Postural Stability of Animals of Different Sizes, Shapes, and Neural Delays
Bartlett, Harrison Logan
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An important issue in the area of biology is form following function. It is evident that animals have wide variation in morphology, but what functions do these forms follow? The postural stability of an animal decreases as the neural delay increases. This delay increases with animal size because signals must travel across a longer distance at a constant speed. Despite this increase in delay, large animals typically do not fall. In addition to the neural components, animal morphology also affects stability. Therefore it is possible that stability is a guiding principle of morphology. An animal may have a particular shape in order to function in its niche in an ecosystem while maintaining a stable morphology. It is proposed that in order to maintain postural stability, large animals have adapted different morphologies to counteract their longer neural delays. The postural stabilities of animals of different shapes and sizes will be examined using a mathematical model of balance. The effects of neural delay and morphology on postural stability were studied using a four-bar linkage model of frontal plane balance applied to previously- published morphological data from horses and dogs. The postural stability was quantified by calculating the maximum allowable neural delay for an animal in order for the animal to prevent falling via corrective action. This measure was compared to the calculated neural delay for each animal. It was found that maximum allowable delay scales proportionally to neural delay, indicating that postural stability may scale across animal size and morphology. The model has limitations in that it does not incorporate animal width into the calculation of neural delay, therefore excluding the effects of animal width. These results may reveal a scaling relationship for the stability of biological systems across sizes, morphologies, and species.