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The Occasional Evolutionist II

“We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey.”

—Charles Darwin, On the Origin of Species, Ch. 3

Evolution has resulted in so many exquisite adaptations that it is sometimes tempting to fall into an admiring swoon at its apparent efficiency and precision. But unlike human engineering, which aims for optimal efficiency, failsafe design, and continual progress, evolution has no point. “Whatever works!” might be considered its rallying cry. This leads to some stunning examples of apparently gross, and, from a human point of view, tragic waste. Indeed, if you prefer your nature fuzzy and endearing, you might want to go read another story.

To illustrate, I give you the turtles of Heron Island. These are the large (think coffee-table sized) and determined mama turtles, familiar from many a nature documentary, that return year after year to the beaches of their birth to lay hundreds of eggs. Graceful as dancers underwater, the turtles are ungainly on land, struggling up the sand like tanks powering through heavy mud. But up they come, and once they’ve found a soft spot past the high tide mark, they spend hours laboriously digging a pit into which they lay 60 to 100 eggs. It takes another 20-30 minutes to cover the eggs, after which they lumber back down to the water. This multi-hour workout is repeated approximately every two weeks throughout a laying season that lasts about three months. Each year an individual mother turtle may lay 400-600 eggs.

The 2013-2014 season was a bumper year for turtles on Heron Island. A walk around the island revealed the turtles’ characteristic tracks leading out of and back into the water every few yards. Because it was such a busy year, it took minimal dedication to spot the turtles; on our first morning walk, we were rewarded with the sight of two mother turtles returning to the water.

But it is at sundown that the real drama of the turtles plays out. The eggs hatch after about 60 days. Because the laying season began in December, many, many baby turtles were emerging during our visit in February. After hatching, the baby turtles remain under the sand until the falling temperatures and fading light of sunset provide the cues that it is time to dig their way out of their nests. Once on the surface, they head to the water, using light as their guide.

Unfortunately, the baby turtles are not the only ones to notice that the sun is going down. Sharks and seagulls notice too, and as far as they’re concerned, sunset means suppertime. Seagulls line the beach, and bare yards from the shoreline are dozens of reef sharks, giving concrete meaning to the phrase “teeming with sharks.” The baby turtles, powering over the pitted sand, face stupendous odds. If a seagull does not pick them off, they are snatched up by a shark. We watched dozens of baby turtles head into the water, but we didn’t see a single one make it past the sharks. Clearly some of them get through, because turtles have been around for upwards of 200 million years, but the odds for an individual baby are low. Research suggests that about one egg in 1000 reaches adulthood.

When you see all of those hard-working turtle mamas and doomed babies, it sure seems like a wasteful system. But of course evolution doesn’t “care” that the situation is wasteful; as long as enough mother turtles manage to lay enough eggs to keep the chain unbroken, there is no “reason” for the process to become more efficient from a human point of view. And while our hominid sympathies are piqued by those adorable baby turtles, the thousands of babies that don’t survive are an important food source for the gulls and sharks that, after all, are also just trying to make a living.

Evolution results in what is “good enough.” And that has some serious implications for another topic dear to the hearts of NCSE and its members: climate change. In the next “Occasional Evolutionist”–life on the edge: the precarious nature of extreme ecosystems.