3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Nongeographic variation in the striped mouse Rhabdomys dilectus chakae (Rodentia: Muridae)(2018) Neves, Candice NikitaThe 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.Item The behavioural ecology of bachelor male groups in the African striped mouse, Rhabdomys pumilio(2018) Kanyile, Sthandiwe NomthandazoMales in several animal species vary in traits which confer competitive ability. Younger and old, small and large males thus regularly make use of different behavioural tactics (i.e. alternative reproductive tactics, ARTs) in order to secure mates and increase their fitness. In the Succulent Karoo, males of the African striped mouse Rhabdomys pumilio adopt one of three main ARTs, i.e. philopatric, roaming, or territorial tactics; the tactic chosen is influenced by body mass. Additionally, the occurrence of bachelor groups (two or more males sharing a nest without any female) in striped mice has recently been observed. The present study was concerned with investigating the composition and function of these bachelor groups in striped mice, especially to assess whether they represent a fourth ART. For this, I used data collected from 2009 to 2016 to determine the season (breeding versus non-breeding) during which bachelor groups occurred and how they originated. At the start of the breeding season, I compared bachelor males with the known ARTs with regard to their scrotality, body mass, and age. I also determined the tactics of bachelor males before and after they were bachelors, and whether these tactic changes were associated to changes in body mass. My results indicate that bachelor groups are mainly formed by unrelated philopatric males which have dispersed from their natal groups. These groups most frequently occur in the breeding season, when population density is low to intermediate. Bachelor males occupy the intermediate position in the body mass spectrum in striped mice, being heavier than philopatrics but lighter than breeders, and do not differ in body mass from roamers. After the bachelor tactic, more males employed the roamer than the territorial breeding tactic. I hypothesise that the bachelor tactic is a “transitional tactic” which facilitates the change from a low fitness tactic (philopatric) to a higher fitness tactic (roaming or breeding) by allowing relatively small males to cooperate in social thermoregulation. These findings provide valuable insight on a phenomenon which has not been studied before in striped mice. Keywords: African striped mouse, alternative reproductive tactics, bachelor, behavioural plasticity, group-living, social flexibilityItem Behavioural adaptive variation in the striped mouse Rhabdomys(2017) Mackay, Megan KirstenUnder current and previous global climate change, environments are changing and have changed at a rapid rate. Species with the potential to undergo adaptive radiation are likely to survive environmental change. The genus Rhabdomys is widespread in southern Africa, occurring along the east-west rainfall gradient in South Africa. Rhabdomys may have undergone adaptive radiations in the past, which may have resulted in the current suite of species in various habitats of different aridity. Some Rhabdomys species also occur in sympatry in some locations in South Africa. The aim of my study was to investigate adaptive variation in Rhabdomys by studying the behaviour of 5 populations, representing 3 Rhabdomys species, across South Africa. Using selected taxa, my approach was, firstly, to describe variation in two traits, personality and spatial cogntion, well known for showing environmentally-linked (i.e. adaptive) variation. Secondly, I manipulated the development of exploratory and anxiety behaviour to assess the limits of the adaptive variation (i.e. test the nature of the reaction norm of the characters measured). I first established the taxon-level personality of 4 taxa (2 sympatric) in 5 standard behavioural tests. Generally, the semi-desert living R. pumilio was the boldest together, surprisingly, with R. d. dilectus occurring in grasslands of central South Africa, contradicting previously published results. Comparatively, R. bechuanae from central South Africa and R. dilectus from far north-eastern South Africa, also occurring in grasslands were less bold, even though R. bechuanae is sympatric with R. dilectus in central South Africa. My data indicate adaptive variation at the extreme populations and possibly character displacement in the sympatric populations. In the next chapter, I investigated whether early rearing environment shapes exploratory behaviour and anxiety responses of R. pumilio and R. bechuanae. I predicted that using an interspecies cross-fostering protocol would reveal a gene x environment interaction on behaviour, so that fostered offspring would display an intermediate behaviour phenotype compared to their non-fostered siblings. I showed that a novel rearing environment mostly did not influence the adult behaviour of cross-fostered inidividuals. This indicates genetic constraints on exploratory behaviour and anxiety responses. Next, I tested whether physical rearing environment shapes exploratory behaviour and anxiety responses. I reared semidesert R. pumilio, sympatric R. bechuanae and R. dilectus and allopatric R. bechuanae under either no cover or high cover for 2 generations. The taxa were mostly similar and altering the phyical housing condition did not alter behaviour, but there were small differences between the taxa in exploratory behaviour. In the final experimental chapter, I established whether the environment predicts the spatial cognition in semi-desert R. pumilio, sympatric R. bechuanae and R. dilectus and an allopatric population of R. dilectus from far north-eastern South Africa. The populations showed very similar performance in a modified Barnes maze, indicating a possible phylogenetic constraint on spatial cognition. Overall, my study suggests that there is adaptive variation in personality but not spatial cognition. In contrast to previous studies in the genus, alterations to the social and physical environments failed to separate out genetic and environmental effects (i.e. reaction norm) that would potentially provide the mechanisms for adaptive variation within and between species. The similarity in spatial cognition between taxa and similar responses to environmental modification indicate phylogenetic constraints on traits that were predicted to vary geographically.