Abstract
The need to vary body temperature to optimize physiological processes can lead to thermoregulatory behaviours, particularly in ectotherms. Despite some evidence of within-population phenotypic variation in thermal behaviour, the occurrence of alternative tactics of this behaviour is rarely explicitly considered when studying natural populations. The main objective of this study was to determine whether different thermal tactics exist among individuals of the same population. We studied the behavioural thermoregulation of 33 adult brook charr in a stratified lake using thermo-sensitive radio transmitters that measured hourly individual temperature over one month. The observed behavioural thermoregulatory patterns were consistent between years and suggest the existence of four tactics: two “warm” tactics with both crepuscular and finer periodicities, with or without a diel periodicity, and two “cool” tactics, with or without a diel periodicity. Telemetry data support the above findings by showing that the different tactics are associated with different patterns of diel horizontal movements. Taken together, our results show a clear spatio-temporal segregation of individuals displaying different tactics, suggesting a reduction of niche overlap. To our knowledge, this is the first study showing the presence of behavioural thermoregulatory tactics in a vertebrate.
Highlights
Temperature is recognized as one of the most important environmental factors controlling the energetics of ectotherms, affecting, among other things, their growth, reproduction, and distribution [1,2]
It is noteworthy that the periodicities in thermal behaviour identified by the selected principal coordinates of neighbour matrices (PCNM) at the population level were qualitatively similar between the two years, as shown by the scalograms (Figure 1)
The observed cycles were clearly not related to daily oscillations in the temperatures of the water column since the daily variation measured by thermographs (Figure S11) was one order of magnitude lower than those experienced by fish (Figures S7, S8, S9, S10)
Summary
Temperature is recognized as one of the most important environmental factors controlling the energetics of ectotherms, affecting, among other things, their growth, reproduction, and distribution [1,2]. Given the pervasiveness of temperature effects on these organisms, thermoregulation is key to their survival [3] Because they cannot rely on endogenous thermoregulatory mechanisms (e.g., thermogenesis) to control their body temperature, ectotherms mainly use behavioural thermoregulation—the active selection of a given temperature—to optimize their metabolic processes in heterogeneous thermal habitats [3]. Basking reptiles and fish seeking thermal refuges are examples of organisms relying on behavioural thermoregulation to improve their locomotor performance [4] or to avoid conditions close to their tolerance limits [5] Since processes such as growth or digestion can differ in their thermal optima [6,7], organisms face trade-offs in their behavioural thermoregulatory decisions. The need to vary body temperatures to optimize cyclic events (e.g., diurnal feeding and nocturnal digestion) could lead to cyclic thermoregulatory behaviours
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