Brain, Eye, and Skull Growth in Embryonic Geese

We tested the hypothesis that bill proportions at hatching are a byproduct of overall skull proportions which are a result of constraints of brain growth. We measured brain and eye weights, eye diameter, and some skull dimensions in domestic geese (Anser anser f. domestica) varying in age between embryonic day 7.5 and 29.5 (hatching), and in five adult geese. With respect to age, weights show divergent growth patterns resulting in complex allometry for eye but simple allometry for brain weight. This differs from the situation in Galliforms and probably reflects the high cerebralization of Anseriforms. Length measurements show similar growth patterns. Cranial length realizes a larger part of overall growth during incubation when compared to facial length. Cranial width is similar to eye diameter and brain weight. Bill width exhibits a unique growth pattern. Bill proportions are similar to the whole head but differ from those of the bony brain capsule. Therefore, although all craniometric measurements strongly correlate to brain size, brain growth probably influences growth of the cranial but not of the facial part of the skull.