Was your turtle living in a warm place? If so, your intuition was good. Most reptiles, like turtles, regulate their temperature by basking in warm environments, and moving elsewhere when they get too hot. In other words, reptiles thermoregulate behaviorally because they are not capable of producing body heat to keep warm, and must move to shade to keep cool.
Speaking of cool, I’d like to introduce you to the one of the coolest reptiles around: Leatherback Sea Turtles, Dermochelys coriacea. In fact, they keep themselves from becoming giant ice cubes while swimming in frigid water as low as 0.4 degrees Celsius.(James, Davenport & Hayes, 2006).
These guys also have the largest migratory path of any reptile, feeding in temperate waters and breeding in tropical waters for years on end. Here we’ll focus on the amazing suite of physiological and behavioral adaptations that allow a reptile to take advantage of frigid habitat.
Just how does a turtle keep warm when it must forage in cold environments? Well first off, they’ve got size on their side.
|Just look at the size of this fella!|
Leatherbacks are the largest turtles in the world. Their huge size contributes to heat retention in cold environments. By having a low surface area- to volume ratio, the turtles dissipate heat much slower than a smaller counterpart. Leatherbacks also have a fatty insulation layer, similar to that found in marine mammals. However, they do not have this insulation in their fins, which make up as much as 30% of their exposed surface area. Scientists have found that heat loss in flippers is much less than expected in these environments (Bostrom et. al 2010), suggesting a reduction in bloodflow to the extremeties or a countercurrent heat exchanger mechanism, much like that described in aquatic birds. Penick et al 1998 found that the pectoral muscles in leatherbacks had thermally independent metabolism, allowing the pectoral muscle to function normally in temperatures from 5 C to 25C. This was not the case in Green Sea Turtles, whose pectoral muscles decreased in function as temperature decreased.
|Warm blood stays near the body core, so heat is conserved|
This combination of physiological adaptations has been termed gigantothermy. And as awesome as the term sounds, it just doesn’t seem to explain how leatherbacks can manage to keep warm. Gigantothermy assumes that leatherbacks have low metabolic rates, just like other reptiles. If so, how can these turtles really combat heat loss? After all, these turtles live in cold water for months at a time, so are constantly losing heat. These turtles aren’t just losing heat through the outside of their bodies. They are also losing heat at the very core their giant bodies are supposed to keep warm—every time they ingest their prey! The jellyfish they hunt are about the same temperature as the water they are diving in.
|Leatherback eating a jelly at the surface|
By eating cold prey, leatherbacks spend energy (heat) to warm the prey to body temperature. In fact, it is a crucial survival tip to NOT eat snow when thirsty and stranded in arctic environments, because more energy is spent warming the snow to water than gained by the resulting gulp! Similar to a stranded hiker melting snow over a fire before drinking, there are a few accounts of turtles seemingly combating this heat differential by carrying prey to the surface to ingest. This would allow the prey to warm at least to the temperature of the surface—but this does not seem to be a main strategy.
|Don't touch them. They're cold.|
A recent study found that Leatherbacks keep their body temperatures within a 3 degree temperature range, and at times this is up to ten degrees warmer than water temperatures (Casey, James, & Williard 2014). Modelling suggests that these turtles must have higher metabolic rates than other reptiles to maintain a body temperature that high while ingesting cold prey. Casey, James and Williard 2014 proposes that leatherbacks use the heat produced from specific dynamic action to warm prey. Specific dynamic action (SDA) refers to energy above resting metabolic rate that is required to break down and store food. A byproduct of these energy processes is heat- and it is estimated up to 90% of the heat produced by SDA in leatherbacks goes to warming prey to body temperature. Leatherbacks also possess brown fat- which could aid in keeping warm as well. For more info on brown fat, check out this recent post. This is really cool, because it means these reptiles are able to produce metabolic body heat—something a true ectotherm cannot rely on to survive.
|Above:Casey, James, & Williard 2014. Sea Surface Temperatures in October. Below: Water temperature of dives by a leatherback sea turtle, James, Davenport & Hayes,2006.|
|Body temperatures of a female sea turtle. Yellow circles represent daytime readings and black nighttime readings. Casey, James, & Williard 2014.|
So, to sum up: Leatherbacks have lots of adaptations to prevent heat loss, retain and even produce heat, and adjust their body temperature behaviorally. They are truly unique among reptiles, and deserve more research attention.
Oh, and they’re endangered, too.
Seebacher, F. 2005. A review of thermoregulation and physiological performance in reptiles: what is the role of phenotypic flexibility? Journal of Comparative Physiology B 175:453-461
Sato, K. 2014. Body temperature stability achieved by the large body mass of sea turtles. The Journal of Experimental Biology. 217:3607-3614.
Albright, B., Woodburne, M.O., Case, J.A., and A.S. Chaney. 2003. A leatherback sea turtle from the Eocene of Antarctica: implications for antiquity of gigantothermy in Dermochelyidae. Journal of Vertebrate Paleontology. 23(4):945-949.
Casey, J.P., James, M.C., and A.S. Williard. 2014. Behavioral and metabolic contributions to thermoregulation in freely swimming leatherback turtles at high latitudes. The Journal of Experimental Biology. 217: 2331-2337.
Hamelin, K.M., Kelley, D.E., Taggart, C.T., and M.C. James. 2014. Water mass characteristics and solar illumination influence leatherback turtle dive patterns at high latitudes. Ecosphere. 5(2):1-20.
Bostrom, B.L., Jones, T.T., Hastings, M. and D.R. Jones. 2010. Behavior and physiology: the thermal strategy of leatherback turtles. PLoS one. 5(11):1-9.
Bostrom, B.L. and D.R. Jones. 2006. Exercise warms adult leatherback turtles. Comparative Biochemistry and Physiology, Part A. 147:323-331.
James, M.C., Davenport, J. and G.C. Hays. 2006. Expanded thermal niche for a diving vertebrate: a leatherback turtle diving into near-freezing water. Journal of Experimental Marine Biology and Ecology. 335:221-226.
Penick, D.N., Spotila, J.R., O’Conner, M.P., Steyermark, A.C., George, R.H, Salice, C.J., and F.V. Paladino. 1998. Thermal independence of muscle tissue metabolism in the leatherback turtle, Dermochelys coriacea. Comparative Biochemistry and Physiology Part A. 120:399-403.
Wallace, B.P. and T.T. Jones. 2008. What makes marine turtles go: A review of metabolic rates and their consequences. Journal of Experimental Marine Biology and Ecology. 356:8-24.