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The Lake Where You Live

How old is that turtle?

July 03, 2020 by Ted Rulseh

As recently as last Sunday I have seen painted turtles crossing town roads on their way to dig nests into sandy soil.

It seemed late in the year for that to be happening. But that aside, what happens to those newly hatched turtles if they manage to avoid being eaten by predators, drying out from the sun, or otherwise perishing on their way from the nest to the water their mother came from?

More specifically, how fast do painted turtles grow? And for that matter, what about the snapping turtles we also see laying eggs beside the roads around here? 

In part, the turtles’ growth rates are determined while the eggs incubate in the nest. Eggs that develop at temperatures below the mid-80s Fahrenheit yield males, and those that develop at warmer temperatures yield females, which then grow faster than the males. 

Painters are about as big around as a nickel when they emerge from the eggs, but they grow quickly — in the first years of independence they might double in size, the actual growth rate depending on factors such as water temperature and the amount and quality of food available.

They continue to grow fairly fast until they reach sexual maturity. For males that is generally three to five years, by which time the bottom of the shell (plastron) is about 2.75 to 3.75 inches long. Females mature after six to 10 years, when the plastron is about four to five inches long.

After that, the turtles grow more slowly, about half an inch per year under good environmental conditions. They may eventually stop growing: females top out at around eight inches, and males are smaller. As for how long they live, I’ve seen figures such as 11 years of average life expectancy in the wild; individuals can live 30 years or more in captivity (many people keep painters as pets). 

If you really want to see how old a painted turtle is, you can examine the scales (scutes) that form the upper shell (carapace). There are 13 of these, and they have growth rings much like those of the scales on a fish or the cross section of a tree trunk. 

As for snapping turtles, the basic growth pattern is similar: They grow rapidly after they hatch, reaching five to six inches long after about two years. Then growth is more slow and steady; a typical snapper will reach 12 to 14 inches in 15 to 20 years. Ultimately they can reach up to 19 inches in shell length.

In all turtles, growth rates differ based on environmental conditions. In addition, some individuals can grow faster than others even when the conditions are identical. I suppose that’s all in the genes.

Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected]

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