Turtles thrive in depths of winter
Nature's Notebook
While visions of basking turtles represent spring and summer at its best, who thinks of them in the midst of winter? "Out of sight, out of mind," wouldn't you agree?
Today let's turn our attention to wintering turtles, which spend the season in hibernation, safely embedded in mud and muck.
Worldwide there are more than 240 turtle species. Just under 50 live in Canada and the U.S., with Florida an especially turtle-rich state. Nine species are found in Minnesota.
Our turtles are divided into three categories: pond, river and marsh. Each describes the type of habitat in which it's found. Let's take a look at each species.
Almost everyone is familiar with two common pond turtles: painted and snapping. The painted is also known as the mud turtle. They spend summer days basking on rocks and logs on the edges of lakes and ponds.
River turtle species include common, false, Ouachita false, smooth, spiny and wood, which live in the large rivers of the Mississippi drainage. Soft-shell turtles, also called pancake turtles, live in medium to large river shallows, burying themselves in the bottom and exposing only their heads as they prey upon small fish.
The wood turtle, one of two threatened species in Minnesota, is found along small, clear streams. In spring and summer the "woody" spends considerable time in adjacent forests.
The only marsh turtle in Minnesota is the threatened Blanding's. In some places it's referred to as the "box turtle," but true box turtles are found in southern states.
Where turtles winter depends on the species, but most hunker down at the bottom of ponds and streams or in burrows, beaver lodges and muskrat mounds. Terrestrial and semi-terrestrial turtles usually bury themselves in sandy soil.
Young turtles are vulnerable to predation and some stay in their nests even after hatching in late summer or early autumn. According to an old Nature Conservancy newsletter article by Ethan Perry, in northern areas all painted turtle hatchlings winter in the nest, partly because they do not always develop fully before cold weather arrives. Adults select a resting spot with water deep enough water for survival.
Although painted turtles can survive partial freezing, severe cold can be fatal for turtles, particularly if water levels are unusually low and ponds freeze to the bottom. When ice seals a pond, oxygen is dissolved and depleted.
Some turtles bury themselves in the mud as early as September and stay there until spring. Others change positions a few times until the ice settles in. Inactive winter turtles might not breathe for up to seven months.
These air-breathing creatures can survive without air because their metabolism slows dramatically, reducing the quantity of air needed to survive. In addition, underwater turtles absorb some oxygen through their skin. Without adequate oxygen a turtle's body produces lactic acid that accumulates in its blood. Too much acid is fatal. Some researchers speculate that the northern limit of the turtle range is not determined by the severity of winter, but how long the turtles can live without oxygen.
When enough ice has melted turtles emerge from hibernation, hungry more for air than food. They're sluggish and unable to forage until days lengthen and the warmth of the sun increases the water and air temperatures, enabling them to eat. During this time they're especially susceptible to predation.
Painted turtles are among the first to resume activity in spring and sometimes are seen swimming under the ice in March. Both young and old produce natural protectants that prevent cell damage during spring.
Some years are better for turtles. Last year's lack of snow and frost depths to 12 feet and more in some areas of Minnesota caused turtle hatchlings, juveniles and adults to freeze to death. This year the lack of snow at the outset of winter has allowed turtles to fare better.