3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The behavioural and neuroendocrinological mechanisms affecting group formation in the African striped mouse, Rhabdomys(2018) Hartman, Kirsty-JaneSocial systems vary within and between species, resulting in some populations living in complex social groups while others adopt a solitary lifestyle. Social systems are ultimately emergent properties of the interactions and relationships between individuals within a population. Social flexibility (a form of phenotypic plasticity) describes hormonally mediated reversible switches in social tactic in response to prevailing environmental conditions. I investigated the mechanisms underlying social interactions, and ultimately social structure in the striped mouse genus, Rhabdomys. It is widespread in southern Africa, occurring along an aridity gradient, which influences the social structure of constituent species. A desert species, R. pumilio, switches from group to solitary-living, and I asked whether a solitary-living grassland species, R. dilectus, could form stable groups. Four experiments were undertaken. 1) To examine the behaviours underlying group formation initiation, age-matched triads of unrelated, sub-adult conspecific female grassland and desert striped mice were formed in captivity. Nest-cohabitation occurred in all desert and 90% of grassland triads, yet desert triads were more aggressive than grassland triads on initial meeting. Both species spent more time interacting amicably as time in triads progressed. 2) Measurements were taken of circulating corticosterone and testosterone concentrations, and 3) neuropeptide arginine vasopressin (AVP) and oxytocin concentrations in the brain of triads and singly-housed controls. Grassland females in triads had greater corticosterone and AVP concentrations than desert female triads, but levels did not differ between triads and solitary-housed controls of both species; testosterone and oxytocin were unaffected. 4) I experimentally injected females of both species sequentially with a saline control, a low and a high dose of AVP, and measured their social motivation (i.e. propensity to spend time with a conspecific) and recognition of familiar unrelated conspecific females. Exogenously administered AVP enhanced the motivation to be social, by increasing the approaches toward the conspecific by focal females in the high AVP treatment in grassland mice and the low AVP treatment in desert mice. Neither species displayed a preference for an unfamiliar conspecific, but amicability toward familiar females was greater in grassland mice, but decreased in desert mice, under high AVP. Solitary grassland striped mice displayed tolerance of conspecifics, underpinned by different inherent hormonal control. My study has demonstrated that the naturally solitary R. dilectus can form similar social groupings to that of the facultatively social R. pumilio. Switching between social tactics might have potential survival benefits in unpredictably changing environments predicted for southern Africa.Item 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.