DOI: 10.5061/dryad.5dv41nscz
title: A global analysis of field body temperatures of active squamates in relation to climate and behaviour

description: 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.
description: We (all authors of this work) caught active squamates in the
 field, in many sites across the world, and measured their body
 temperatures (T<sub>b</sub>). We then measured substrate
 temperatures (T<sub>sub</sub>) and/or air temperatures
 (T<sub>a</sub>) 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.
 <em>“heliotherms”</em> (heliothermic lizards), 2.
 <em>“non-heliotherms”</em> (diurnal non-heliothermic lizards),
 3. “<em>diurnal snakes”</em>, and 4.
 “<em>nocturnal</em> <em>species”</em>. 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).
description: # A global analysis of field body temperatures of active squamates in
 relation to climate and behaviour | Column title | Type | Description | |
 :--------------- | :------ |
 :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Species | factor | Binomial name, updated to fit the Reptile Database 2022 | | Category | factor | \[Based on the data in rows 4-6]. Helio\_liz = heliothermic lizard. Non\_helio\_liz = non-heliothermic lizard. Snake\_diur = diurnal snakes. Nocturnal = nocturnal lizards and snakes. | | Taxon | factor | Lizard or snake | | Activity | factor | Diurnal or nocturnal. Cathemeral species were assigned to the time of day they had been documented | | Behaviour | factor | Heliothermic or not. According to the literature and the researcher's \[see row 12] personal expertise | | Tb | integer | Body temperature (degrees Celsius) | | Tsub | integer | Substrate temperature (degrees Celsius, at the location where Tb was taken) | | Ta | integer | Air temperature (degrees Celsius, at the location where Tb was taken) | | Latitude | integer | Decimal degrees. If exact location could not be provided (e.g. in protected species where location is not publicly available), rounded to the nearest 0.1 degree | | Longitude | integer | Decimal degrees. If exact location could not be provided (e.g. in protected species where location is not publicly available), rounded to the nearest 0.1 degree | | Research group | factor | Initials of the researchers who measured this individual. People working together and using the same methodology were grouped together. | | Tb device | text | Model of the device | | Tb\_method | factor | Tb device separated into three categories: cloacal probe, skin (infrared), and radio transmitter | | Ta device | text | Model of the device | | Ta height | text | Height (in cm unless otherwise indicated) of the Ta measurement device above ground | | Ta\_height | Factor | Ta height separated into three categories: &lt;5cm, 5-15cm, and &gt;50cm | | Measur radiation | text | Was the animal location when caught sunlit, shaded, etc. | | Tsub device | text | Model of the device | | Country | factor | Country where the animal was measured (no political statement is intended, in the case of disputed territories) | | Date | text | When the measurement was taken. Exact dates, if known, are in dd/mm/yyy format. | | Time | text | Hour of the measurement, if known | | Age | factor | Adult, subadult, juvenile, or unknown | | Sex | factor | Male, female, or unknown | | Locality | text | Name of the region or location | | Weather | text | Weather observations at the time of measurement | | log mean mass | integer | log10 of the mean species mass (in grams). Mass was calculated from our data if available, or from snout-vent length data using the allometric equations from Feldman et al. (2016) and Meiri et al. (2021) | | Notes | Text | Any further information | | Active? | Factor | Yes/No. Was the animal active, or not (e.g., sleeping, thermoregulating, resting under cover, etc.) | | Tsub\_use | Factor | Yes/No. Did the data in this row fit the criteria to be used in the Tsub analyses (n&gt;20 active individuals, phylogenetic data present) | | Ta\_use | Factor | Yes/No. Did the data in this row fit the criteria to be used in the Ta analyses (n&gt;20 active individuals, phylogenetic data present) | *NOTE: blank cells indicate that no data is available for that variable.