A crevice spiny lizard (Sceloporus poinsettii) in the San Mateo Mountains, NM, USA. Photo credit: C. Loughran.
Although popularly referred to as ‘cold blooded’, active land-dwelling ectotherms are anything but cold. These creatures derive heat from their surroundings and feel relatively warm to the touch. However, as global temperatures rise thanks to human activity, many cold-blooded creatures are at risk of overheating and have to resort to activities such as sheltering and panting like dogs to protect themselves from the heat; unlike us, lizards cannot sweat. But Caleb Loughran from University of New Mexico, USA, warns that creatures that pant to keep their temperature down run the risk of dehydration. Curious to find out how costly this is to the animals, Loughran teamed up with Blair Wolf (University of New Mexico) and headed out into the wilds of New Mexico and Arizona in search of panting lizards to discover how hard some of them work to keep cool and how serious their risk of dehydration is.
‘Going out to collect lizards was always the most enjoyable part of the study, because they occur in a broad array of habitats’, says Loughran, who spent most of the summer months during 2017 and 2018 roaming the local lowland deserts, uplands and woodlands in search of lizard species with different lifestyles, from frenetic foragers to easy-going sit-and-wait predators. Back in the lab, Loughran gently warmed each lizard individually from 35°C until the animal could stand the heat no more, measuring the amount of water that they were losing and energy they were consuming as the mercury rose, in addition to filming their activity to record when they began panting to remain cool. ‘The hardest part of this study was making sure that all of the analysing instrumentation was properly calibrated …because you don't want all the work you put into collection of animals be for nothing’, says Loughran.
Sure enough, as the animals began to pant, their water loss rates rocketed, with some of the smaller individuals [ornate tree lizards (Urosaurus ornatus) and side-blotched lizards (Urosaurus stansburiana)] losing as much as 6% of their body mass per hour in the form of water. ‘It was surprising that some individuals had the capacity to evaporate that much water’, says Loughran. Even though the larger lizards, such as chuckwallas (Sauromalus ater) lost more water, it only amounted to 1.4% of their body mass, placing them at lower risk of dehydration. However, the lizards that lost more water managed to keep their body temperature down better, with eastern collared lizards (Crotaphytus collaris) remaining as much as 3°C cooler than the air temperature when it became dangerously high, allowing them to spend longer searching for food on hot days. And when Loughran checked how much effort panting required, it was relatively little, even though the metabolic rate of some lizards rocketed, thanks to the heat.
Yet, Loughran warns that panting to keep cool may not be a viable alternative for lizards that make their homes in sizzling low-lying deserts: ‘Increased rates of water loss may be difficult to recover from’, he explains. But panting may give species that move to higher altitudes a head start over more vulnerable mountain species that do not pant. ‘Lizards that can effectively evaporate water to cool their body temperature may have a thermoregulatory tool in their toolkit to resist the effects of climate change’, says Loughran. But panting can only provide so much protection until water resources run low.
