THE ONTOGENIC COMPLEXITY OF DEVELOPMENTAL CONSTRAINTS

Developmental constraint is a theoretically important construct bridgi ng ontogenetic and evolutionary studies. We propose a new operationali zation of this notion that exploits the unusually rich measurement str ucture of landmark data. We represent landmark configurations by their partial warps, a basis for morphospace that represents a set of local ized features of form. A finding of developmental constraint arises fr om 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 constrain t: canalization (constraint to relatively constant form age by age); c hreods (reduction of variance orthogonal to the mean trajectory over a ges); and opposition (reduction of age-specific variance along the mea n trajectory over ages). While canalization and chreodic constraints h ave 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 displ ay a remarkable spatiotemporal complexity in patterns of variance redu ction. The specific assortment of constraints observed may be related to the precociality of Sigmodon. We suggest that Waddington's diagramm atic presentation of the ''epigenetic landscape'' may be misleading in quantitative studies of developmental regulation.