Saturday, June 13, 2015

Old Turtle

The old turtle scraped at the sand with her naily toes as the kids gathered in a wide circle around her. Sometimes I get questions about dinosaurs on field trips, but they don’t fit into the Museum’s focus on Northern Wisconsin species. Today, instead, the first- and second-graders got a close-up look at a creature who has existed on Earth for over 40 million years, with direct ancestors much older than dinosaurs.

Quietly and respectfully, the students observed as the mother slowly finished excavating a depression for her precious cargo at the edge of the boat ramp’s asphalt. We commented on her smooth, algae-covered shell and enormous claws on her webbed feet. Once, I caught a glimpse between her hind leg and knobby tail of a smooth, white eggshell sliding into the nest.

The size and age of a female snapper, and the number of eggs she lays each year, are all connected. A mother turtle will only lay a clutch of eggs equal to about 7% of her body mass each year, and some years not at all. This helps make sure she’ll have enough energy to survive the winter, and translates into somewhere between 11 and  87 eggs, with an average of 34 eggs per clutch in northern populations.
Because of this trend, female snapping turtles don’t mature until they are eight inches long -- big enough support a clutch of about 22 eggs. With our short growing season in the north, that can take 19 years. This big mamma was well over a foot long. How many years must it have taken her to grow that big?

One reason that snappers grow so slowly is that they are ectotherms who use their environment to regulate body temperature. In the summer, they sun themselves to warm up. After a long winter, they have to wait until the shallows reach at least 40 degrees in order to become active.  Even then, they don’t start eating until the water temperature reaches about 60 degrees. This means that in cold northern lakes, snapping turtles may go nine months without eating.

Once they do warm up enough to eat, over half of their diet is vegetation. Snapping turtles are important scavengers, and may improve fisheries by eating the slow, bottom-feeding fish (which are generally unpopular among anglers). Although baby ducks do make the occasional tasty snack, they are a much less common part of the snapper diet than many people think. Over the course of a year, a snapper will only eat its own bodyweight in food. That isn’t a recipe for quick growth.

One consolation for their stingy diet may be that snapping turtles rarely become food for something else. The eggs and little guys are vulnerable, of course, but once their carapace reaches three inches long, they have no more natural predators.

Getting there is the tough part.

The eggs we just watched being laid have almost no chance of reaching maturity. For one, their location at the edge of a driving surface is pretty risky. But even in a good location, only about 14% of clutches hatch each year. Nest predation, temperature variation, and dehydration are all constant dangers.

Temperature is especially important. The embryos won’t develop at temperatures cooler than 68 degrees Fahrenheit. Above that, interesting things happen. Because turtles evolved before x and y chromosomes, they developed a system to use temperature to determine the sex of the babies. At 83 degrees Fahrenheit, the number of males and females will be equal. Cooler temperatures will produce males, and higher ones will develop females.

It is amazing that this lumbering matriarch even survived to this point in time. But turtles are survivors. Fossils of the most primitive turtle put the age of this group at 215 million years old – about 100 million years older than dinosaurs. Turtles survived the meteorite impact and the dinosaurs’ great extinction.

Reluctantly, the students left the great mother to her important task, and began one of their own. With nets and enthusiasm, they caught critters in the weedy shallows. Soon a shout rose above the rest, “I caught a turtle!” With its spiky shell, just over an inch long, this new stage in the life of a snapper – a baby hatched last year – captivated their attention just as thoroughly as the one before it.
This baby snapper, caught the same day, must have hatched last year. It gives hope for the future. Photo by Emily Stone

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