Effects of Body Morphology on Vertebrate Rotational Inertia

Walter, R. *; Otterstrom, J.; Carrier, D.: Effects of Body Morphology on Vertebrate Rotational Inertia

Because the force required to rotate a body about an axis is directly proportional to its rotation inertia about the axis, it is likely that animals with high rotational inertia would be constrained in their turning abilities. Using the fact that a physical pendulum’s period is proportional to its rotational inertia^1/2, we measured the rotational inertia in two groups of vertebrates with greatly different body shapes: murian rodents and lizards (Iguana iguana and Varanus exanthermaticus). A small lizard has more than double the rotational inertia about its center of mass as an equivalent size rodent. Because, in geometrically similar animals, rotational inertia scales with mass^5/3, while muscle force scales with mass^2/3, larger animals are ^ected to have more difficulty turning. Rotational inertia scaled nearly isometrically for the muiran rodents, and with slight negative alometry for the varanids and iguanas. This suggests that either these animals are not constrained by rotational inertia due to their relatively small size, or that larger rodents and lizards are poorer turners than smaller ones. The basal synapsids from which cynodonts and mammals are derived had a body morphology best represented by recent lizards such as varanids or iguanas. The loss of the large muscular tail and elongated body form during synapsid evolution reduced rotational inertia and likely improved turning ability.

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