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Competition and mate choice are fundamentally important components of social systems. We investigated intra-sexual competition and inter-sexual competition (mate choice) in Rankinia diemensis, an agamid lizard with female-biased size dimorphism. We examined intra-sexual interactions during contests and mate choice in relation to body size for both males and females. In male-male competition trials, proportions of two display types differed depending on body size, with more tail flicks produced by bigger males, and more hand-waves displayed by smaller males. These behaviours hold particular biological significance for agamid lizards; tail-flicks convey aggressiveness and, therefore, dominance, while hand-waves often denote submissiveness. In female-female competition trials, a greater difference in body size between the two conspecifics resulted in the larger female directing more pushes towards the smaller female. This female competition may be important in the social system and could be involved in resource defence. We found no indication of size-based mate choice for males or females. This suggests mate preferences may not be based on body size in this species. This may be linked to female-biased size dimorphism in this species, but it also supports previous studies that have failed to demonstrate female choice in reptiles.

Fecundity selection is one of the most influential underlying driving forces responsible for body size differences between the sexes of a species. Reproductive output is one of the most important aspects of an animal's life-history strategy, and any trait that acts to improve this will be under strong selection. Body size is one potential trait that can influence fecundity and when a species exhibits female-biased size dimorphism, fecundity provides an ideal starting point for understanding why dimorphism in body size exists. Female-biased sexual size dimorphism is uncommon in vertebrates, including lizards. To explore the relationship between female-biased size dimorphism and fecundity, we examined maternal size and clutch data collected over four years from a temperate-zone agamid, Rankinia (Tympanocryptis) diemensis. We measured the following descriptors of reproductive output: clutch size and mass, relative clutch mass (RCM), average egg mass and offspring size. We found a positive relationship between maternal size and clutch size and mass, but no relationship between maternal size and RCM, average egg mass or hatchling size, demonstrating that the relative reproductive output is not influenced by female size, and that the only way to increase reproductive output is for the female to attain a greater body size. There exists an overall strong relationship between maternal body size and fecundity, thereby providing a potential explanation as to why female size is under selection in this species.

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout - native fish interactions has not been experimentally evaluated. 2. An in-lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat-dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat-dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout - native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.

The habitat used by animals plays an important role in their interactions with predators and prey. By using complex habitats such as areas of dense macrophyte cover in response to elevated predation risk, small fishes may reduce their foraging success. Because the threat of predation by introduced brown trout increases the use of complex habitats by the threatened Galaxias auratus (Johnston), we experimentally examined its foraging in different habitats to estimate indirect impacts of brown trout presence. The lakes in which G. auratus lives have recently become more turbid, so the experiment was also conducted under different turbidity levels. Laboratory feeding trials in which planktonic and epibenthic prey were simultaneously offered to G. auratus in the presence or absence of artificial macrophytes and at three turbidity levels (0, 50 and 100 NTU) revealed that its overall foraging success was unaffected by habitat complexity; however, in trials with artificial macrophytes, G. auratus consumed a greater proportion of planktonic prey than in the absence of artificial macrophytes. Neither overall foraging success nor prey selection by G. auratus was affected by high turbidity, indicating that water clarity does not appear to directly negatively impact its feeding. The switch in prey types would probably not be detrimental to G. auratus in the long term, and thus it appears that there is no substantial feeding cost associated with its increased use of complex habitats. It could, however, affect lower trophic levels in the lakes to which it is endemic.

Fecundity selection is one of the most influential underlying driving forces responsible for body size differences between the sexes of a species. Reproductive output is one of the most important aspects of an animal's life-history strategy, and any trait that acts to improve this will be under strong selection. Body size is one potential trait that can influence fecundity and when a species exhibits female-biased size dimorphism, fecundity provides an ideal starting point for understanding why dimorphism in body size exists. Female-biased sexual size dimorphism is uncommon in vertebrates, including lizards. To explore the relationship between female-biased size dimorphism and fecundity, we examined maternal size and clutch data collected over four years from a temperate-zone agamid, Rankinia (Tympanocryptis) diemensis. We measured the following descriptors of reproductive output: clutch size and mass, relative clutch mass (RCM), average egg mass and offspring size. We found a positive relationship between maternal size and clutch size and mass, but no relationship between maternal size and RCM, average egg mass or hatchling size, demonstrating that the relative reproductive output is not influenced by female size, and that the only way to increase reproductive output is for the female to attain a greater body size. There exists an overall strong relationship between maternal body size and fecundity, thereby providing a potential explanation as to why female size is under selection in this species.

Competition and mate choice are fundamentally important components of social systems. We investigated intra-sexual competition and inter-sexual competition (mate choice) in Rankinia diemensis, an agamid lizard with female-biased size dimorphism. We examined intra-sexual interactions during contests and mate choice in relation to body size for both males and females. In male-male competition trials, proportions of two display types differed depending on body size, with more tail flicks produced by bigger males, and more hand-waves displayed by smaller males. These behaviours hold particular biological significance for agamid lizards; tail-flicks convey aggressiveness and, therefore, dominance, while hand-waves often denote submissiveness. In female-female competition trials, a greater difference in body size between the two conspecifics resulted in the larger female directing more pushes towards the smaller female. This female competition may be important in the social system and could be involved in resource defence. We found no indication of size-based mate choice for males or females. This suggests mate preferences may not be based on body size in this species. This may be linked to female-biased size dimorphism in this species, but it also supports previous studies that have failed to demonstrate female choice in reptiles.