THE ONTOGENIC COMPLEXITY OF DEVELOPMENTAL CONSTRAINTS

Developmental constraint is a theoretically important construct bridging ontogenetic and evolutionary studies. We propose a new operationalization of this notion that exploits the unusually rich measurement structure of landmark data. We represent landmark configurations by their partial warps, a basis for morphospace that represents a set of localized features of form. A finding of developmental constraint arises from the interplay between age-varying means and age-specific variances in these subspaces of morphospace. Examination of variances and means in 16 ventral skull landmarks in the cotton rat S. fulviventer at ages 1, 10, 20, and 30 days yielded three types of developmental constraint: canalization (constraint to relatively constant form age by age); chreods (reduction of variance orthogonal to the mean trajectory over ages); and opposition (reduction of age-specific variance along the mean trajectory over ages). While canalization and chreodic constraints have been noted previously, the oppositional type of constraint appears novel. Only one of our characters, relative length and orientation of the incisive foramen, appears to be canalized. Although skull growth becomes increasingly integrated through ontogeny, our characters display a remarkable spatiotemporal complexity in patterns of variance reduction. The specific assortment of constraints observed may be related to the precociality of Sigmodon. We suggest that Waddington's diagrammatic presentation of the ''epigenetic landscape'' may be misleading in quantitative studies of developmental regulation.