Temperature variability and thermal performance in ectotherms: acclimation, behaviour, and experimental considerations

Abstract

Background: Among the predictions of the effect of future climate change, the impact of thermal conditions at local levels on the performance of individuals and their acclimation capacities is key to understanding animals' responses to global warming. Woodlice (the terrestrial isopod Porcellio laevis) exhibit a readily observed behaviour that may reflect their acclimation capacities. When they find themselves on their back, they sometimes roll over (i.e. right themselves). Whether they do, and how fast they do it, are measures of their behaviour and performance that vary with ambient temperature.

Goal: Measure experimentally the effect of acclimation to environmental thermal variability on the rollover behaviour of Porcellio laevis.

Methods: Maintain woodlice in 24 degrees C laboratory environments. Each individual lived in one of three thermal treatments: constant temperature regime (delta = 0); a regime with alternating temperatures of 27 degrees C during daytime and 21 degrees C at night (delta = 3); and a regime with alternating temperatures of 32 degrees C during daytime and 16 degrees C at night (delta = 8). After 3 months, measure the rollover behaviour (percentage that rolled over) and rollover speed of the woodlice at 5, 15, 25, 32, and 38 degrees C. Rollover speed is the inverse of the time it took an individual to right itself.

Results: At 5 and 38 degrees C, most woodlice seemed to be in thermal coma and had very slow rollover speed. At 15, 25, and 32 degrees C, rollover speed was in the range 0.12-0.18 rollovers per second. At 15, 25, and 32 degrees C, 52% of individuals righted themselves in similar to 4 s. At these temperatures, most individuals either rolled quickly or remained inactive for the entire 10-min test period. When we examined rollover speed in the cooler (night) and warmer (daytime) periods, individuals subjected to the three regimes of temperature variation differed: woodlice acclimated to delta = 8 roll over very fast when temperature (T) is low and very slowly when T is high; woodlice acclimated to delta = 3 roll over fairly fast when T is low and somewhat slowly when T is high; and woodlice acclimated to delta = 0 roll over quite quickly when T is low and quite slowly when T is high. However, the higher and lower speeds at different regimes neutralized each other so that average rollover speed over the course of a whole day did not depend on the size of the daily fluctuation.

Conclusions: The impact of temperature variability must be taken into consideration. Behavioural responses that are seemingly costly in the very short term may be beneficial in the longer term.