Oxygen supply did not affect how lizards responded to thermal stress

Zoologists rely on mechanistic niche models of behavioral thermoregulation to understand how animals respond to climate change. These models predict that species will need to disperse to higher altitudes to persist in a warmer world. However, thermal stress and, thus, thermoregulatory behavior may depend on atmospheric oxygen as well as environmental temperatures. Severe hypoxia causes animals to prefer lower body temperatures, which could be interpreted as evidence that oxygen supply limits heat tolerance. Such a constraint could prevent animals from successfully dispersing to high elevations during climate change. Still, an effect of oxygen supply on preferred body temperature has only been observed when oxygen concentrations fall far below levels experienced in nature. To see whether animals perceive greater thermal stress at an ecologically relevant level of hypoxia, we studied the thermoregulatory behavior of lizards (Sceloporus tristichus) exposed to oxygen concentrations of 13% and 21% (equivalent to PO₂ at 4000 m and 0 m, respectively). In addition, we exposed lizards to 29% oxygen to see whether they would accept a higher body temperature at hyperoxia than at normoxia. At each oxygen level, we measured a behavioral response to heat stress known as the voluntary thermal maximum: the temperature at which a warming animal sought a cool refuge. Oxygen concentration had no discernable effect on the voluntary thermal maximum, suggesting that lizards experience thermal stress similarly at all 3 levels of oxygen (13%, 12% and 29%). Future research should focus on thermoregulatory behaviors under ecologically relevant levels of hypoxia.