Changes in Ontogenetic Allometry and their Role in the Emergence of Cranial Morphology in Fossorial Spiny Rats (Echimyidae, Hystricomorpha, Rodentia)

All evolutionary modifications of morphology in adult animals presuppose occurrence of changes in developmental programming. While some developmental changes affect rates of trait growth during the entire ontogeny, other developmental changes modify timing and growth rates during limited stages, usually in early development. Identifying which kind of these alterations are more frequent during evolution is crucial for understanding processes influencing the emergence of phenotypic diversity and specializations. Here, we used an allometric approach to assess the relative impact of these two kinds of ontogenetic alterations in the emergence of specialized skull morphology in fossorial spiny rats, comparing them with closely related, more generalist, terrestrial species. Univariate and multivariate analyses of adult shape consistently showed that fossorial spiny rats remarkably differed from terrestrial species, mainly by showing shorter and lower rostrum and more expanded auditory bullae, a set of traits usually considered specializations for life underground. Slopes and elevations of allometric trajectories of cranial traits were estimated for each species and compared with Analysis of Covariance, Likelihood-ratio tests, and Analysis of Variance based on Burnaby-corrected data. These tests showed that changes in allometric elevations were more recurrent during evolution and more congruent with the change in adult morphology than change in allometric slopes. These findings indicated that developmental changes modifying timing and growth rates during limited stages of early development were more frequent than alterations of trait covariation patterns along the entire ontogeny. This kind of developmental change accounts for a large effect on diversification of adult morphology and emergence of burrowing specializations in spiny rats.