Female-biased sexual size dimorphism and mate choice in Tarrkawarra (Notomys alexis)

Female-biased sexual size dimorphism (FSSD) is an unusual phenomenon in mammals, which are typically male-biased sexually size dimorphic (SSD). Mammalian FSSD is also poorly accounted for by theory and, in particular, by sexual selection theory (Chapter 1). In this thesis I demonstrate that tarrkawarra (Notomys alexis or spinifex hopping mouse), is an excellent model system with which to investigate mammalian FSSD for the following reasons: Firstly, captive tarrkawarra and some field populations demonstrate a high level of FSSD relative to other FSSD mammals. Secondly, sexual size parity in other tarrkawarra populations, implies intra-specific variation in the degree of FSSD expressed. Studies of intra-specific variation have some advantages over inter-specific studies. Thirdly, tarrkawarra are small, easy to maintain in captivity and common in their natural habitat (the Australian arid zone). Based on the field data published, I determined that intra-specific variation in the degree of FSSD tracks the environmental fluctuations that increase tarrkawarra populations 40 to 100-fold between long ‘bust’ and short ‘boom’ periods (Chapter 2). Spatial distribution of resources and potential mates is usually a strong determinant of optimal mating system and flexibility in tarrkawarra social organisation therefore seems likely. Flexibility would also explain why attempts to categorise the mating system have never satisfactorily accounted for all observations of the animals’ reproductive physiology and behaviour. To test the flexibility of FSSD in tarrkawarra, I assessed developmental and environmental effects on the degree of FSSD in a captive tarrkawarra population. I found evidence for divergent male and female growth from birth, mitigated by both population density (Chapter 2) and by neonatal litter composition (Chapter 3). Specifically, litter sex-ratio effects on female growth and reproductive success suggest sex differentiated maternal investment. Litters sired by larger males were also found to have a higher sex ratio, suggesting that, despite low inter-male competition, sexual selection may still be important in this species via mating preferences. Studies of first female (Chapter 4), then male mate choice (Chapter 5) support there being a role for sexual selection in the mating system of tarrkawarra. Both sexes demonstrated large-partner preferences, although male preferences reflected their own sexual experience and phenotype more than female quality. Consequently, these results cannot explain FSSD in tarrkawarra by themselves. Whilst there is now evidence of fecundity and mate choice selection for larger female size, selection on smaller male size has yet to be adequately described.