Nongeographic variation in the striped mouse Rhabdomys dilectus chakae (Rodentia: Muridae)

Neves, Candice Nikita
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The morphological variation between populations is shaped by adaptive responses to prevailing environmental conditions and/or not adaptive stochastic effects. Within-population variation is mainly related to age and sexual dimorphism, as well as temporal and spatial variation in environmental conditions. The aim of this study was to investigate patterns of variation in the skull, mandibles, and dentition in a population of the African four-striped mouse Rhabdomys dilectus chakae. Geometric morphometrics was used to assess the variation related to allometry and age, sexual dimorphism, and the inter-annual variation between specimens collected in different years (1975, and 1994-1997). A review of the literature on the application of geometric morphometrics to rodent morphological variation was conducted and landmark morphometrics were concluded to be the most appropriate methods for the skull and the mandible analyses, with a separate analysis using landmarks and semi-landmarks to analyse the variation of mandibular curves, and outline morphometrics with an Elliptic Fourier analysis of the upper molars. Images of the skull (ventral and lateral view at 5x magnification), mandible (at 6.8x magnification), and the three left upper molars (at 10.5x magnification) were captured using a stereoscopic microscope. Landmarks were digitized on the skull and mandible images, with semi-landmarks digitized on mandible images, and outlines were digitized on molars. Multivariate analysis of variance was used to analyse shape and size variation due to age, sex, and year of sampling and a multivariate regression was used to analyse allometry. A PCA was used to visualize shape variation, and boxplots of log-centroid size to visualize size variation between age classes, sexes, and years of sampling. Shape variation was significantly predicted by age and year of sampling, while size was significantly predicted by age, sex, and year of sampling. Size significantly contributed to shape variation, although size alone did not appear to explain much of the variation present. Most age-related variation in shape was due to differences between the first three age classes and age class IV, while size increased with increased age (i.e. growth). Males were larger than females although no sexual shape dimorphism was evident between sexes. Typical of species with male-male competition, Rhabdomys dilectus chakae demonstrated sexual size dimorphism; larger males have greater mating opportunities. Specimens from 1994 and 1997 had more variable shapes and were consistently smaller than those collected in 1975, 1995 and 1996, except for dentition where these latter specimens were smaller than those collected in 1994 and 1997. Annual variation in morphology may be a plastic response to prevailing local climatic conditions (i.e. rainfall and temperature), Masters Dissertation Candice Neves iv resulting in annual variation in diet, affecting the skull and dentition. Further studies should consider more populations in the species to assess the generalizability of the findings, particularly annual variation, and to consider spatial variation across the distribution of a population.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2018
Neves, Candice Nikita, (2018) Non-geographic variation in the striped mouse, Rhabdomys dilectus chakae (Rodentia: Muridae), University of the Witwatersrand, Johannesburg,