A basketful of baby musk turtles and a backyard eyesore introduced me to the complex way some creatures survive the winter under water.
When we were traveling long distances, my parents would often humor me by turning onto side roads, stopping to allow me to poke around for toads, snakes and other lowly creatures of youthful interest. If I was lucky enough to find something and capture it, I always begged to take it home. (It took me some years to realize that the more important my find, the less likely it would he allowed in the car.) Once in south Florida, I discovered an abandoned fish trap under a bridge, retrieved it from the water and found it full of young musk turtles. To my delight I was allowed to keep them. They were carefully released in a concrete backyard pool that I had designed and constructed specifically for aquatic turtles at my home in Maryland. I had created what was unanimously recognized as the number one neighborhood eyesore, but my turtles liked it and so did I.
Winters came and went, and each spring after the ice melted, I'd clean the pool and count my turtles. I was pleased to see that all of the musk turtles survived season after season.
How did these turtles from south Florida survive sub-freezing Maryland winters. Like most northern turtles, they hibernated. I did not make much of it then, but thinking back, it is interesting to ponder how a genetic stock of south Florida turtles had the behavioral instincts and physiological abilities to hibernate for four months in an ice-covered Maryland pond. Although musk turtles range well to the north of Maryland, these were able to use a previously untested ability to cope with winter.
For animals that cannot make a living during the winter, hibernation is a great way to avoid the problems of cold and unavailability of food. The process—as explained in high school biology classes—is quite simple. Store up lots of fall fat reserves and when things get unbearable, lower the metabolic rate and sleep through the bad weather. This takes a lot less energy than trying to relocate buried hickory nuts under 16 inches of snow or migrating thousands of miles to some South American country.
Yet, though the principle of hibernation may be simple, the execution of the process is complex. Built in problems must be addressed if the animal is to see the spring.
Those problems become magnified in air-breathing animals that escape the cold by burrowing in the mud at the bottom of ponds. Examine the situation from the point of view of my musk turtles. Excluding some mud turtles that seem compelled to crawl out onto land to hibernate underground, nearly all North American turtles spend the winter underwater. Even the wood turtle that lives for the summer as a terrestrial woodland reptile withdraws to aquatic habitats for hibernation. There are in fact advantages to wintering in ponds, the most striking of which is the behavior of water itself.
Water is capable of absorbing and storing tremendous amounts of heat with only a slight increase in temperature. It cools off just as slowly as it warms up, meaning that aquatic life undergoes relatively moderate fluctuations in temperature. Another quirk of water is that at 39 degrees Fahrenheit water achieves its greatest density and sinks to the bottom of ponds. At all other temperatures, both warmer and colder, it rises. For our musk turtles this means that as long as they are in a pond deeper than the local frost line, the bottom temperatures in winter will stabilize at about 39 degrees. This microclimate is quite comfortable for turtles and other creatures hibernating at the bottom of ponds. If the water becomes cooler than 39 degrees, the cooler water rises. Thus the turtle can never become frozen. When the bottom finally begins to warm in spring, the change sets off a biological alarm clock in the turtle and it awakens.
But the property of water that keeps the coldest layers above the bottom of the pond potentially could be deadly to an air-breathing creature like the turtle. At 32 degrees water expands and freezes, leaving an impenetrable self-sealing layer of ice between the pond's surface and the hibernating creatures trying to get a good winter's nap. In northern latitudes ice may persist for up to four months. That's a long time to hold a breath—too long, as a matter of fact. Living cells need oxygen, and holding your breath—or, in the turtle's case, simply not breathing—works only for short periods. With lots of practice an animal like ourselves can go for perhaps five minutes without breathing. Some seals can dive for an hour or so before they have to surface to breathe, and large whales have been timed at an hour and a half.
How do turtles get enough oxygen to survive their winter's nap? For one thing, their 39-degree blanket at the bottom of ponds has remarkable benefits for k~w oxygen use. If the water were warmer, the turtles would use more oxygen; colder, and their cells—being made primarily of water—would freeze. Jagged ice crystals rupture cell walls from within, and if enough cells were broken the turtle would die.
Thus a hibernating turtle is sleeping in a snug zone of perfect temperature for its temperamental cells. Yet with its submerged lungs unable to breathe, the turtle still faces slow oxygen starvation and a lethal buildup of carbon dioxide. Hibernating amphibians, such as frogs, solve this problem by breathing directly through their skin and are thus able to retrieve oxygen from the water. Turtles and other reptiles, however, have skin clothed with thick scales and, of course, a major portion of the turtle's body is covered with a shell. Absorbing oxygen through the skin is no more an option than flying south, although migration might seem easier than solving the built-in problems of hibernation.
But the turtle solved them by means of a couple of remarkable adaptations. Aquatic turtles, like the musk turtle, have two sources of oxygen to satisfy their winter needs. One is their throat cavity, which is lined with lots of minute blood vessels that permit oxygen to be extracted from the water. The second is a similar type of tissue present in two thin walled sacs near the anus. A small amount of food coloring placed near a captive turtle sleeping on the bottom of an aquarium will show faint independent pumping movements of water at each end of the turtle. To demonstrate the turtle's effectiveness at breathing through its anus and its pharynx, researchers submerged captive turtles for up to eight days. All the turtles survived. Cold water traps and holds more dissolved oxygen than warm water, thus assuring that additional oxygen is available under ponds.
While these capabilities augment oxygen, they do not, by themselves, get a hibernating turtle through an entire winter. Perhaps the threat of slow oxygen starvation in wintering musk turtles can be explained by the example of a long-distance runner. Runners who push themselves to their limits often experience the consequences of muscle cells trying to perform on insufficient oxygen. Normally, muscle tissues are fueled by oxygen-burned carbohydrates. But as the system is stressed, runners experience shortness of breath because it is difficult to take in enough oxygen. In times past when running abilities were used for more immediately rewarding activities, like escaping large predators, it was important to continue to function even when you knew you couldn't. In these emergency situations, the skeletal muscles can operate short-term on an alternative process known as anaerobic glycolysis. In simpler terms, you burn sugar and derive energy without oxygen. Though this fuel-burning process won't keep you going for very long, the survival value is obvious. The mammalian heart and brain can go anaerobic (without oxygen) for only a few minutes. Any longer deprivation results in rapid energy shortage and a by-product of lactic acid that can lead to leg cramps, among other drawbacks. An extreme result is a chemical imbalance called acidosis which is potentially fatal.
The hibernating musk turtle, however, can get away with oxygen deprivation for much longer periods because its metabolism is lower than mammals' Turtles don't do much and live for relatively long times (perhaps there's a lesson here). Their metabolic rates are normally 10 times lower than those of warm-blooded animals of similar size. In hibernation this drops by 10 to 20 percent of the normal tranquil rate. Turtle hearts that beat 40 times a minute on a warm day in July drop to one beat every 10 minutes in the winter.
So it's simple enough, after all. A musk turtle's anaerobic metabolism gets it through the winter, right? Well, almost. There is still a buildup of lactic acid to deal with for the months the turtle is sealed under the ice. Naturally, the turtle slows the buildup by doing next to nothing throughout the winter, but its body functions still produce enough toxin to kill it before winter runs its course. Again, the solution is quite simple. Minute amounts of calcium salts from the turtle's shell are slowly dissolved into its bloodstream. These salts act as a buffering agent, neutralizing the lactic acid.
Researchers at Brown University in Rhode Island, using enough sophisticated equipment to intimidate a space scientist, have been tracking the internal chemical balances of turtles for some time. In tracing the calcium, they discovered these salts perform another invaluable service. Acidosis can depress the heart rate so much that it stops, but the calcium keeps the pump on schedule.
One day, these studies may have implications for enhancing knowledge of clinical problems of more interest to us, but for the moment the researchers are simply trying to understand turtles. Meanwhile, under the ice, and above it, the life of gray winter days goes on, with each plant and animal using its own mechanisms to survive. The days grow longer, water in its solid state melts, and sometime in March or April the turtles once again poke their noses into the air, filling their lungs with oxygen. While they can neither sigh nor smile, the first breath of spring must be an emotional moment, even for a sleepy turtle. Then, as the first sunny days arrive, our turtle neighbors will haul themselves out on a log to bask and raise their body temperatures above that of the water.
This will allow. . . well, that's another story.
David S. Lee. 1999. Terrapin Tails 3(3):1-3 and 1991. Wildlife in NC 55(2):24-27.
|"The tortoise, like other reptiles, has an arbitrary stomach as well as
lungs; and can refrain from eating as well as breathing for a great part of
Gilbert White 1772