Imagine for a moment that you’re a sea snake and you’re feeling a little chilly. That’s quite understandable for an ectotherm. Your body temperature rises and falls with that of your environment, and spending all day slither-swimming through the water can wick away your precious body heat.
You could bask in the sun to remedy the cold. That’s a classic reptile way of working some warmth back beneath those scales. But there’s another option. You could steal your warmth. All you’d have to do is find some seabirds.
There’s a specific term for this warmth-sucking behavior – kleptothermy. The conditions, laid out by François Brischoux and colleagues, are really quite simple. There has to be a warm animal in a relatively cool environment and another animal that can use that body heat to raise their own body temperature. For example, a blue-banded sea krait that got nice and cozy in a wedge-tailed shearwater burrow.
When not in the underground nest, the researchers found, the sea krait had a body temperature of about 89ºF. Pretty warm for an ectotherm. But when the snake coiled up inside the seabird nest, out of the way of the owners, its body temperature was more stable and rose to about 99ºF. This was definitely because of the birds. When the accommodating avians weren’t at home, the lack of their body heat caused the nest temperature to dip to 82ºF.
The snake wasn’t the only reptile to borrow a little body heat. Other reptiles, from lizards to crocodiles, have been known to inhabit the burrows of warmer-bodied animals, as well as termite mounds where the activity of all the little insects keeps the colonies on the toasty side. And in a much broader study published this year, Ilse Corkery and colleagues found that tuataras are little kleptotherms, too.
Tuataras – which look like lizards but belong to a different group of reptiles called rhynchocephalians – face heating problems just like the sea snakes. In fact, scientists have found that the spiny reptiles are most comfortable with body temperatures between 67 and 73ºF. The trouble is that the temperature in the nighttime forest can dip below their preferred range, and basking back to a more tepid temperature the next day can take a while. So many tuataras seek out bird burrows to spend their nights.
After following the reptiles and taking temperature readings over three years, Corkery and coauthors found that many tuataras clambered into burrows made by fairy prions. Those that did so maintained higher body temperatures thanks to the bird-warmed air of the burrows. This probably let the reptiles start the day with a higher body temperature, reducing the time needed to bask in the sun the next morning and increasing the tuataras’ chances to spend much of the day foraging for insects and frogs.
Of course, mammals show similar behaviors. The difference is that mammals typically maintain high, constant body temperatures and share the warmth with each other. Although that knowledge is cold comfort when your significant other snuggles up to you with ice cold hands or feet. In such moments, you may be the victim of kleptothermy.
Brischoux, F., Bonnet, X., Shine, R. 2009. Kleptothermy: an additional category of thermoregulation, and a possible example in sea kraits (Laticauda laticaudata, Serpentes)Kleptothermy: an additional category of thermoregulation, and a possible example in sea kraits (Laticauda laticaudata, Serpentes)Kleptothermy: an additional category of thermoregulation, and a possible example in sea kraits (Laticauda laticaudata, Serpentes). Biology Letters. doi: 10.1098/rsbl.2009.0550
Corkery, I., Bell, B., Nelson, N. 2014. Investigating kleptothermy: A reptile-seabird association with thermal benefits. Physiological and Biochemical Zoology. 87 (2): doi: 10.1086/674566