Crocodilian Biology Database - Locomotion (high walk)

High Walk

General Biology / Moving around: Crocodiles need to move around in order to survive.

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GENERAL BIOLOGY

Locomotion - High Walk

Hind limb supporting body (left), high walking freshie (below)

TYPE:	Behaviour
FUNCTION:	Moving the body from one place to another to satisfy requirements (e.g. thermoregulation, finding food, social interactions, nesting, escape from threats). Used when emerging from water and stepping over obstacles. Normally slow except when used for escape - may turn into fast belly crawl.
INVOLVING:	Front and hind limbs. The whole body and tail undulates rapidly from side to side when escaping

The "high walk" is so called because the crocodile lifts its entire body trunk and at least the anterior half of its tail clear of the ground when it walks. It is usually a relatively slow gait, not exceeding 2 to 3 kph. However, the crocodile can speed the gait up when required to exceed 5 kph.

During the belly crawl, crocodilians hold and move their limbs very much like a typical lizard would (crocodiles are not lizards, by the way!) - splayed out to the side (see left), giving the body a low centre of gravity, and causing it to flex in a sinusoidal manner at slightly higher speeds. Like lizards, most or all of the ventral body surface is in contact with the ground, which adds friction over some substrates like grass or rock. The high walk differs because the whole posture of the limbs change - the body is lifted up and the limbs positioned virtually underneath it. This kind of posture is unique amongst reptiles, and the reason lies in the structure of the ankle joint.

In a typical lizard, the ankle joint (along which the plane of movement runs) passes in roughly a straight line between the upper (astragalus and calcaneum) and lower bones of the ankle. This means that the foot can only really move in one plane. In crocodilians, however (and their ancestral thecodonts - which also gave rise to the dinosaurs and birds), the plane passes between the astragalus and calcaneum bones. The astragalus does not move when the calcaneum (effectively the heel) moves. This gives the ankle the ability to twist.

So what difference does this make? The arrangement of the joint means that crocodiles, unlike lizards, can easily rotate the foot about 90 degrees so that it is facing forwards rather than out to the side. This enables the crocodile to move the legs much closer to the body so that they project downwards (see right), almost beneath the body trunk (see alligator below) - the crocodile can therefore lift its entire body trunk clear of the ground and provide good support. The limbs move forwards and backwards without swinging out to the sides very far, although the alternating diagonal of the "belly crawl" remains (see alligator below). At slow speeds, there is hardly any sinusoidal movement of the body trunk, although the tail falls first to one side and then to the other as the axis of the body changes slightly - in other words, crocodiles wiggle their hips slightly as they walk.

[top] A. mississippiensis viewed from above during a high walk. The front left leg is moving forwards with the rear right leg, and the right front leg is moving backwards with the rear left leg. The legs which are moving backwards, and hence providing most support, are almost directly beneath the body trunk. [above] A. mississippiensis viewed from the side. The pose is taken shortly after the top picture, where weight has been transferred to the front left and rear right legs. The other two legs are starting to move forwards again.

[top] C. johnstoni high walking rapidly to the water. The rear left and front right feet are on the ground supporting the body, moving backwards and pushing the body forwards. The front left and rear right feet are at the back of their cycles and are about to be lifted up and moved forwards. [above] C. johnstoni during a high walk. This pose occurs slightly after the one above it. The rear left and front right feet have pushed the body forwards and are now at the back of their cycles about to be lifted up and moved forwards. The front left and rear right feet are now supporting the weight again and about to push backwards. If you look carefully, you can see the swing of the hips (and the base of the tail) from one photograph to the next.

At higher speeds, the limbs are thrown slightly further out from the body, although the trunk is still lifted clear of the ground. The sinusoidal movement of the body trunk also becomes more pronounced. This fast high walk cannot usually be sustained for very long as it requires a considerable amount of effort. It is also inherently less stable as the centre of gravity is lifted higher off the ground. As the crocodile accelerates, therefore, the body eventually falls to the ground (see left) and the gait turns into a rapid "belly crawl" with the limbs cycling and the whole body and tail thrashing from side to side.

It is reasonable to assume that the high walk is simply a remnant of the style of walking used by ancestral thecodonts and extinct crocodyliforms, many of which were more terrestrial and often able to run and gallop for some distance over land. Modern crocodiles can still employ this method of locomotion, the advantage of which is that it provides a means of lifting the body clear of the ground, unlike the belly crawl. The stride length increases, and friction decreases. Crocodiles use the high walk to step over rough ground, or raised obstacles such as rocks, branches, or other crocodiles, or when climbing a steep bank out of water. The ability to high walk varies from one species of crocodile to the next. Most species of crocodilians have relatively powerful limbs, enough to sustain high walk for several metres, although most walks are short, slow and to the point. Crocodiles often travel overland for distances of 1 km or more, and the high walk is frequently employed, although it is probably intermittant. Larger adults are less able to high walk for long distances than small animals because the power of the legs is not sufficient to support the weight of the body for long periods. The Indian gharial, Gavialis gangeticus, apparently cannot perform the high walk at all as an adult (although it is likely that the juveniles still can) because its limbs simple aren't strong enough to lift the body clear of the ground. The gharial is adapted to spend more time in the water, generally emerging only to bask and nest, so the loss of the high walk is not surprising.