HETEROCHRONY WITHIN SPECIES - CRANIOFACIAL GROWTH IN GIANT, STANDARD,AND DWARF RABBITS

Change in developmental timing is one source of heritable variation up on 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 g rowth and adult morphology of several structures (one soft tissue and 15 skeletal) were quantified and analyzed. We took two views of radiog raphs at close time intervals throughout ontogeny, from one week of ag e through adult size. Measurements from the radiographs were analyzed using a Gompertz growth model. Between-breed differences in model para meters were tested using one-way ANOVA. Few significant differences ex isted between the white and giant rabbits, but several differences wer e found between the white and dwarf breeds. Similarly, comparisons of adult morphology showed that white and giant rabbits are the same shap e, while dwarf rabbits have shorter and broader snouts than white rabb its. The variation in size among breeds appeared to be due to differen ces in the length of time spent growing at rates near the maximum grow th rate. While no one parameter of this model quantifies this pattern, differences in duration of maximum growth rate can be seen in the fir st derivative of the growth trajectory. Small changes in the model's p arameters that measure rate and timing of growth have large morphologi cal consequences, indicating that heterochronic changes are important sources of variation.