Animals must consume nutrients in optimal amounts and ratios to maximize their fitness. However, most animals face various constraints to foraging optimally in their natural habitats. While studies conducted in the lab and mesic habitats suggest that animals can sense and meet their transient and long-term needs, we still have little understanding of the nutritional ecology of vertebrates in extreme environments. In this thesis, I attempt to fill this gap by examining the nutritional ecology of the desert-dwelling Indian spiny-tailed lizards Saara hardwickii under various ecological contexts.
In the first chapter, I adopt a global approach to understand whether variation in life-history traits across lizards can be explained by nutritional intakes. Lab based studies on multiple species suggest a strong link between nutrition and life-history traits. However, the results from my study suggest that these associations generally do not reflect in the relationship between nutrition and life-history at an inter-specific level. Absence of a significant relationship between nutrition and life-history at an evolutionary scale might indicate that nutritional responses are more sensitive to demands imposed at ecological timescales.
In the second chapter, I examine whether lizard diet is sensitive to specific nutritional requirements from key life-history events across seasons. For this, I quantified nutritional responses (nutrient consumption and retention) in Indian spiny-tailed lizards Saara hardwickii across four seasons in the Thar desert of northwest India. The results from this work show that S. hardwickii uses both behavioural diet choice and post-ingestive physiology to match seasonal nutritional needs by differentially consuming and retaining nutrients in an extreme environment.
In addition to the long-term demands of life-history traits, animal nutrition is also sensitive to more transient nutritional needs due to various ecological factors, such as predation risk. Lab based studies show that fear of predators can modulate nutritional responses via the physiological stress response. In the third chapter, I examine whether the risk of predation from a feral predator affects stress physiology, and consequently, nutritional responses in S. hardwickii in their natural habitat. Lizards in high-risk habitat adjust both intake and retention of carbon and nitrogen. The lack of physiological stress and changes in diet composition in this species hints to a significant role of behaviour, not physiology, in mitigating predation risk.
I test this in my final chapter by examining the mechanistic links between antipredator responses and their downstream costs on fitness in S. hardwickii. To this end, I quantified behavioural and physiological antipredator responses in S. hardwickii across habitats varying in predation risk and food resources. Using a structural equation modelling approach, I examine how the costs associated with these antipredator responses can result in varying fitness outcomes in heterogenous environments.
Together, this thesis integrates extensive field observations, lab experiments, modelling approaches and a global synthesis to understand the nutritional underpinnings of behavioural, physiological, and life-history trait variation. Understanding the nutritional ecology of these traits can provide mechanistic insights into species responses to various natural and anthropogenic changes in their environment.