Abstract:
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 or
together, and for selecting adult specimens for subsequent data recording and analysis. However, such studies have
largely 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 morphometric
analyses are considered to offer a more appropriate tool for assessing shape-related morphometric variation. In this
study, we used geometric cranial morphometric analysis to assess the nature and extent of sexual dimorphism and age
variation within the Tete veld rat, Aethomys ineptus (Thomas and Wroughton, 1908) from southern Africa and the
African Nile rat, Arvicanthis niloticus (Desmarest, 1822) from Sudan. The results obtained were in turn compared with
previously published results based on independent geometric and traditional cranial morphometric data from the same
sampled populations examined in the present study. While our geometric morphometric results detected statistically
significant sexual dimorphism in cranial shape within Ar. niloticus only, previously published results based on
traditional morphometric data failed to detect significant sexual dimorphism within this species. However, similar to
previously published traditional morphometric data, our geometric morphometric results detected statistically
significant age-related variation in cranial shape and size within both Ae. ineptus and Ar. niloticus, with individuals of
age classes 5 and 6 being considered to represent adult specimens. Our results highlight the importance of carefully
evaluating 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 in
the interpretation of geographic and evolutionary processes that may be responsible for morphological differences at
both the inter- and intra-specific levels.