by Joseph DeSisto
The Jurassic World movie really got me thinking about tree frogs (warning: spoiler minefield ahead). In the movie, park management decides to boost ticket sales by genetically engineering a brand-spanking new dinosaur: Indominus rex. Indominus combines DNA from Tyrannosaurus and Velociraptor to create a monstrous killing machine — intelligent, powerful, color-changing, and able to consciously alter its body temperature to evade heat-sensing cameras.
Wait, what? Henry Wu, Jurassic World’s top scientist and dinosaur-engineer, explains by recalling that Indominus also contains tree frog DNA. Tree frogs, he tells us, can change their body temperatures at will.
I love tree frogs, and I had to know if this was true. I spent the next few days sifting through papers, trying to find an answer. What follows is the result.
Here’s the problem: tree frogs, like all amphibians, are ectothermic or “cold-blooded.” Warm-blooded or endothermic animals, such as humans, burn calories to keep their bodies a constant, ideal temperature (for you, that’s around 98.6° F). Frogs can’t do that. Instead, they have to move from place to place, seeking out warm spots if they need to raise their body temperature, and vice versa.
All that moving around sounds like a lot of effort, but in fact frogs and other ectotherms spend a lot less energy regulating their body temperature than mammals do. This means frogs can get by on much less food than, say, mice can, even if they are the same size and eat the same type of food.
But are any frogs capable of changing their body temperature at will, without moving around? As early as 1970, California herpetologist Bayard Brattstrom thought so. He studied how amphibians adapt to different environments, conducting experiments to see which species could survive at the hottest and coolest temperatures. During a study of Australian frogs, he found that one species in particular was extremely adaptable. That species was the White’s tree frog.
The White’s tree frog (or Australian green tree frog) is common across northern and eastern Australia. It lives in rain forests, deserts, farmland, and pretty much everywhere in between. This frog even makes a hardy and forgiving pet, and captive-bred specimens are sold in pet shops around the world.
When sweat evaporates off your skin, it takes some heat with it — this is called evaporative cooling. Amphibians are subject to evaporative cooling too, not by sweating, but because of the water evaporating through their permeable skins. Brattstrom (1970) believed that the White’s tree frog could maintain a constant body temperature under rapidly changing conditions, by controlling how quickly water left its body.
Decades later an experiment would prove him wrong — the White’s tree frog can’t change its own rate of water loss to regulate its body temperature. But a closely related species, the Australian red-eyed tree frog, can (Buttemer 1990). Buttemer used these two species to study how Australian tree frogs are adapted to different environments.
In the wild, red-eyed tree frogs are only found in rain forests. So, Buttemer expected the more versatile White’s tree frog to be better at regulating its rate of water loss. The opposite was true. Not only was the red-eyed tree frog able to control evaporative cooling, it also had tougher skin, with a rate of water loss almost as low as that of an alligator.
Could Indominus rex have gotten its temperature-changing abilities from the Australian red-eyed tree frog? Probably not. Even though these frogs can regulate their body temperature, they can only do so because of water evaporating through their skin. Dinosaurs didn’t have the breathable skins of amphibians. Related to modern-day birds, both Tyrannosaurus and Velociraptor would have had largely water-tight skins, covered with insulating … feathers.
Brattstrom B. H. 1970. Amphibia. In G. C. Whittow (ed.), Comparative physiology of thermoregulation 135-166. Academic Press, New York.
Buttemer W.A. 1990. Effect of temperature on evaporative water loss of the Australian tree frogs Litoria caerulea and Litoria chloris. Physiological Zoology 63(5): 1043-1057.