Ever since Darwin’s theory of sexual selection was first published, understanding the evolutionary mechanisms that lead to sex-specific traits and quantifying patterns therein has been a large focus of evolutionary ecologists. Sexual variation in phenotypes in form and function is ubiquitous in nature, and its extent varies widely, within closely related groups and even species. Understanding patterns and the circumstances under which sexually divergent phenotypes evolve requires an understanding of both the proximate and evolutionary mechanisms involved. While studies have primarily focused on sex-specific morphological traits such as size and secondary trait dimorphism, less attention has been given to the functional differences between the sexes. From an evolutionary perspective, sex-specific traits result from many factors that act differentially on each sex, modifying phenotypic and behavioural components that influence the capacity of fulfilling reproductive roles. Therefore, sexual selection alone does not define the magnitude of phenotypic divergence between the sexes, but it is rather the interplay between sexual and natural selection and their interactions with the environment. Intraspecific studies investigating associations between form and function of sex-specific traits provide a useful tool to understand the fundamental mechanisms that lead to the evolution of complex traits in males and females. The overall aim of this thesis was to gain a better understanding of the evolutionary mechanisms that drive patterns in sex-specific phenotypes, using a territorial gecko species (Hemidactylus frenatus) distributed over a large latitudinal cline.
The first aim of this thesis was to examine the potential trade-offs between performance functions associated with reproductive success and to test whether compensatory mechanisms may obscure the detection of such costs (Chapter 2). Fighting capacity and escape performance of male H. frenatus are likely to pose conflicting demands on the optimum phenotype for each task. Highly territorial and aggressive males may require greater investment in head size/strength but such an enhancement may reduce overall escape performance. To test this idea, the second aim (also in Chapter 2) was to determine the role of morphological and functional traits in determining the outcome of male-male combat (reproductive success) and prey-capture ability (survivorship). Among male geckos, I found that larger head size resulted in greater biting capacity, but at a cost of reduced spring performance. Females, however, showed no evidence of this trade-off. The sex specificity of this trade-off suggests that the sexes differ in their optimal strategies for dealing with the conflicting requirements of bite force and sprint speed. Unlike males, female H. frenatus had a positive association between hind-limb length and head size, suggesting that they have utilised a compensatory mechanism to alleviate the possible locomotor costs of larger head sizes. It appears that there is greater selection on traits that improve fighting ability (bite force) for males but it is viability traits (sprint speed) that appear to be of greater relative importance for females.
The third aim of this thesis was to investigate geographical patterns of sexual variation in morphological and performance across a wide latitudinal range of H. frenatus (Chapter 3). This chapter tested if (i) patterns of sexual variation across latitudes support Rensch’s rule, (ii) if abiotic and biotic environmental factors are correlated with patterns of sexual dimorphism, and, (iii) sexual variation in morphology reflect variation in functional performance. We found that body size and head shape in H. frenatus demonstrated a systematic relationship between the magnitude of sexual dimorphism with the latitude and population density. Males were consistently the larger sex and more divergent in head shape, probably due to the association of these traits with better fighting capacity. Of all traits tested, head shape and sprint speed were the only traits to demonstrate potential support for the allometry in patterns of Rensch’s rule with male head shape and sprint speed being more divergent than females. In general, we found that latitudinal patterns of intraspecific sex-specific phenotypes in H. frenatus are associated with environmental factors, both abiotic and biotic, that co-vary with geographic variation. Such dimorphism and divergence therein may be driven by both variations in the plasticity of phenotypes and sexual selection on both males and females and is trait dependent.
The fourth and final aim of this thesis was to examine the sex-specific variation in thermal sensitivity of performance across several geographical populations of H. frenatus along a latitudinal cline (Chapter 4). This was examined by determining if: (i) thermal sensitivity of functional performance traits differed between latitudinal populations, (ii) if thermal sensitivity of performance traits are sex-specific and, (iii) if the degree of sexual variation in thermal sensitivity diverged among populations. H. frenatus that experience the most variable thermal environments displayed the broadest thermal performance range, whereas the more stable lower latitudes appear to exhibit the greatest degree of divergence in their thermal sensitivity of performance. We also found significant differences between the sexes in whole-organism performance with males producing greater bite forces and faster sprint speeds than females, independent of body size. We found sex-specific thermal sensitivities in the tropical population, with female H. frenatus exhibiting a larger reduction in sprint performance at higher temperatures compared to males. Also sexes from the tropical population demonstrated divergence in their thermal sensitivity of activity at higher temperatures, although this was non-significant.
Taken together the results of these studies show that only by examining the underlying functional traits and the effects of environmental factors on both form and function that we can really gain a better understanding of the evolutionary mechanisms that drive sex-specific phenotypes.