Aim: Squamate fitness is affected by body temperature, which in turn is influenced by environmental temperatures and, in many species, by exposure to solar radiation. The biophysical drivers of body temperature have been widely studied, but we lack an integrative synthesis of actual body temperatures experienced in the field, and their relationships to environmental temperatures, across phylogeny, behaviour, and climate.

Location: Global (25 countries on six continents)

Taxa: Squamates (210 species, representing 25 families)

Methods: We measured body temperatures during activity for 20,231 individuals, and examined how body temperatures vary with substrate and air temperatures across taxa, climates, and behaviours (basking and diel activity).

Results: Heliothermic lizards had the highest body temperatures and those most weakly correlated with substrate and air temperatures. Body temperatures of non-heliothermic diurnal lizards were similar to heliotherms in relation to air temperature but to nocturnal species in relation to substrate temperatures. Diurnal snake and non-heliothermic lizard body temperatures were more strongly correlated to air and substrate temperatures than in heliotherms. Correlation parameters of all diurnal squamates vary with mean annual temperatures, especially in heliotherms, so that the thermal relations of the various categories are disparate in cold climate but convergent in warm climate. Non-heliotherms and nocturnal body temperatures are better explained by substrate temperature than by air temperature. Body temperature distributions become left-skewed in warmer-bodied species, especially in colder climate.

Main conclusions: Differences in squamate body temperatures, their environmental relationships, and frequency distributions are globally influenced by behavioural and climatic factors. Differences between behavioural categories are smaller in warm climates where environmental temperatures are generally favourable, but heliotherm body temperature remained consistently higher than all others.

We (all authors of this work) caught active squamates in the field, in many sites across the world, and measured their body temperatures (T_b). We then measured substrate temperatures (T_sub) and/or air temperatures (T_a) at the specific location where each individual was found. The method of measurement varied among groups. Most of us took cloacal temperatures using either a digital thermocouple or an analogue thermometer, but in a few cases, body temperature was measured using an infrared thermometer (measuring skin temperature) or temperature-sensitive radio transmitters. Cloacal temperatures were taken immediately (no more than 1 minute) after the individual was caught. Note that these environmental temperature data are used here in the absence of measurements of other thermal properties of the environment. Thus, they do not enable to qualify thermal quality and thermoregulatory strategy and efficiency (Hertz et al., 1993). Protocols were consistent for each species and therefore could be corrected for in the statistical models.

We filtered the data to include only species with records from at least 20 individuals per species. To account for phylogenetic non-independence in the subsequent statistical analyses, we used the full imputed phylogenetic tree of Tonini et al. (2016). Species absent from this phylogenetic tree were inserted into it manually when possible (in place of a sister species or into an existing polytomy) and otherwise were excluded from the analysis. Since the Tonini et al. tree contains several polytomies, which are known to affect phylogenetic analyses (Molina-Venegas & Rodríguez, 2017), we repeated all of the analyses using the tree from Zheng & Wiens (2016) which has 42 fewer species but is fully resolved.

We divided species by diel activity and basking behaviour, according to the literature and our own observations. We did not base the partitioning of species on the temperature measurements to prevent circularity of the definitions (Vitt et al., 2008). We classified species according to these behavioural categories, rather than between thermoregulators versus thermoconformers, because the latter is unknown for many species, and because discerning between thermoconformers and actively regulating thigmotherms is difficult (Doan et al., 2022; Hertz et al., 1993). We categorized species that are not commonly observed exhibiting basking behaviour as “non-heliothermic” rather than “thigmotherms”, since we classified them by observable behaviour and not according to the sources of heat gain and loss, of which we cannot be sure without direct testing. That is, each researcher or group classified the behaviour of the species which they contributed to the database, according to the literature and their own observations and expertise. This classification, while qualitative and to an extent subjective, was carried out before any of the analyses to prevent them from being biased by the authors’ hypotheses. Diurnal snakes were placed in a separate category despite basking since their thermal biology is considered distinct from that of the more commonly studied lizards (Gibson & Falls, 1979; Avery, 1982; Whitaker & Shine, 2002). We did not have measurements of enough nocturnal snake species to include them as a separate category and grouped them with the nocturnal lizards. Species were classified into four categories: 1. “heliotherms” (heliothermic lizards), 2. “non-heliotherms” (diurnal non-heliothermic lizards), 3. “diurnal snakes”, and 4. “nocturnal species”. We derived the mean annual temperature, as a proxy for the macroclimatic conditions, at the site where each species was measured (1970-2000 average, data from BIO1 in WorldClim; Fick & Hijmans, 2017). When we had no body mass data for a species from measurements of the individuals used in the temperature measurements, we estimated it from mean species snout-vent length data (either from the individuals measured or from Meiri et al., 2021) using allometric equations from Feldman et al. (2016) and Meiri et al. (2021).