Lifespan changes in the index of cephalization.

This study compares the lifespan changes in respect to several brain-body weight parameters between several rat strains and human population groups. In the rat, following an initial potential decline, these parameters remain essentially constant throughout the lifespan. In the human, following a similar postnatal decline, there is a further progressive decline after about age 50. However, this decline with age disappears when correction is made for percent body fat. Thus, while both brain and body weight decline with age in humans, they do not decline proportionately. In both species brain and body weights are greater in males than in females, but when the data are translated into the several brain-body parameters females generally show higher indices than males, and this correlates with their superior longevity. These findings are discussed in respect to the role of the brain in aging.