Heterochrony within species: Craniofacial growth in giant, standard, and dwarf rabbits

Change in developmental timing is one source of heritable variation upon which selection can act. However, the amount of variation possible in ontogenetic trajectories is often unknown. We used three different-sized conspecific breeds of domestic rabbits to investigate the extent of variation in growth trajectories of craniofacial morphology. The growth and adult morphology of several structures (one soft tissue and 15 skeletal) were quantified and analyzed. We took two views of radiographs at close time intervals throughout ontogeny, from one week of age through adult size. Measurements from the radiographs were analyzed using a Gompertz growth model. Between-breed differences in model parameters were tested using one-way ANOVA. Few significant differences existed between the white and giant rabbits, but several differences were found between the white and dwarf breeds. Similarly, comparisons of adult morphology showed that white and giant rabbits are the same shape, while dwarf rabbits have shorter and broader snouts than white rabbits. The variation in size among breeds appeared to be due to differences in the length of time spent growing at rates near the maximum growth rate. While no one parameter of this model quantifies this pattern, differences in duration of maximum growth rate can be seen in the first derivative of the growth trajectory. Small changes in the model's parameters that measure rate and timing of growth have large morphological consequences, indicating that heterochronic changes are important sources of variation.