Lizard thermoregulation revisited after two decades of global warming

José Díaz , Raúl Izquierdo-Santiago, Alejandro Llanos-Garrido

This is a plain language summary of a Functional Ecology research article which can be found here.

Global warming should have a deep impact on thermoregulation by ectothermic animals such as lizards, because they rely on behavior to select the microhabitats that allow them to keep their body temperature within their preferred -and physiologically optimal- range of temperatures. However, there is a paucity of studies that use the same methods, after a long enough period of environmental warming, to evaluate how much the body temperatures and regulatory behaviors of a given population of lizards have been affected by climate change.

We chose a population of Psammodromus algirus lizards inhabiting a well-conserved temperate open forest, and we compared several thermoregulation parameters between the summers of 1997 and 2017:

1. Body temperatures (T_bs) of field-active lizards,
2. Environmental operative temperatures, which are the temperatures ‘available’ to lizards and can be estimated using randomly distributed physical models that integrate environmental heat loads (air and substrate temperature, solar radiation, wind speed, etc.) in the same way as lizards do,
3. The extent to which both T_bs and operative temperatures deviate from the summer ‘target’ range, which for this species is 32.8-36.9 ºC (the target range [T_sel] is measured in a laboratory thermal gradient where lizards can select their T_bs with no physical or biotic restrictions); and
4. The selection (use as opposed to availability) of sunlit and shaded patches along the day, which can be estimated by comparing the distributions of lizards (use) and randomly positioned physical models (availability).

A male Psammodromus algirus lizard perching in the sun. © Antón David Pérez-Rodríguez.

Both T_bs and environmental operative temperatures increased over the 20-years period examined; in 2017, 52% of field T_bs (N = 65) were above T_sel, in sharp contrast with the T_bs selected by lizards on 2-4 August 2017 in a laboratory thermal gradient  where only one of 43 T_bs was above T_sel. Also, the selection of shaded patches in the field (87% of lizards in full shade, against less than 1% in full sun) was more intense than in 1997. In spite of this, both T_bs and their average distance to T_sel were larger in 2017 (when only 8% of all available operative temperatures were within or below T_sel) than in 1997 (when most shaded patches offered favourable thermal opportunities, with plenty of operative temperatures within or below T_sel).

Thus, our data suggest that the low availability of operative temperatures within or below T_sel, at least on mid-summer days, is forcing lizards to reach T_bs above their preferred range. This implies that the effects of global warming are already hindering the ability of reptiles to buffer environmental change by behavioral means (although lizards could compensate these detrimental effects by being active earlier in the year and/or enlarging their daily activity time in early spring or late autumn).

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