No abstract

Temperature is an important factor determining distribution and abundance of organisms. Predicting the impact of warming climate on ectotherm populations requires information about species' thermal requirements, i.e. their so-called 'thermal niche'. The characterization of thermal niche remains a complicated task. We compared the applicability of two indirect approaches, based on reaction norm (aerobic scope curve) and optimality (preferred body temperature) concepts, for indirect estimation of thermal niche while using newts, Ichthyosaura alpestris, as a study system. If the two approaches are linked, then digesting newts should keep their body temperatures close to values maximizing aerobic scope for digestion. After feeding, newts maintained their body temperatures within a narrower range than did hungry individuals. The range of preferred body temperatures was well below the temperature maximizing aerobic scope for digestion. Optimal temperatures for factorial aerobic scope fell within the preferred body temperature range of digesting individuals. We conclude that digesting newts prefer body temperatures that are optimal for the maximum aerobic performance but relative to the maintenance costs. What might be termed the 'economic' thermoregulatory response explains the mismatch between thermal physiology and behaviour in this system.

Many organisms seasonally modify their standard metabolic rates (SMR). However, the diversity of cues triggering the acclimatization response remains little understood. We examined the influence of experimentally induced aquatic-to-terrestrial habitat shift on the thermal sensitivity of SMR in newts. Standard metabolic rates increased with temperature (13-23°C), although consistently lower in terrestrial than aquatic individuals. Motor activity during respirometry trials decreased with temperature at similar rates in both groups. We conclude that in newts, a habitat shift might represent an important modulator of the seasonal acclimatization response in SMR. Lowered SMR suggests the potential to reduce newt maintenance costs and depletion of caloric reserves during the activity-limited period on land.

Metabolic rates (MRs) vary consistently among individuals within a population, providing raw material for natural selection. Although individual energy demands may play an increasingly important role for ectotherm survival under warmer and more variable winter conditions, whether individual variation in MRs persists during overwintering is virtually unknown. Here, we repeatedly measured MR in wintering Alpine newts Ichthyosaura alpestris to (1) confirm the consistent individual variation in this trait; (2) test whether the individual differences in MR affect body mass loss during overwintering. The individual identity of newts explained 72% of variation in mass-and-activity-corrected MR. Newts with a high MR lost a higher proportion of their initial body mass than individuals with lower metabolic demands. We conclude that the consistent individual variation in MR during overwintering is an important predictor of spring body condition in newts. This provides a new perspective on intraindividual variation in MRs as a mediator of winter climate change on the dynamics of ectotherm populations.