Ecomorphology, microhabitat use, performance and reproductive output in tropical lygosomine lizards

Ecomorphology is the study of correlations between morphology and habitat(s) in organisms. If morphology is tightly correlated with habitat, then differences in morphology should directly affect fitness via their effect on performance within specific habitats. Despite the generality of this approach, clear correlations between habitat use, morphology, and performance have been established for few vertebrate groups. Furthermore, no study has examined whether correlations between habitat use and morphology may affect fitness via an effect on reproductive output. This thesis examines the relationships between microhabitat use, morphology, performance and reproductive output among scincid lizards from tropical north east Australia. My examination of microhabitat use, temporal activity and size for 21 skink species from five assemblages (Alligator Creek, Cairns, Chillagoe, Cooktown and Mt. Bartle Frere), revealed that species separated along two gradients of structural microhabitat use: one that ranged from large rocks to leaf litter, and a second that ranged from closed habitats (high in leaf litter, ground cover, undergrowth, proximity to vegetation and increased canopy cover) to open habitats (low in all these characteristics). Species used microhabitats non-randomly, with species from the same ecotype (arboreal, generalist, litter-dwelling, rock-using) clustering in multivariate ecological space. Despite considerable niche overlap, null-model comparisons revealed only one assemblage (Chillagoe) had greater niche overlap than expected by chance. Assemblages with more species occupied smaller niche space, indicating species packing, however, species with more diverse niches were less evenly packed. Most species overlapped in activity time and size, suggesting that structural microhabitat is the dominant axis decreasing competitive interactions, allowing coexistence within these assemblages of tropical skinks. Sexual differences in morphology were examined for 18 skink species that occupy a range of habitats. Phylogenetic analyses revealed that females from rocky environments evolved longer limbs and shorter abdomens compared to those from leaf litter. In males, use of rocky habitats was correlated with the evolution of a flatter, shorter body. The use of more open habitats was correlated with an evolutionary increase in limb length and a decrease in abdomen length in females, and an increase in limb length in males. Phylogenetic comparisons among generalist, leaflitter and rocky habitat species revealed that males from rocky habitats were flatter than generalist and leaf-litter species, with females less stockier than males. Selection for body flattening in females appears constrained, or weaker than for males, presumably due to the antagonistic effect of fecundity selection to maximize space for eggs. The more extreme flattening of male lizards from rocky habitats may assist locomotor performance, male-male contests or the use of refugia. Phylogenetic analyses of males from 18 species revealed a tight positive correlation between sprinting and climbing ability, and climbing and clinging ability. There was no trade-offs among these performance traits, such as that observed in studies of arboreal lizards. Morphologically derived species were better at sprinting, climbing and clinging, which are presumably sufficiently similar tasks for scincid lizards that no trade-offs were observed. Although biomechanical models predict that flatness should enhance climbing speed, there was no evidence that a flat body assisted in climbing in this study. Similarly, biomechanical models predict that long limbs should enhance jumping ability, but no such correlation was observed in my study. Five conceptual models of lizard locomotion relating to habitat use and morphology (limb length) were examined using 18 species of skink. Both differences and similarities between the sexes in the relationships between microhabitat use and performance were observed. Male and female skinks both responded to increased habitat openness by evolving greater sprint speeds. However, males in open habitats also had faster climbing speeds, and better clinging ability than those from closed habitats; enhanced clinging ability is likely beneficial for increased climbing speed, or correlated selection on these two traits. While these relationships were in the same direction, they were less robust or non-significant for females. Intersexual differences in performance resulting from natural selection for improved locomotor function in particular habitats may be eroded in females by sexual selection (e.g., for increased fecundity). Moreover, specialized leaf-litter dwelling species had poor performance at all performance traits examined, suggesting that these traits were not relevant to specialisation to a leaf litter habitat, or that selection on these traits is relaxed as there is more reliance on crypsis. Body flattening was negatively correlated with abdominal volume, such that flatter species had lower abdominal volumes. Abdominal volume was strongly correlated with reproductive output (RCM), and flatter species had lower reproductive output. Thus, body shape determines reproductive output by imposing a constraint on clutch mass. The tight correlation between abdominal shape and both RCM and habitat, suggests changes in body shape are adaptive and may have a functional role (e.g., using rock-crevices). Thus, adaptive changes in morphology can influence fitness without affecting performance. This study shows that for this group of Lygosomine lizards there is a clear evolutionary pathway between clutch mass and body shape, with body shape acting as a constraint on clutch mass and therefore, reproductive output. Structural microhabitat use provides the dominant axis separating and allowing coexistence among this group of diurnal tropical skinks in northeast Australia. Morphological adaptation in this group of tropical lizards to two habitat gradients and in response to three categories of habitat use has led to convergence in morphology and performance, however, sexual differences were detected. The increased use of rocky and/or open habitats by species has led to evolutionary increases in running speed, climbing speed and cling ability, with performance of females lagging behind that of males. These sexual differences in morphology appear to be the result of the antagonistic effects of natural selection for performance, offset by sexual selection that affects the sexes differently. Finally, adaptive changes in morphology and body shape in response to these habitats have led to changes in reproductive output.