Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [63]
In one study by Gordon R. Ultsch and colleagues (2000), map turtles (Graptemys geographica) were equipped with tracking tags emitting radio signals and found to range up and down the Lamoille River in Vermont and into Lake Champlain over at least a dozen kilometers. In the autumn as water temperatures drop quickly from 22°C in August to 11°C in September, and 2°C in November, the turtles congregate in one assembly about three kilometers up from the mouth of the river. This communal map turtle hibernaculum (which also includes softshell turtles, Apalone spinifera) was investigated by the biologists using scuba gear. They saw turtles pile on top of each other in a deep depression where there is negligible current. The turtles stay at the same site from November to the end of March. After the ice melts and when water temperatures warm up from 0.1° to 12°C, the turtles again leave and return to Lake Champlain for the summer (Graham et al. 2000).
After ice covers the Lamoille in December, the turtles are unable to come up for air for about five months. Do they experience stress of submersion? The biologists studying these turtles (Crocker et al. 2000) returned monthly to the communal hibernation site throughout the winter. Using a chain saw, Carlos Crocker (from balmy Alabama) cut a hole through the ice and then dove down and retrieved turtles and gathered environmental data such as water temperature and oxygen tensions. He sampled the turtles’ blood to measure acidity, lactate, and oxygen and carbon dioxide concentrations. The conclusion drawn from the data was that these large thick-shelled turtles remain essentially aerobic (oxygen-breathing) all winter long despite their inability to breathe with their lungs. They avoid the progressive acidosis that results from anaerobic metabolism, suffering no apparent diving stress because their low metabolic needs for oxygen are met despite inability to take a breath of air into the lungs for months. Their oxygen needs are low due both to their physical lethargy and their low body temperature that reduces resting metabolism. How they accomplish any oxygen uptake at all is not clear. However, the hibernating turtles rest with their heads and legs fully extended on the river bottom and may thus be exposing as much skin as possible to take up dissolved oxygen from the water.
Our best understanding of the hibernation dive physiology of turtles comes from the painted turtle, Chrysemys picta (Ultsch et al. 1999). This species, like other water-inhabiting species studied, also shows no apparent diving stress under simulated hibernation dives in the laboratory at 3°C in normal, i.e., unaerated water. That is, they show relatively little rise in lactic acid and also no change in blood glucose. These results indicate that gas exchange through the skin is sufficient in these turtles as well, at least if they lay on the pond bottom at near 3°C. However, these turtles normally hibernate by burying themselves in the mud, which is nearly devoid of oxygen so that they apparently even deprive themselves of breathing through the skin.
In order to find out how the turtles respond to oxygen lack, the researchers (Ultsch et al. 1999) brought them into the laboratory and sealed them into water bubbled with nitrogen gas to drive off all the dissolved oxygen. The almost totally oxygen-deprived turtles then survived for “only” about four months at 3°C. Their blood lactate increased steadily throughout the whole time of immersion. Blood pH declined from slightly basic 8.0 to near-lethal levels of 7.1. The acidification of the blood (to near-neutral pH) was compensated for in part by increases in concentrations of positive ions (magnesium, calcium, and potassium) that buffer the acidity.
Southern populations of these turtles reached near-lethal