Physiological responses to daily temperature variation and ultraviolet radiation in amphibian larvae

Abstract

Understanding how environmental variability influences species fitness is important for predicting species persistence in changing environments. Environments are highly variable and organisms have discrete physiological limits which determine the range of conditions they can tolerate. Daily thermal fluctuations (DTFs) impact the capacity of ectotherms to maintain performance and energetic demands due to thermodynamic effects on physiological rates. However, organisms can also flexibly alter their phenotype in order to maintain fitness in variable environments. Ectotherms experiencing DTFs would benefit from mechanisms which reduce the thermal sensitivity of physiological traits and buffer energetic costs. Furthermore, energetic consequences of DTFs may be exacerbated by the presence of additional stressors. Exposure to ultraviolet radiation (UVR) can increase energy requirements due to the repair of UVR induced damage. The costs of UVR exposure may cause energy trade-offs which exacerbate consequences of DTFs, and reduce the capacity to flexibly alter phenotypes. Importantly, temperature and UVR are also linked in their effects on cellular function as exposure to temperature extremes and UVR causes cellular damage that induces common protective mechanisms. Interactive effects of these environmental drivers on mechanistic traits may explain whole animal responses. Understanding what drives responses to temperature variation and the interactive effects between environmental stressors will broaden our understanding of how animals respond to environmental variation. The overarching aim of this thesis was to investigate physiological mechanisms underlying responses to DTFs and exposure to UVR in amphibian larvae. The capacity to flexibly alter the thermal sensitivity of traits in response to DTFs may depend on the degree of environmental DTF species experience. The first specific aim of this thesis was to investigate whether plasticity in response to DTFs is influenced by the thermal variability of a species' habitat (Chapter 2). Tadpoles of three related species whose habitats vary in the degree of DTF were raised in stable temperatures and conditions in which temperatures fluctuated daily. Plasticity of upper thermal limits and the thermal sensitivity of swimming performance, resting metabolic rate and metabolic enzyme activity were examined. Environmental variation in species habitats did not predict the capacity to alter physiological rate processes. Tadpoles were unable to reduce the thermal sensitivity of physiology traits, which led to smaller body sizes and altered the length of development in a species specific fashion. DTFs increased upper thermal limits which may ii buffer tadpoles from the lethal consequences of temperature extremes. The effects of DTFs on growth and development however, will likely negatively impact individual fitness. The second specific aim of this thesis was to investigate whether UVR exposure alters the physiological response to DTFs (Chapter 3). Tadpoles were raised in stable or dai...