Support and locomotion in land animals

1Elbows out or elbows in?

Try this. Get into a push-up position with your arms straight under your shoulders: you can hold it for a long time. Now bend your elbows out sideways until your chest is just above the floor, and hold that. Within seconds your chest and arm muscles burn. You have just compared the two ways land vertebrates hold their bodies up: the sprawling posture of a lizard and the upright posture of an antelope. On land there is no water to hold the body up, so the skeleton and muscles must carry its full weight with every step. This lesson looks at how land animals solve that problem and how they move.

2The problem of support on land

Air gives almost no upthrust, so a land animal's weight must be transmitted through its skeleton to the ground. In a four-legged vertebrate (a tetrapod), the vertebral column works like a bridge: it is held up at the front by the forelimbs and pectoral girdle, and at the back by the hindlimbs and pelvic girdle, with the body hanging from it. Strong ligaments and the muscles of the back and belly keep the "bridge" from sagging. The pelvic girdle is fused firmly to the vertebral column, because the hindlimbs push the body forward; the pectoral girdle is held mainly by muscles, which cushion the shock when the forelimbs land.

Smaller land animals solve the problem differently: an earthworm has a hydrostatic skeleton (fluid under pressure in each segment), and insects and spiders have an exoskeleton, which you met in S1 and will meet again in lesson 5.

3Posture: sprawling and erect

Figure 1. Sprawling and erect postures, front views (simplified).

In a sprawling animal such as a lizard or crocodile, the ground pushes up on a foot that is far out from the shoulder or hip joint. That force has a large moment about the joint, so muscles must work constantly to stop the body sagging. Lizards therefore rest with their bellies on the ground and move in short bursts. Mammals and birds hold their limbs under the body: the upward force passes almost straight up through the bones, its moment about each joint is small, and the bones take the load in compression. This saves energy, allows long legs and longer strides, and frees the chest for breathing while running.

4How limbs move the body

A limb is a system of levers: the bones are the levers, the joints the pivots, and the muscles the effort. In each stride, a foot is placed on the ground and the limb muscles push it backwards; by Newton's third law, the ground pushes the animal forwards (friction stops the foot slipping). The limb is then lifted and swung forward for the next step. Antagonistic pairs of muscles, which you met in S2, bend (flex) and straighten (extend) each joint.

Fast runners have long limbs, and many stand on their toes (Figure 2). Lifting the heel lengthens the leg, so each stride covers more ground, and keeping the heavy muscles near the body with long, light tendons running down to the feet makes the leg quicker to swing.

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To go faster, animals lengthen their stride and take strides more often, and they change gait. In walking, each foot is on the ground for more than half of the stride (duty factor above 0.5) and there is always at least one foot down. In running, each foot is on the ground for less than half of the stride (duty factor below 0.5), and there are moments when no foot touches the ground.

Snakes have no limbs. In lateral undulation, waves of muscle contraction bend the body into S-shaped curves that press sideways against stones, twigs and uneven ground; the reaction from these points pushes the snake forward. Large, overlapping belly scales grip the ground and stop the body sliding backwards.