Thermoregulation by Bullsnakes (Pituophis catenifer sayi): do burrows make life easier on the prairies?

Abstract

Reptiles living in the grasslands of Western Canada cope with extremely variable environmental temperatures. We studied body temperatures (Tb) and operative environmental temperatures (Te) for bullsnakes (Pituophis catenifer sayi Schlegel, 1837) to address uncertainties regarding challenges posed by the thermal environments of northern grasslands, and to evaluate conflicting hypotheses regarding thermoregulatory strategies. Despite potentially extreme surface temperatures (x̄min = 9.3, x̄max = 31.4°C), mammal burrows remained within voluntary limits (15 – 35°C) for 93 % of the active season and created thermal gradients (up to 27.2°C) which may facilitate thermoregulation by creating high thermal heterogeneity. This evidence suggests that grasslands may actually be less challenging for thermoregulation than comparable forests. 57.8 % of the variation in observed Tb (x̄min = 20.1, x̄max = 29.1°C) was explained by 3 simple variables: time of day, day of year, and bare ground Te. Bullsnakes thermoregulated by conforming to Te near their preferred range (21 to 27°C), selecting locations that enabled heating up below this range, and avoiding warmth above this range. Our results support broad hypotheses of reptilian thermoregulation that predict increased thermoregulation when a) environmental temperatures deviate further from preferred ranges and b) costs of thermoregulation are lower due to thermal heterogeneity.

The two environmental drivers of thermoregulatory costs: Interactions between thermal mean and heterogeneity influence thermoregulation

“…However, excluding 'thermally avoidant' behaviour may be overlooking a biologically meaningful component of thermoregulation that is not accounted for in theory (e.g. using cool retreats for metabolic conservation or choosing suboptimal thermal microhabitats that have reduced locomotory costs or predation risk, Greenville & Dickman, 2009;Kearney et al, 2009;Johnson et al, 2022).…”

“…The arena combines strengths of previous studies by teasing apart the separate effects of thermal mean and heterogeneity (Schwanz et al, 2018;Sears et al, 2016) within a tractable lab-based setting (Basson et al, 2017) Cadena & Tattersall, 2009;Herczeg et al, 2006), while even fewer have distinguished between the independent effects of mean and heterogeneity (Schwanz et al, 2018;Sears et al, 2016). Similarly, field-based studies typically focus on either mean temperatures or heterogeneity (Díaz et al, 2022;Johnson et al, 2022;Lymburner & Blouin-Demers, 2020;Neel & McBrayer, 2018;Rozen-Rechels et al, 2021). Thermal heterogeneity in nature is multi-faceted and arises from diverse sources, making it difficult to measure or replicate experimentally (Christian et al, 2006).…”