To rise to temperature: Variation in temperature effects within and among populations

Temperature drives physiological changes on three timescales: acute,
acclimatory and evolutionary. Acutely, passive temperature effects often
dictate an expected two-fold increase in metabolic processes for every
10°C increase (Q10). Yet for acclimatory or evolutionary time scales,
selection often mitigates these acute effects. This selection also should
concomitantly reduce interindividual variation. However, the individual
variation in physiological trait thermal sensitivity is not well
characterized. Here we quantified physiological responses to a 16°C
temperature difference across nine thermally distinct Fundulus
heteroclitus populations. Traits included whole animal metabolism (WAM),
critical thermal maximum (CTmax), and substrate-specific cardiac
metabolism measured in approximately 350 individuals. These traits exhibit
high variation among both individuals and populations that depends on
acclimation temperature. Thermal sensitivity or Q10 variation is
unexpected and ranges from 0.6 to 5.4 for WAM. Thus, with a 16°C
temperature increase, some individuals have the same or lower metabolic
rates while others have metabolic rates almost seven-fold higher (Q10 =
5.4). Furthermore, a significant portion of this variation is related to
habitat temperature, such that warmer populations have a significantly
lower Q10 for WAM and CTmax than colder populations. These data support a
novel hypothesis: individual variation in thermal sensitivity reflects
different physiological strategies to respond to environmental temperature
variation and provides the potential for many different adaptive responses
to temperature.