Estimating the adaptive potential of critical thermal limits: Methodological problems and evolutionary implications

1.Current studies indicate that estimates of thermal tolerance limits in ectotherms depend on the experimental protocol used, with slower and presumably more ecologically relevant rates of warming negatively affecting the upper thermal limits (CTmax). Recent empirical evidence also gives credence to earlier speculations suggesting that estimates of heritability could drop with slower heating rates. 2.Using published data from the fruit fly Drosophila melanogaster, we show that empirical patterns can be explained if flies' physical condition decreases with experimental time in thermal tolerance assays. This problem could even overshadow potential benefits of thermal acclimation, also suggesting that a drop in CTmax with slower heating rates does not necessarily rule out beneficial acclimatory responses. 3.Numerical results from a simple illustrative model show that no clear conclusions can be obtained on how the phenotypic variance in CTmax will be affected with different rates of thermal change. Conversely, the genetic variance and estimated heritabilities are expected to drop with slower heating rates, hence ramping rates in experiments aiming to study the evolutionary potential of thermal tolerance to respond to global warming should be as fast as possible (within the range in which measurement accuracy and physical condition are not affected). 4.Measurements under ecologically realistic warming rates should also consider the impact of other physiological and behavioural strategies that might partly compensate the negative effects of slow heating rates. However, there are situations in which slow heating rates closely mimic natural conditions, as those encountered by some aquatic ectotherms. These heating rates may be an issue of major concern in these species, given its negative impact on CTmax and its adaptive potential. © 2010 The Authors. Functional Ecology © 2010 British Ecological Society.Peer Reviewe

Supercooling point frequency distributions in Collembola are affected by moulting

1. Many arthropods depress the freezing point of their body fluids (supercool) to avoid freezing at subzero temperatures. This is normally a seasonal response and is achieved by the production of specific biomolecules including cryoprotectants, a cessation in feeding, and the removal or masking of ice-nucleating material from their bodies. 2. In springtails, the mid-gut is shed during moulting which results in the complete evacuation of the gut and a concomitant reduction in the supercooling point (SCP). We determined whether this non-adaptive explanation could account for the variability observed in the SCP of summer-acclimatized springtails. 3. Moult preparation resulted in a highly significant reduction in the SCP. Feeding after moulting restored the SCP to previous high levels. 4. Significant differences in SCP between springtails sampled from vegetation and the soil surface, on different days, and at different sites on the same day were also documented, demonstrating that not all variation in SCP is environmentally induced. 5. Investigations of the responses of the SCP to environmental variation in springtails and other arthropods should take into account the effects of moulting before solely adaptive conclusions are drawn