As we race to understand how increased heat caused by climate change will affect animals, the potential ‘winners and losers’ of this climate change marathon are beginning to emerge. Many of these struggling animals cannot generate body heat on their own, relying on the temperature of their surroundings to warm themselves. Lizards, for example, must move in and out of shade to maintain a body temperature that allows them to move quickly and helps them digest their food. But hotter is not always better. Heating up lizards speeds up their metabolism, forcing them to burn through energy faster and producing more harmful by-products that damage tissues. But when the environment heats up to the point that even shade temperatures become too hot, lizards must find another way to slow down their metabolism. Xingzhi Han, Baojun Sun and Qiong Zhang at the Chinese Academy of Sciences, China, along with Fushun Zhang at the Chinese Academy of Agricultural Sciences, China, and Liwei Teng and Zhensheng Liu at the Northeast Forestry University, China, wanted to see whether a desert-living lizard could adjust its metabolism and reduce tissue damage when the temperature gets too warm.
To do this, Han and colleagues caught female and male toad-headed lizards (Phrynocephalus przewalskii) from Shierliancheng in Inner Mongolia, China in July 2021 and brought them back to their lab in Beijing. Normally, the lizards live in a habitat that is cooler in the morning and hotter in the afternoon (21–39°C). After 2 weeks, the researchers changed the daily temperatures of half of the cages to mimic the heatwaves in China during 2016 and 2018 (21–58.7°C). On the third day of the artificial heatwave, Han and colleagues measured the body temperature of each lizard every hour between 08:00 h and 18:00 h. Unsurprisingly, lizards living under heatwave conditions were hotter than those experiencing normal temperature conditions. Five days later, the researchers measured the resting metabolism of each lizard at 34°C and 38°C. The team found that the lizards that experienced the heatwave had lower metabolic rates at both temperatures, suggesting the heat-stressed lizards could slow their energy use. The team also recorded how fast each lizard could run at 34°C and 38°C. The heat-stressed lizards only ran at about a third of the speed of the lizards from cooler conditions when running at the higher temperature. Although having a lower metabolism when resting can be a good way to save energy on hot days, lizards that have lived through heatwaves do not have as much energy available for running, making them slower when the weather is too warm. This means that these lizards will have a harder time catching their insect prey and avoiding being eaten themselves, but having a slower metabolism also means their tissues are less damaged by turning food into energy.
So, after the 14 day heatwave, the team measured how much damage their metabolic rate caused the lizards’ livers. The researchers found that heat-stressed lizards not only slowed their metabolism, which reduced the amount of overall tissue damage, but their bodies also produced more antioxidants – molecules that help get rid of the harmful by-products of metabolism. The team concluded that making more of the protective antioxidants further helps lizards deal with the heat by shrinking the negative consequences of having a faster metabolism.
Han and colleagues show that toad-headed lizards can adjust their metabolism when it is too hot, but at the cost of being easier to catch. Although these lizards can adjust to living in the heat, the impacts these adjustments have on their survival will be crucial for predicting how these – and other – reptiles will endure the climate change marathon.
