When challenges dictate choices: Environmental risks alter lizard foraging and nutritional ecology

Abstract

Foraging is an indispensable task for all animals, providing energy for all fitness-relative activities. While foraging is often viewed as maximizing food intakes while minimizing time and energy costs, this food-centric foraging approach overlooks the fact that food is a complex mixture of multiple nutrients, each with distinct functional goals. In the wild, animals face various environmental challenges that can trigger behavioral and physiological responses, such as glucocorticoid driven increases in metabolism that can alter nutrient demands and shape animals’ dietary choices. Since, optimal nutrient balance is essential for all individuals to function efficiently, diet selection should occur at the level of nutrients and foraging decisions of animals should also be readily explicable at this nutritional level. In my thesis, I explored the effects of environmental risks, such as resource uncertainty, seasonal challenges, and predation risk on foraging decisions and nutritional requirements in the tropical lizard, Psammophilus dorsalis, studying how nutritional intakes change with environmentally relevant contexts and exploring various strategies employed to meet changing nutritional requirements. Reptiles exhibit various physiological adaptations for torpor and are therefore expected to combat energetic shortages in the wild. Hence, I first tested whether tropical lizards adjust their foraging choices in response to resource uncertainty risk using a choice experiment and altering their energetic states through manipulation of their starvation state and determined their foraging preference. I found that lizards avoided risk when satiated and took greater risks when starved, providing novel evidence for risk-sensitive foraging in a tropical reptile. I then explored how seasonal variation in glucocorticoids is linked to life-history stages of lizards and affect their dietary nutritional intakes and faecal excretion in the wild. I measured glucocorticoids, energy metabolites, dietary elemental intakes, and faecal elemental excretions (carbon and nitrogen) in wild lizards across multiple seasons representing different breeding stages. I found that stress-induced corticosterone levels fluctuated seasonally, but lizards maintained a consistent intake ratio (carbon:nitrogen), showing no shift in dietary elemental composition. However, glucocorticoids negatively correlated with faecal elemental composition, indicating post-ingestive retention of elements as a possible strategy to meet physiological demands. Expanding on the findings from previous chapters, I then explored post-ingestive elemental retention in response to physiological stress as an adaptive strategy to meet energetic demands during challenging conditions. In a lab experiment, I manipulated stress levels of lizards and continuously measured their ingested and egested elemental compositions, to calculate the elemental retention of carbon and nitrogen. I found that lizards from both the treatments exhibited comparable stress levels and ingested and eliminated similar amounts of carbon and nitrogen. Although the study did not find support for stress-induced post-ingestive elemental retention, possibly because the stress of captivity itself had masked the potential effects on retention, it highlighted the need to study such compensatory strategies that animals can employ to meet their stress-induced energetic demands. Lastly, in my final chapter, I explored prey dietary nutritional choices in response to predator risk testing two competing hypothesis – food safety trade-off and nutritional optimization. While the former hypothesis posits predators constrain foraging, the later states that physiological responses of prey drive prey towards energy-rich food resources. By manipulating ‘what to eat’ and ‘where to eat’, I studied their dietary choices and measured the macronutrient ingestion by lizards. The study found novel evidence that both the hypotheses govern dietary choices of prey faced with predation risk providing novel understanding of how predators shape the nutritional decisions of prey in the wild. Overall, my thesis highlights how animals integrate environmental cues with their physiological needs to guide foraging decisions and nutritional choices, drawing attention towards different foraging strategies animals employ to optimize their nutritional intakes in response to varying ecological challenges.