Organisms invest in a multitude of traits shaped by different selection pressures to maximise fitness. As resources are limited, extensive investment in costly traits can trade-off against investment in other traits that affect survival or reproduction. Given the huge costs of such investment, why are exaggerated traits so widespread in nature?
In this thesis, I studied the evolution and maintenance of costly exaggerated traits under positive allometry in Psammophilus dorsalis, a short-lived socially polygynous lizard. I investigated how costly exaggerated traits under different selection forces change with fine-scale varying environment, how they affect the use of signals and aggression in fights, and examined trait evolution by measuring inheritance, plasticity, and fitness consequences in a wild population of these lizards.
Changing environments can impose strong selection on traits, yet our knowledge of costly trait response across dynamic environments is limited as longitudinal studies across generations are rare. In the first chapter, I use a long-term 11-generation dataset from 2011 to 2021, to examine how fine-scale spatial and temporal variation in ecological and demographic conditions modify positive allometry in traits across sex and selection process. Positive allometry in male head width (under sexual selection) varied dramatically over generations and space. Limited rainfall, harsh temperatures, and greater competition promoted positive allometry in male head width. In stark contrast, positive allometry in female interlimb length (under fecundity selection) only weakly correlated with environmental conditions. Our study demonstrates that costly traits are sensitive to changing environments depending on the underlying selection pressure and sex.
Contest competition for acquiring resources – territories and mates - typically involves individuals using multiple morphological and behavioural traits. In the second chapter, I explored how male Psammophilus dorsalis balance investment in aggression – between long-term investment in body size and exaggerated weapons, and “labile” and costly aggressive behaviours during intra-sexual contests. Behavioural experiments simulating contests were conducted on males in the wild using 3D-printed lizard models of varying sizes across four breeding seasons (2016 to 2023). The results indicate that males use aggressive displays as honest indicators of competitive ability in Psammophilus dorsalis. Our results suggest that males use a combined suite of aggressive traits, behavioural and morphological, to signal the motivation to fight and the ability to win contests.
Costly exaggerated traits are hypothesized to be maintained through reproductive advantages that outweigh the survival costs. In the third chapter, we empirically investigated the mechanisms maintaining exaggerated traits under different selection processes in a natural population of Psammophilus dorsalis. To enable this study, I developed and optimised 6 microsatellite markers for P. dorsalis. Using two parent–offspring generation pairs (N = 325) sampled across four breeding seasons (2015–2016 and 2021–2022) from six sheetrocks, I quantified heritability, phenotypic plasticity, and selection gradients under varying environmental conditions.
I show that exaggerated traits under sexual selection in males are maintained through inheritance and fitness consequences, which varied with years and environmental conditions. In contrast, exaggerated traits under fecundity selection in females seem to show high environmental plasticity, low inheritance, and negligible fitness consequences. Furthermore, our predicted responses to selection based on heritability were less accurate when environments varied and shifted trait allometry, highlighting the importance of accounting for environmental effects when estimating trait evolution.
Overall, this thesis demonstrates that the maintenance of exaggerated traits is shaped by interactions among environmental variation, selection process, and sex. Our findings emphasise the importance of long-term, longitudinal studies across environment, sex, and selection, for understanding the ecology and evolution of exaggerated traits and predicting evolutionary responses in wild populations.