Thursday, March 11, 2010

Turtles in Winter

As a curious but not entirely cautious kid, I used to catch adult snapping turtles bare-handed while swimming.

I'd lure them in by pulling wild grape leaves down onto the lake's calm surface, and then I'd wait motionlessly nearby in water up to my nostrils. When a hungry turtle appeared and started feeding, I would slowly sink underwater, swim upside-down with my eyes open, and grab the turtle by the sides of its shell -- hopefully beyond the reach of its long neck and powerful mouth. But that was in the warm waters of my Texas childhood.

Abbey posing with a three-year-old Painted Turtle (c) John AshleyIn the comparatively cold waters of Montana, we enjoy the much friendlier Painted Turtles. Our primary dog, Abbey, likes to stand motionlessly in water up to her waist, watching the turtles bask on their favorite log. But by early October her turtle friends stop coming to their log, dissappearing for the winter.

How do these turtles survive the northern winter? Well, they just hold their breath for six months and wait for spring. Simple, right?

Sea turtles migrate to avoid winter waters. Dry land turtles spend the winter in burrows. But most freshwater turtles overwinter at the bottom of ponds and lakes for 5-6 months at a time -- which means that they spend almost half their lives not breathing.

That's quite a feat for any animal.

How can a non-breathing turtle survive? The first part is easy for a cold-blooded reptile -- become very inactive. A hibernating turtle's metabolism plunges by almost 90% and the heart beat drops to once every 5-10 minutes. Dormancy greatly reduces the body's need for oxygen.

A different northern turtle, the eastern Map Turtle, overwinters in communal groups at the bottom of rivers. By stretching out its legs and head before going dormant, a Map Turtle in a slight current can absorb enough oxygen through its skin to meet its reduced demands.

Painted Turtles can also absorb oxygen through their skin, even without a current. But in the wild, the adults typically bury themselves in the mud, where there is little or no available oxygen.

Not so for the juvenile Painted Turtles. The eggs are buried in late June or July in sandy soil near the pond, and they hatch two or three months later. But most of the soft, inch-long hatchlings will remain buried in their nest for the first winter, finally emerging the following spring to head for the pond -- and their first meal.

A few of the young turtles may go to the lake after hatching, apparently spending their first winter resting on the bottom in shallow water and absorbing oxygen through the skin. But these youngsters are more prone to predation, especially by wading birds and raccons in the fall and spring, when the turtles are sluggish and there is no surface ice to protect them. And this, apparently, is why the adults bury themselves in the mud in the same shallow waters.

But how do the adults survive, buried for six months without oxygen?

To survive, animals must metabolize food (or food stored internally as "fat"). Normally, this takes place as aerobic respiration -- a process that requires oxygen. But as oxygen levels drop, turtles can switch to anaerobic respiration, or energy production without oxygen. A by-product of anaerobic respiration, however, is lactic acid which lowers Ph levels and makes the blood acidic.

Red and yellow markings on the ventral plastron of a friendly Painted Turtle (c) John AshleySo now they have a different problem to deal with. To prevent acidic blood, dormant turtles must pull off a shell-game maneuver, switching minerals and acids between different parts of their bodies.

From their bones, and especially their shell (which is actually another bone) turtles move potassium and calcium ions into the blood to neutralize the acidity. They also move some of the lactic acid into the shell, to be dealt with later on next summer, during aerobic respiration.

So the bigger and more calcified the shell, the better a turtle is at maneuvering through the chemical demands of hibernation. Because juvenile turtles have a smaller and softer shell bone, they must spend their first winter buried on dry land, or in underwater hibernation sites that do not become severely depleted of oxygen.

All aquatic turtle species show some tolerance to reduced oxygen levels. But northern species are better adapted than eastern species, which in turn are better adapted than southern species. And turtles with highly ossified shells, like snapping turtles, are better adapted than turtles with less shell bone buffer, like softshell turtles.

An isolated population of Spiny Softshell turtles manage to survive in the larger rivers of eastern Montana. And Snapping Turtles range eastward across the U.S. and into the southwestern corner of Montana. Painted Turtles live across much of the northern U.S., including just about all of our state.

Our friendly Painted Turtles are pretty common here in northwestern Montana, at least during the warmth of summer, when Abbey and I watch for them in the lake. Fortunately, during the winter, Abbey doesn't realize that her ice-covered and mud-buried friends are resting right under her nose -- holding their breath and waiting for spring.

Behind the lens: We rescued three young Painted Turtles that washed ashore on a cold, blustery spring day. We warmed them up in the bathtub overnight and released them the next day, after posing for a picture with Abbey.