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Size is one of the most important features governing how an animal functions: there are no elephant-sized mice or mouse-sized elephants for myriad reasons. Big animals have different metabolic and structural demands than small animals. One of the archetypal examples of a size-related biological trait is the posture of terrestrial vertebrates. As weight is proportional to volume (a length cubed), and strength is proportional cross-sectional area (a length squared), as size increases, weight increases faster than the ability to support it. That is why the biggest terrestrial vertebrates need stiff, thick, columnar legs and must maintain an upright posture to support their own weight. This contrasts with small animals, which usually crouch. This raises the question, ‘Why?’ If small animals stood upright like their bigger companions, they could have lighter limbs, with the same ability to support their weight. So why do small animals crouch?

In a recent study published in Biology Letters, James Usherwood of the Royal Veterinary College, London, asked just this question. By looking at existing locomotor scaling relationships and the gaits of differently sized animals, and employing numerical models, Usherwood posited that there are size-dependent differences in the demand for muscle work and muscle power for a given gait. To minimize the amount of active muscle required to produce work and power, animals of different sizes may use different gaits.

Large animals tend to use gaits that minimize the length of time each foot is in contact with the ground. This is energetically cheap: little work is wasted slowing down and speeding up with each step. However, despite the reduction of mechanical work, short ground contact time can be a problem when it comes to the demand for muscle power. As power is the rate at which work is performed, reducing the amount of contact time with the ground increases the power required for push-off.

Usherwood thought that small animals crouch to increase the amount of time the foot is in contact with the ground during each stride, reducing their power requirements. After observing that smaller animals do in fact use gaits with longer foot contact times, Usherwood used a numerical model based on data from flightless birds of different sizes to model the effects of size on the muscular requirements of different gaits.

According to Usherwood's model, if small animals were to use the upright, stiff-legged, high power gait, they would need to activate a wastefully large volume of muscle. Reducing their power requirements by having longer periods of contact with the ground allows small animals to minimize their active muscle volume. Maintaining longer contact time requires bent legs. According to this logic, the crouched posture in small animals is merely a way of giving the leg more time to push off the ground and compensating for the large power demands (relatively speaking) of small size.

This is not to say that crouching is ideal. In exchange for their lower push-off power requirements, small animals do lose energy as they change speed within the stride. That's why if you walk with crouched legs, it's more exhausting than if you walk normally. But, in the unlikely event you find yourself shrunken to mouse proportions, try crouching.

References

Usherwood
J. R.
(
2013
).
Constraints on muscle performance provide a novel explanation for the scaling of posture in terrestrial animals
.
Biol. Lett.
9
,
20130414
.