A ring-necked snake (Diadophis punctatus) on a rock in the Appalachian Mountains, USA. Photo credit: Christian Cox.
A ring-necked snake (Diadophis punctatus) on a rock in the Appalachian Mountains, USA. Photo credit: Christian Cox.
Although ‘cold blooded’ is popular shorthand for ectothermic creatures, most are anything but chilly. Basking in the sun or sheltering beneath warm rocks raises their body temperatures. Yet, some ‘cold-blooded’ animals can also elevate the temperature of specific body parts, such as the head, to keep alert and active. However, maintaining the warmth of a body region is particularly challenging when you're long and slender with a relatively large surface area to lose heat over. And it's even more problematic when you're tiny. Fully grown ring-necked snakes (Diadophis punctatus) are only ∼20 cm long, yet their heads can be ∼3°C warmer than their chillier rear ends. But it wasn't clear how the diminutive snakes maintain this temperature difference; are their heads composed of tissues that retain heat better than their tail ends and how is this temperature difference shaped by the temperature of their surroundings? Heading to a rocky hillside in the Appalachian Mountains, where the snakes appear to hibernate, Christian Cox (Florida International University, USA) and Albert Chung (Princeton University, USA) with undergraduate students from a University of Virginia reptile and amphibian field course set about measuring the head and rear end temperatures of ring-necked snakes.
When the students found a snake, the race was on to measure its head and tail temperature quickly, before it began to cool or warm up. Cox and Chung wore gardening gloves, to ensure that the heat from their hands didn't affect the snakes, as one of them gently inserted an electronic thermometer into the animal's mouth while the other cautiously inserted a thermometer in the snake's cloaca – the end of the snake's digestive system through which they excrete waste – to measure the two temperatures simultaneously. Having also recorded the temperature of the ground and rock beneath which the snake was sheltering, the duo carefully placed each snake in a stocking, before spiriting it back to the lab, to measure its head and cloaca temperatures after an hour at 15, 20, 25, 30 and 32°C. ‘Fortunately, these gentle little snakes were quite tolerant of having their temperature taken’, Cox recalls.
Sure enough, the snakes were always considerably warmer when the rock beneath which they were resting was hot and, remarkably, their heads were always warmer than their rear ends, despite their diminutive size. Yet, the temperature difference between each animal's head and cloaca was largest beneath the coolest rocks. And when the researchers re-measured the snakes’ temperatures in the lab, before returning them to their rocky homes, it was clear that the head temperature became more similar to the local temperature when the latter was high, reducing the temperature difference between the head and tail end. The team also measured the cloaca and head temperatures of a dead ring-necked snake, which had been found nearby, and discovered that the two were indistinguishable. So, the tissues comprising the snake's head were not able to hold more heat than the rear end to produce a measurable temperature difference.
The snakes were definitely doing something to make their heads warmer than their tail ends and Cox suggests that altered blood flow may allow them to retain more heat in the head to keep it warm. Unexpectedly, the tiny snakes were also able to maintain a head temperature that was 2.1°C warmer than the 25°C room in which they had been housed for a day. ‘This was surprising for such a small ectotherm’, says Cox, who suspects that the petite snakes may generate some internal heat, possibly during digestion, which could be shunted to their heads to keep their brains warm.
