Daily time constraints limit behavioural capacity to cope with thermally increased metabolic demands

Abstract

Increased environmental temperatures result in greater energy demands for ectotherms, however, it is currently not clear if these energy demands can effectively be met by increased foraging effort. Here, we tested the temperature dependence of foraging effort and metabolic rate in an aquatic ectotherm across its entire natural thermal range. We developed a novel hidden Markov model to detect behavioural states in long-term body acceleration data collected from free-ranging bull sharks between 19 and 33 {degrees}C, and found that increasing temperature altered both the timing and extent of foraging effort. Our data revealed asymmetrical increases of metabolic demands and foraging effort; standard metabolic rates increased exponentially with temperature, but foraging effort increased logarithmically. The observed decoupling of foraging effort and energy demand suggests individuals face increased energy deficits at higher temperatures, confirmed by concomitant reductions in body condition measured in this population with increasing temperatures. We suggest that alterations in the well-established trade-offs between foraging and predation risk coupled with time constraints imposed by high temperatures limit the capacity of animals to cope with environmental temperatures well below critical temperatures.