Skull growth in equids beyond domestication

Te size of body parts often co-vary through exponential scaling, this is known as allometry. Allometric changes are important to the generation of morphological diversity. To make inferences regarding the evolved responses in allometry to artifciial selection in the genus Equus, we compared allometric parameters (slope and intercept) among 18 domestic breeds (11 for horses and 6 for donkeys) and 7 wild species, attempting to interpret the dfferences in allometric parameters (body length, withers height and head length). Te allometric values were not different among domestic equids. Breeds of similar sizes have similar head lengths. The elongation of head length is related to overall body size, indicating that allometry was invariant and did not change under specific selection in the breed formation.Head elongation (dolicocephaly) is probably focused on the preorbital region (dolicoprosopial) rather than on basicranial region. A remarkably higher correlation among donkey breeds can be explained by its strong similar morphological evolution. These findings provide evidence that changes in the allometry pattern point to modifications of ontogenetic processes derived from breeds differentiation and evolution. Further analysis should focus on the relationship between ancestral ontogeny and adult morphology in equids.