Size dimorphism in the agamid Rankinia diemensis : patterns, adaptive significance &amp; evolutionary pathways
Creators: name: Stuart-Smith, Jemina
Publisher: name: University Of Tasmania
Year: 2023

description: Body size is one of the most important life history traits of an organism, with links to, and influences on, almost all areas of an organism's ecology and evolution. Size divergence between males and females of the same species, termed sexual size dimorphism (SSD), reflects three major processes acting on body size: sexual selection (intra- and inter-specific competition), natural selection (fecundity advantage), and selection related to niche divergence. Each of these selection pressures has the potential to magnify or reduce the degree of size difference between the sexes. This thesis examines SSD in an agamid lizard, Rankinia diemensis, which exhibits female-biased dimorphism within a taxonomic group that is typically dominated by male-biased SSD. This investigation of size dimorphism addresses the above ultimate processes as well as determining the potential proximate mechanisms (the physical process producing the size difference) between males and females. Assessment of overall size and other morphological traits (e.g. head, limb and tail sizes) revealed that although females have larger overall body size, other traits are proportionally longer in males, suggesting that several selection pressures are at work. Geographic comparison also revealed that these sizes are not static - larger individuals (overall size) were found in the northernmost population studied, and trait size also varied geographically, but the overall degree of size difference between sexes from each site did not vary. This suggests that the net selection pressures acting on geographically distinct populations are similar. It also indicates that a combination of genetic and environmental factors may influence size, but there are constraints on the degree of size divergence that can occur between the sexes. Analysis of size at hatching, growth trajectories and size asymptotes between the sexes revealed that males and females hatch at a similar size and grow at similar rates prior to maturity. Males and females begin to diverge in size (including overall size and trait sizes such as head, limb and tail measurements) at three to four years of age. Male growth rate asymptotically approaches zero at maturity, while females continue growing throughout life; however both sexes live to approximately the same age. It is therefore the combination of delayed maturity and post-maturity growth in females that allows them to attain larger size than males. Size-specific reproductive output exists in R. diemensis with a strong positive relationship between female body size and fecundity, but no other aspect of reproductive output in females is dependent on size (e.g. average egg mass, relative clutch mass). This suggests that larger size does not permit females to put more energy into individual eggs, it only allows them to increase the number of eggs they can lay - suggesting that fecundity selection is a major driving force behind SSD in R. diemensis. As males are the smaller sex, it suggests that there is no strong positive selection on male body size. This is despite such selection being common in other agamids, where size determines contest success with conspecifics. Competition experiments revealed typical agamid male-male antagonistic interactions, with large size linked to aggressive/dominant behaviour. In males this could be a function of trait size (not overall size), since limb and head movements are the primary forms of communicative display in agamids. It could also suggest that there are opposing selection pressures acting on males to mature smaller. This is also reinforced by the field study component of this .thesis, where the use of microhabitat differed between males and females, and is most likely linked to rival or courtship displays. Males used higher structural habitat, typical for male agamids, which engage in displays from these posts. Males also perched in more exposed areas than females. Mature females utilized sites with higher temperatures, analogous to previous research that has found that a female can have considerable influence on her offspring during egg development in utero. These sex-specific behaviours are likely to promote niche divergence between the sexes, potentially reducing intra-specific competition. Overall this thesis reveals that a complex interplay of different selection pressures acting on size is responsible for the observed SSD in R. diemensis. My findings demonstrate the, influence of morphological variation on key life history components - such as reproductive output, mating system and microhabitat use. Furthermore, this thesis emphasizes the importance of viewing SSD evolution in the context of not only why one sex is large, but also why the other is small, and further demonstrates the complexity of trait evolution.