Seasonality provokes a shift of thermal preferences in a temperate lizard, but altitude does not (original) (raw)
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Functional Ecology, 2004
1We studied seasonal changes in the thermoregulatory behaviour of the lacertid lizard Psammodroums algirus in a Mediterranean evergreen forest. Body temperatures (Tb), environmental operative temperatures (Te) and upper and lower limits of the selected thermal range (Tsel) were lower in May than in July.2On average, mean deviations of Tb from Tsel (0·7 °C in both seasons) were much smaller than those of Te (8·3 °C in both seasons). Thus both the accuracy (average difference between Tb and Tsel) and effectiveness (the extent to which Tb are closer than Te to Tsel) of thermoregulation were high, and similar in both seasons.3However the thermoregulatory contribution of two distinct behavioural mechanisms varied markedly between seasons. Daily activity contributed significantly to the regulation of Tb in May (when a population of Te thermometers matching lizard activity patterns would be, on average, 1·0 °C closer to Tsel than were randomly available Te), but not in July (when such a population would be only 0·2 °C closer to Tsel than were randomly available Te). The selection of sun–shade patches, the contribution of which was larger than that of daily activity in both seasons, was more important in July (when it produced a distribution of Te that would be, on average, 3·1 °C closer to Tsel than were randomly distributed Te) than in May (when a population of thermometers matching the lizards’ pattern of exposure to sunlight would be 1·3 °C closer to Tsel than were randomly available Te).4These changes are discussed in the light of seasonal differences in the daily patterns of among-patch variation in Te. In spring, lizard activity was low in the early morning because even the selection of sunlit patches was of limited utility to attain Tb within Tsel; in summer, lizards could remain active at midday, despite low overall thermal suitability, by selecting shaded patches. Thus the contribution of patch selection to thermoregulation was important in the early basking period of both seasons, and at summer midday hours.5Our data suggest that shuttling between sun and shade, rather than selecting sun or shade, may be an additional mechanism of behavioural thermoregulation, the importance of which would be greatest at times of day when lizards use patches at random (e.g. spring midday hours), and that their mean Tb is closer to the grand mean of full sun and full shade Te than to the mean equilibrium Te within any type of patch.We studied seasonal changes in the thermoregulatory behaviour of the lacertid lizard Psammodroums algirus in a Mediterranean evergreen forest. Body temperatures (Tb), environmental operative temperatures (Te) and upper and lower limits of the selected thermal range (Tsel) were lower in May than in July.On average, mean deviations of Tb from Tsel (0·7 °C in both seasons) were much smaller than those of Te (8·3 °C in both seasons). Thus both the accuracy (average difference between Tb and Tsel) and effectiveness (the extent to which Tb are closer than Te to Tsel) of thermoregulation were high, and similar in both seasons.However the thermoregulatory contribution of two distinct behavioural mechanisms varied markedly between seasons. Daily activity contributed significantly to the regulation of Tb in May (when a population of Te thermometers matching lizard activity patterns would be, on average, 1·0 °C closer to Tsel than were randomly available Te), but not in July (when such a population would be only 0·2 °C closer to Tsel than were randomly available Te). The selection of sun–shade patches, the contribution of which was larger than that of daily activity in both seasons, was more important in July (when it produced a distribution of Te that would be, on average, 3·1 °C closer to Tsel than were randomly distributed Te) than in May (when a population of thermometers matching the lizards’ pattern of exposure to sunlight would be 1·3 °C closer to Tsel than were randomly available Te).These changes are discussed in the light of seasonal differences in the daily patterns of among-patch variation in Te. In spring, lizard activity was low in the early morning because even the selection of sunlit patches was of limited utility to attain Tb within Tsel; in summer, lizards could remain active at midday, despite low overall thermal suitability, by selecting shaded patches. Thus the contribution of patch selection to thermoregulation was important in the early basking period of both seasons, and at summer midday hours.Our data suggest that shuttling between sun and shade, rather than selecting sun or shade, may be an additional mechanism of behavioural thermoregulation, the importance of which would be greatest at times of day when lizards use patches at random (e.g. spring midday hours), and that their mean Tb is closer to the grand mean of full sun and full shade Te than to the mean equilibrium Te within any type of patch.
As climate change marches on, rapidly rising temperatures shatter records every year, presenting ever-growing physiological challenges to organisms worldwide. Ectotherms rely on behavioral and physiological plasticity to contend with environmental fluctuations. Nonetheless, our understanding of thermal plasticity has been largely limited to laboratory settings. Here, we test whether aspects of thermal physiology respond to interannual shifts in thermal environment in a natural population of Phrynosoma orbiculare, a montane horned lizard, from Hidalgo, Mexico. At our field site, 2019 was markedly warmer than the year that preceded it. We detected population-level increases in three key thermal physiological traits: preferred temperature, the critical thermal minimum, and the critical thermal maximum. Thus, thermal phenotypes appear to shift in tandem in response to environmental fluctuations. A subset of individuals were resampled across years, allowing insight into plastic shifts within an organism's lifetime. We detected parallel increases in these lizards for the preferred temperature and the critical thermal minimum, but not for the critical thermal maximum. Our results support a growing body of literature indicating that preferred conditions and cold tolerance can be highly labile over the course of an organism's lifetime, whereas hardening over shorter time periods is more common for heat tolerance. Given that heat tolerance increased at the population-level, but not in resampled individuals, it is possible that rapid evolution occurred due to temperature increases. In short, physiological shifts can be observed in natural populations over relatively short timespans, and these shifts might reflect a combination of evolutionary and acclimatory responses. K E Y W O R D S body temperature, critical thermal limits, phenotypic plasticity, physiological acclimatization, preferred body temperature, thermal physiology