Swimming with Predators and Pesticides: How Environmental Stressors Affect the Thermal Physiology of Tadpoles

To forecast biological responses to changing environments, we need to understand how a species's physiology varies through space and time and assess how changes in physiological function due to environmental changes may interact with phenotypic changes caused by other types of environmental variation. Amphibian larvae are well known for expressing environmentally induced phenotypes, but relatively little is known about how these responses might interact with changing temperatures and their thermal physiology. To address this question, we studied the thermal physiology of grey treefrog tadpoles (Hyla versicolor) by determining whether exposures to predator cues and an herbicide (Roundup) can alter their critical maximum temperature (CT(max)) and their swimming speed across a range of temperatures, which provides estimates of optimal temperature (T(opt)) for swimming speed and the shape of the thermal performance curve (TPC). We discovered that predator cues induced a 0.4°C higher CT(max) value, whereas the herbicide had no effect. Tadpoles exposed to predator cues or the herbicide swam faster than control tadpoles and the increase in burst speed was higher near T(opt). In regard to the shape of the TPC, exposure to predator cues increased T(opt) by 1.5°C, while exposure to the herbicide marginally lowered T(opt) by 0.4°C. Combining predator cues and the herbicide produced an intermediate T(opt) that was 0.5°C higher than the control. To our knowledge this is the first study to demonstrate a predator altering the thermal physiology of amphibian larvae (prey) by increasing CT(max), increasing the optimum temperature, and producing changes in the thermal performance curves. Furthermore, these plastic responses of CT(max) and TPC to different inducing environments should be considered when forecasting biological responses to global warming.