Abstract
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.
Highlights
Animals are not isometric; that is, their organs generally do not scale in a linear fashion with their bodies [1]: as they increase in body size, they tend to change the proportions of various body parts relative to the body as a whole [2]
To compare allometric relationships among breeds, paired measurements of body length (BL), withers height (WH) and head length (HL) were collected for the following extant breeds of equids: 5 horses, 6 ponys, 6 donkeys, and 7 wild species belonging to the genre Equus (Table 1)
The allometric slopes did differed between groups, both for WH to HL (p=0.017) as for BL to HL (p=0.038), whereas no significant differences were found among domestic breeds (p>0.05), for which there were homogeneous slopes (F>0.4, p>0.2), for both traits
Summary
Animals are not isometric; that is, their organs generally do not scale in a linear fashion with their bodies [1]: as they increase in body size, they tend to change the proportions of various body parts relative to the body as a whole [2]. The term ‘allometry’, in its broadest sense, designates the differences in proportions correlated with changes in absolute magnitude of the total organism or of the specific parts [3] Allometric differences, those among closely related species, often directly relate to performance requirements [2]. The skulls offer a morphological diversity, given its complexity in form (shape and size) and embryological origin In such cases, the significance of allometry could be investigated by examining how allometric variation impacts variables that may be important for organismal performance, such as those related to feeding [2]. We attempted to interpret the differences in relation to their courses of differentiation, evolution and breed improvement to make inferences regarding the evolved responses in ontogenetic allometry to natural and artificial selection
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