates buried under sand in a zoo enclosure, so large animals could walk naturally across them.
• GoPro Hero3+ cameras at 120 frames per second, synchronised with the force plates by a light-emitting diode (LED) triggered at the start of each recording.
• Marker-free video analysis: points on each giraffe were digitised by hand from the video (DLTDV6 software), because the animals were not used to having markers attached.
• Fourier analysis to describe the shape of force curves and neck movement, allowing comparison with other species, including humans.

7. Implications for conservation (from the paper)

• Lameness is a welfare issue for giraffes in zoos. Understanding normal giraffe gait across speeds brings us closer to measuring gait problems.