ZELDITCH, M.L.; Univ. of Michigan: Canalizing skull shape: balancing the generation and compensation of variance

Over the course of development, variance in skull shape decreases, suggesting that shape is canalized. Although that pattern has been described for three rodents, the processes responsible are still mysterious, partly because the generality of the trend is uncertain and several conflicting hypotheses are equally consistent with the available data. In this study, I examine the ontogeny of variance in a longitudinal sample of laboratory rats, and find that shape variance decreases dramatically over early postnatal development and thereafter is nearly constant. This pattern is consistent with several hypotheses, including: (1) prenatal development is less tightly regulated than postnatal development, (2) variance in developmental timing or growth rate decreases over ontogeny, thereby reducing variance in shape; (3) variance is generated and compensated continually, with deviations from the norm becoming less dramatic as developmental rates decrease. Results of shape analysis show that variation in developmental timing accounts for a small and nearly constant proportion of the variation in shape so a decrease in this source of variance does not indirectly canalize shape. Individuals most deviant for their age are returned to the norm within one week, indicating rapid compensation, whereas other individuals come to deviate from the mean, demonstrating that variation is continually generated. The magnitude of deviations generally decreases over time, as does the number of highly deviant individuals. The results support the hypothesis that variation is both continually generated and compensated at decelerating rates over postnatal growth.