shredded turtle foot

found by the dog

The Question: Look what my dog dragged into our home! Can you help us identify who/what/where this claw came from?? It was about the size of my palm. My son says it looks like a dragon paw!

Submitted by: Susan, Wisconsin, USA

(click on photos and graphics to expand)

The Short Answer: The only wild animal in Wisconsin that this could possibly come from is a large snapping turtle. Three very similar species of snapping turtles are found only in the Americas, with a Central American version (Chelydra rossignonii) and a South American version (Chelydra acutirostrisI) joining our North American species (Chelydra serpentina). The larger alligator snapping turtle (Macrochelys temminckii), is in the same family, the Chelydridae, but its range doesn’t extend into Wisconsin.

The common snapping turtle is a large turtle, with a maximum length of about 45 cm (18 inches) and weight of about 20 kg (45 pounds) in the wild. Snapping turtles, like most turtles and reptiles, have generally been considered to exhibit “indeterminate growth.” In a species with indeterminate growth, individuals develop at a rate that is usually dependent on the amount of food available, and they typically continue to grow throughout their lifetimes, although at a much slower rate as adults. Most fish, for example, continue to grow as long as they live. Animals with “determinate” growth, on the other hand, include mammals and birds, which, if given enough food, grow at a fairly predictable rate until reaching a certain age, at which point they stop growing at all (except for gaining weight).

baby snapping turtle

baby snapping turtle

The generalization that all reptile species, including turtles, exhibit indeterminate growth is definitely not true, and as we’ve learned more about growth patterns, the picture has become more complicated, even for species that do exhibit indeterminate growth (more on that later). But it is mostly true that most snapping turtles grow throughout their lives. They grow much faster as juveniles, with a growth rate of about 1.5-3 cm per year (1/2-1 inch) for the first decade of life, but only about .2 cm per year (less than a 1/16 of an inch) after the age of about 20. This rate can vary quite a bit, depending on the amount of food and the length of the active season. The growth rate of snapping turtles in Algonquin Provincial Park in Ontario, for example, is about half that of more southern turtles in the first few years of life. And the farther north you go, the longer it takes for turtles to reach reproductive maturity.

If you search online, you can find estimates of the maximum age of snapping turtles that range from 30 years to 150. The 30 year estimate is undoubtedly wrong, and the 150 years may be extreme also. But turtles in general can be very long-lived. A recent study (Congdon et al., 2013) estimated that in some populations it would take 75 years after reaching maturity at the average growth rate of adult snapping turtles to reach the size of the largest turtles in the study. Adding that to the typical age of about 15 years at reproductive maturity gives an age of about 90 years for some of the largest turtles. This is a very rough estimate, but it suggests that some big snapping turtles have lived a long time – especially in the northern parts of the snapping turtle’s range, like Wisconsin, where turtles are likely to grow slowly.

Your “dragon paw” may have come from a turtle that hatched before the Great Depression.

It’s not surprising that snapping turtles live a long time if you look at their adult survival rate. By the time a snapping turtle reaches reproductive maturity, it is big enough and formidable enough to have very few natural predators. In fact, in many parts of North America, the biggest cause of mortality for snapping turtles is people:  boat propeller strikes, cars, and trapping by people for food.

If I had to take a wild guess on this turtle, given that your dog presumably found it in a suburban neighborhood, on land, it was probably hit by a car.  It may have later been torn apart by a coyote, a fox or your dog, but that’s probably not what killed it.  Dr. Justin Congdon, at the Savannah River Ecology Laboratory, has studied snapping turtles for more than 30 years. He gave me a different theory. “When people catch and butcher snappers, they typically cut off the head and feet first because reflex movements of the turtle make both a risk to the person doing the butchering. I suspect that’s what happened here and the dog found one of the discarded feet.”

common snapping turtle

common snapping turtle

More on Growth of Snapping Turtles: The Congdon et al. study looked at tagging and recapture data for nine species of North American turtles and found that all had indeterminate growth, in that about 80% of the individuals in the population continued to grow as adults. Interestingly, however, about one out of five adult turtles didn’t grow at all, even across time periods of a decade or more. So it appears that in these turtles, at least, the determinate/indeterminate dichotomy may not be so clear cut.

There is an evolutionary trade-off for animals with indeterminate growth that pits growth as a juvenile versus growth as an adult. The larger a female turtle, the more eggs she can lay. And generally, the larger a male turtle, the more battles he wins against other males in the competition to mate with females. It’s better to be bigger.

The trade-off occurs because once turtles begin mating, the energy expended by males competing and the energy expended by females to form and lay eggs slows their growth rate to a crawl (pun intended). Turtles that postpone mating so they can grow larger before they begin reproducing are at risk of being eaten or otherwise killed without ever producing any offspring. If they survive to that larger reproductive size, however, they may enjoy a long adulthood of prolific reproduction.

Turtles that begin mating at a smaller size, and then continue to grow slowly after that, may take a very long time to reach the size needed to produce large clutches of eggs or win their battles against other males. They are, however, more likely to have at least some offspring before something drastic happens to them.

Let’s hope your turtle had a long and successful reproductive life. Snapping turtles are still common across their range, but their numbers have decreased sharply in some places, as people overharvest them for consumption in the U.S. and shipment to other countries as well. Because few hatchlings make it all the way to adulthood, it will take a long time for snapping turtle numbers to recover to the levels prior to human harvesting.

Sources:  Special thanks to Dr. Ronald J. Brooks and Dr. Congdon for their help in reviewing this article.

Congdon, J D, Gibbons, J W, Brooks, R J, et al. (2013). Indeterminate growth in long-lived freshwater turtles as a component of individual fitness. Evolutionary ecology, 27(2), 445-459.

Galbraith, D A, Brooks R J, & Obbard M E. (1989). The influence of growth-rate on age and body size at maturity in female snapping turtles (chelydra-serpentina). Copeia, (4), 896-904.

Congdon, J D, Dunham A E, & Sels R. (1994). Demographics of common snapping turtles (chelydra-serpentina) – Implications for conservation and management of long-lived organisms. American zoologist, 34(3), 397-408.

Armstrong, D P, and Brooks, R J. “Estimating Ages of Turtles from Growth Data.” Chelonian conservation and biology 13.1 (2014):9-15.

Bennett, A M, & Litzgus, J D. (2014). Injury rates of freshwater turtles on a recreational waterway in Ontario, Canada. Journal of Herpetology, 48(2), 262-266.

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