Cranial Allometry and the Evolution of the Domestic Dog

Domestication is usually defined as a process involving human subjugation of other animal or plant species. From this perspective, it is often presumed that morphological changes in domestic animals are the product of conscious or unconscious human selection. A broader evolutionary perspective does not make this presumption.

The origin of the domestic dog (Canis familiaris) is best understood as a consequence of human adoption of wolf pups (Canis lupus) some 12,000 years ago. Young wolf pups growing up in human society formed their primary social bonds with humans. The radically altered circumstances experienced by these early domestic canids placed them in a new role as ecological colonizers. Selection under these circumstances favored precocious maturation, resulting in evolutionary progenesis, a form of heterochrony. Concurrently, an abrupt shift in diet resulted in rapid size reduction in the new evolving species.

Craniometric data analyzed from modern wild Canis and prehistoric domesticated dogs from North America and northern Europe, all predating 3,000 B.P. The goal is to assess whether or not morphological changes in dogs are allometric consequences of size reduction, brought about by heterochronic alterations. Previous investigations of canid allometry involving wild canids and modern dog breeds serve as a frame of reference.

Bivariate analysis of static data reveals that the dogs exhibit uniquely wide cranial vaults and palates, and are distinct from allometric trends seen among other groups. Anterior cranial length variables are tightly scaled among all groups, with proportional variation a consequence of allometric scaling. Dogs also tend to have proportionally longer teeth than similar sized wild canids. Bivariate analysis of ontogenetic data reveals that wide vaults and palates in dogs are associated with a greater correspondence to wolf ontogenetic regression lines relative to other groups. On anterior cranial length variables all groups exhibit evidence of ontogenetic scaling. Multivariate analysis indicates that dogs are morphologically more similar to juvenile wolves than to any adult group.

Juvenilized morphology in dogs is a consequence of rapid size change with morphology constrained to developmental pathways. Invariance in gestation period in Canis may pose a fundamental morphological constraint on dog morphology. Confinement of morphology to developmental boundaries may be indicative of rapid evolutionary change in general. Heterochronic mechanisms responsible for this mode of change may be important in the evolution of domestic animals other and the dog.