How Do Baby Turtles Know To Crawl Towards the Water?

There is a beautiful park a couple of blocks from my house. Groves of big shady trees line the shores of the lake; the footpath winds through the trees, over bridges and around an open field perfect for flying kites. It is a nice place to go for a walk, or a bicycle ride, although the summer here is too hot to be outdoors except early in the morning or after sundown.

Yesterday we rose early and rode our bicycles down to the park. The air was still cool in the shade, but the sun was up and it would be hot before long. As we turned onto the path, I spotted a large turtle, a red-eared slider, in the grass near the path. It was a good fifteen meters from the water, which was strange; when the turtles pull themselves up onto the bank to bask in the sun, they usually stay right at the water’s edge, ready to slide back into the lake at the first sign of someone approaching. Why was this turtle so far from the safety of its habitat?

RedEarSlider

I stopped and signaled the boys to come slowly and quietly. As we watched, the red-eared slider dug into the ground with its hind legs and began laying a clutch of eggs! Slowly it began to scrape the soil back into the hole to cover them up.

Turtles, like all reptiles, are exothermic (or “cold-blooded”); they depend on the environment to warm or cool their bodies. If a turtle lying on the shore gets too hot, it goes back into the water to cool its body. But this turtle was a long way from the water (at least, for a turtle!) As I watched the turtle covering its eggs, I began to worry that it would overheat. The spot it had picked to dig its nest had been in the shade when it began digging, but now the sun beat down directly on the turtle’s shell. I wondered how long it would take for the turtle to finish covering its eggs and drag itself back down to the lake.

I need not have worried. A few minutes later, the nest covered and nearly invisible, Mother Red-Ear was on her way back to the cool water. She stopped in the shade of a hackberry tree to rest beside a fallen branch. Soon she was back in the water.

RedEarSlider2

Andres asked, “When the baby turtles hatch, how will they find the lake? What if they go up onto the road instead, or get lost in the park?”

The short answer is that aquatic turtles hatch with the instinct to go straight for the water. They don’t know where the water is, or even what it is; they have no experience at all, yet they all head for the water as quickly as a baby turtle can (which is quite a bit faster than the adults, on land anyway). Instinct drives them; they can’t help it. The reason for this instinct is obvious: the ones, long ago, who went in any direction other than the water failed to survive, and never grew up to pass that trait on to their offspring. Today, all aquatic turtles are descended from turtles who survived by seeking the water as soon as they hatched, and so all aquatic turtles have that instinct.

So turtles go to water because of survival instinct. But what makes a hatchling turtle move in the right direction? There must be something a baby turtle can sense in order to trigger the instinct. Since gravity makes water lie in the lowest part of any area, it makes sense to think that newly hatched turtles would follow the slope of a beach or lakeshore down to the water. Another possibility is that the mother turtle leaves a scent trail for the hatchlings to follow, but since the eggs take two to three months to hatch, it seems unlikely that there would be much of a scent left to detect. Finally, researchers have observed that baby sea turtles can become confused by electric lights near the beach, becoming attracted to the lights instead of the water. On a beach with no electric lights, the brightest place is the water because it reflects moonlight or starlight from the sky. Maybe turtle hatchlings find their way by the difference in brightness between land and water.

BabyRedEarSlider

Our turtle’s eggs should hatch towards the end of summer. We will be watching for the baby turtles to crawl down to the lake. One way or another, they will find the way!

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