Allometry of cetacean forelimb bones

This study examines the allometric scaling relationships of the cetacean humerus, radius, and ulna. Bone lengths and diameters were measured for 20 species of odontocete and three species of mysticete cetaceans, representing eight of the nine extant cetacean families. The scaling of individual bone proportions (bone length vs. cranio-caudal diameter, bone length vs. dorso-ventral diameter), and of individual bone dimensions against estimated body mass, are compared to models of geometric and elastic similarity. The geometric similarity model describes the scaling relationship of bone length vs. cranio-caudal diameter and body mass vs. cranio-caudal diameter for the humerus only; geometric similarity also describes the scaling relationship of body mass vs. bone length for all three bones. None of the scaling relationships fits the elastic similarity model. The scaling relationships of bone length vs. dorso-ventral diameter for all three bones, and bone length vs. cranio-caudal diameter for the radius and ulna, exhibit negative allometry, indicating that large bones are less robust than small bones. Negative allometry of structural support elements has not been previously described for terrestrial mammals or plants. The high relative swimming speeds of small delphinids may generate sufficient stresses to require more robust bones relative to those of larger whales.