By Dr. Karen Shaw Becker
Have you ever wondered how turtles, which have lungs and breathe air, survive harsh winters in the icy waters of a pond? Some turtles are able to hibernate in ponds — even those covered with ice — because of a clever adaptation known as cloacal respiration. In simple terms, this is the ability to extract oxygen from the water via its cloaca — the hole in a turtle behind through which it excretes, urinates and lays eggs.
The process efficiently absorbs oxygen without much exertion on the turtle’s part. This allows turtles to survive underwater even over a period of several months without having to come up to the surface for air. But cloacal respiration is only one part of the turtle hibernation story. The fact is, even with cloacal respiration, a pond over winter is a low-oxygen environment.
While aquatic plants lend oxygen to the water in the summer months, this is used up during the winter. In some cases, the pond may become largely depleted of oxygen, but, amazingly, turtles have ways of dealing with this, too.
Turtles’ Metabolism Slows During Winter
A key part of the equation is turtles’ classification as ectotherms, which are animals that are dependent on external sources of heat. This is why you’ll often see turtles sunning themselves on rocks or logs during warmer months. In the winter, however, a turtle’s body temperature will match its environment.
Interestingly, hanging out underwater or even buried in mud at the bottom of the pond when it’s chilly outside may actually help ensure that turtles don’t freeze. Jacqueline Litzgus, Ph.D., professor, department of biology at Laurentian University in Ontario, wrote for The Conversation:1
“Water acts as a temperature buffer; it has a high specific heat, which means it takes a lot of energy to change water temperature. Pond water temperatures remain quite stable over the winter and an ectotherm sitting in that water will have a similarly stable body temperature. Air, on the other hand, has a low specific heat so its temperature fluctuates, and gets too cold for turtle survival.”
The cold water also serves to slow the turtles’ metabolism, Litzgus explains, “which translates into lower energy and oxygen demands.” In this way, many turtles are able to survive in a frozen pond with very little oxygen, simply by breathing through their backsides to support their minimal oxygen requirements.
Some Turtles Can Switch to a No-Oxygen Metabolism
Some turtles, such as snapping turtles and painted turtles, have the ability to survive low- or no-oxygen water by changing their metabolism to one with no oxygen requirement. In lab studies, turtles have been found to survive for more than 100 days in this way, with some even moving calcium from their shells to neutralize the accumulating lactic acid buildup.2
Painted turtles, which are the most tolerant of no-oxygen conditions among air-breathing vertebrates, can survive such conditions for up to five months at just 37 degrees F (3 degrees C).3 According to Litzgus:4
“This ability [to switch to a no-oxygen metabolism] is amazing, but can be dangerous, even lethal, if it goes on for too long, because acids build up in their tissues as a result of this metabolic switch … In the spring, when anaerobic turtles emerge from hibernation, they are basically one big muscle cramp.
It’s like when you go for a hard run — your body switches to anaerobic metabolism, lactic acid builds up and you get a cramp. The turtles are desperate to bask in the sun to increase their body temperature, to fire up their metabolism and eliminate these acidic by-products. And it’s hard to move when they’re that crampy, making them vulnerable to predators and other hazards. Spring emergence can be a dangerous time for these lethargic turtles.”
Interestingly, it’s been suggested that perhaps turtles are comatose and unresponsive during their long winter submergence in ice-covered ponds, but one study found the turtles still respond to light stimuli (although not to vibration). “We conclude that hibernating freshwater turtles are not comatose, but remain vigilant during overwintering in cold hypoxia,” the researchers concluded.5
The safeguards that allow a turtle to stay underwater for long periods in the winter disappear in the summer months, as the warmer water increases their body temperature and metabolism, and therefore their energy and oxygen requirements.6 For this reason, a turtle can easily drown if it’s caught in a fishing net during the spring or summer.
Meanwhile, many turtle species are endangered, including the white-throated snapping turtle, which is one of the species that depends on cloacal respiration. Found only in southeast Queensland, Australia, the white-throated snapping turtle was only described as a species in 2006 and is threatened due to the construction of weirs and dams, along with boat strikes and predators eating its eggs.7
The water in the area’s Connors River — the only place the turtles live — is also full of sediment and slower moving due to the construction, which makes it harder for the turtles to stay underwater, leaving them vulnerable to predators.8 Despite advances in understanding how turtles hibernate and survive winter, there’s still much more to be discovered — and the more humans learn about what turtles need to overwinter, the more targeted efforts can be made toward conservation and protecting their habitat.