bumps on a log

On a sunny Sunday afternoon I walked the perimeter of Seattle’s Green Lake with my good friends, Will and Katie. This small, shallow lake, dredged out by the Vashon Glacier more than 50,000 years ago, is an extremely popular spot for Seattleites out for a weekend stroll. The tableau that day was vernal if not quite warm, replete with flowering cherry trees and joggers sporting sunglasses paired with Gore-tex jackets. Along the looping 2.8-mile path we saw many people, many dogs, some ducks—scaups, mergansers, buffleheads, mallards—and, at the very end of our walk, a group of painted turtles sunning themselves on a log. We stopped to watch.

The painted turtle has been around for 15 million years, and the western subspecies—the one native to the Pacific Northwest—has been basking in our preciously scarce sunshine since the last ice age, between 110,000 to 10,000 years ago. Western painted turtles are the biggest of the four subspecies, the most widespread, and, if I may editorialize, the prettiest of them all. Drab olive above, ornately red-striped below, the western painted has vivid yellow lines running down its throat and along its neck, tracing onto its legs and tail. The stripes even extend to the beak and eyes, giving the turtle a Maori tribesman-sort of look.

Painted turtles, like most reptiles, amphibians, fish, and invertebrate animals, are ectotherms—that is, they control their body temperature externally, via their surroundings. (“Ektos” is Greek for “outside”.) More specifically, turtles and their ilk are poikilotherms, meaning that their internal temperature varies considerably depending on the time of day, season, etc. Endotherms, by contrast, control their body temperature metabolically, a process that occurs within their bodies. Most endotherms are also homeotherms: their internal temperature remains more or less constant throughout life. Humans are homeothermic, as are most other mammals and birds. One system isn’t necessarily better than the other: endothermy and ectothermy both have their pros and cons, and organisms employing either strategy have flourished for many millennia.

Ectothermy, or “cold-bloodedness”, came first, dating back from when life proliferated in the sun-warmed oceans more than 500 million years ago, to the point when, 395 million years ago, the first tetrapods ventured onto land and found it both sunny and warm. Because it is metabolically less expensive to store up energy only as it is needed, instead of constantly burning fuel to maintain a regular body temperature, the ectothermic way of life was the norm all the way up until the early Mesozoic, around 250 million years ago.

Ectotherms must absorb heat from their surroundings in order to become warm and active—enough so to eat, mate, escape predation, and do whatever else strikes their fancy. For most ectotherms, this means sitting out in the sun. Painted turtles wake up at dawn to bask for several hours, usually on logs or other above-water platforms, often in large, huddled groups. For them, real activity doesn’t start until they’re 63-73 degrees Fahrenheit inside, and any heat lost during their deep-water dives for food must be regained in the sun. A typical turtle day involves two or three sustained periods of sunbathing, interspersed with forays to the lakebed for invertebrates and plant matter. The life of an active painted turtle is all sun and waves, protected by a carapace and plastron impenetrable to all but the strongest and craftiest of predators.

But the source of ectothermy’s efficiency is also its shortcoming: The sun’s energy doesn’t reach Earth equably, and our planet’s tilt ensures that latitudes far north and south of the Equator receive substantially less of this energy during the winter months. Things get nippy, and the poikilotherms slow down. Their body temperature drops, and their metabolism grinds to a halt. Without warmth, the poikilotherms would make easy pickings for predators—and without the energy to search for food, they would quickly starve. Thankfully, nature has provided these creatures with an ingenious adaptation: hibernation.

Except for the southern subspecies, all painted turtles hibernate for at least part of the year. They dig into the mud in shallow water or find old muskrat burrows to settle in, and their body temperature sinks to a chilly 40-45 degrees Fahrenheit. They become torpid, dead to the world. Oftentimes they are completely encased in mud or ice, and in these anoxic conditions the turtles stop breathing, relying instead on glycolytic—that is, anaerobic, or oxygen-free—processes to keep their vitals running. In animals, glycolysis converts biochemical nutrients into adenosine triphosphate (ATP), the molecular currency that funds our bodily ventures. The chemical by-product of glycolysis is lactic acid, the source of the lingering “burn” we feel in our muscles after a good sprint, or after lifting weights. In aerobic cellular respiration, any accumulation of lactic acid from glycolysis is quickly eliminated when the acids are oxidized—converted to carbon dioxide—which is then exhaled from the lungs. But without oxygen’s mitigating effect, lactic acid builds up in the tissues and becomes deadly toxic. The turtles, of course, have found a way around this problem, as well.

It turns out that the shell of a painted turtle serves not only as defense against predators—it also protects the creature from its own bodily waste. While hibernating for upwards of four months at a stretch with little to no oxygen, the extreme build-up of lactic acid from glycolysis would normally kill the turtle, if not for two amazing adaptations: the release of carbonate buffers from the shell to neutralize the acid; and the uptake of lactic acid into the shell, where it is buffered and sequestered. The former is similar to taking an antacid for heartburn: the carbonate-based antacid neutralizes the hydrochloric acid and lessens the irritation on stomach tissue. The latter is akin to some forms of nuclear waste disposal, whereby the extremely radioactive waste is rendered as inert as possible and then stored far, far away. Both processes work together to keep the painted turtle alive and well, despite the freezing cold and the lack of air. The turtle’s shell is its aegis, and also its home.

Come springtime, the rising temperatures signal an awakening among the turtles. They dig themselves out of the muck and find a suitable log to haul onto. Once warmed, they ply the shallows for their first meal in months, and then they head back to the log for more sun. Watching the turtles at Green Lake, I couldn’t help but admire their flagrant lack of urgency, their plodding pace of life. The timeless clichés came back to me then: Haste makes waste; slow and steady wins the race. If left unmolested, the painted turtles at Green Lake would live maybe fifty years or more. Perhaps those bumps on a log could teach us something yet.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s