Geometric craniometric analysis of sexual dimorphism and ontogeneticvariation: A case study based on two geographically disparate species, Aethomys ineptus from southern Africa and Arvicanthis niloticus from Sudan (Rodentia: Muridae)

摘要

Non-geographic morphometric variation, particularly at the level of sexual dimorphism and ontogenetic (agerelated)variation, has been documented in rodents, and useful for establishing whether to analyse sexes separately ortogether, and for selecting adult specimens for subsequent data recording and analysis. However, such studies havelargely been based on traditional morphometric analyses of linear measurements that mainly focus on overall size,rather than shape-related morphometric variation. Unit-free, landmark/outline-based geometric morphometricanalyses are considered to offer a more appropriate tool for assessing shape-related morphometric variation. In thisstudy, we used geometric cranial morphometric analysis to assess the nature and extent of sexual dimorphism and agevariation within the Tete veld rat, Aethomys ineptus (Thomas and Wroughton, 1908) from southern Africa and theAfrican Nile rat, Arvicanthis niloticus (Desmarest, 1822) from Sudan. The results obtained were in turn compared withpreviously published results based on independent geometric and traditional cranial morphometric data from the samesampled populations examined in the present study. While our geometric morphometric results detected statisticallysignificant sexual dimorphism in cranial shape within Ar. niloticus only, previously published results based ontraditional morphometric data failed to detect significant sexual dimorphism within this species. However, similar topreviously published traditional morphometric data, our geometric morphometric results detected statisticallysignificant age-related variation in cranial shape and size within both Ae. ineptus and Ar. niloticus, with individuals ofage classes 5 and 6 being considered to represent adult specimens. Our results highlight the importance of carefullyevaluating both size- and shape-related non-geographic morphometric variation prior to the analysis of geographicvariation and the delineation of species. Erroneous conclusions of non-geographic variation may have implications inthe interpretation of geographic and evolutionary processes that may be responsible for morphological differences atboth the inter- and intra-specific levels.