Last month during a Wood Turtle (Glyptemys insculpta) survey I stumbled upon one of the most vibrantly colored turtles I have ever seen. Easily recognized by their brown shells, black heads, and distinctly orange necks and limbs, even the more drably-colored Wood Turtles are quite striking, but this particular animal really stood out to me. She was resting on the bank of a river on a cool rainy day and did not attempt to flee as I approached. As I was handling her to take some measurements she seemed to have good body mass and was in great physical condition, but to my great surprise, was missing both of her hind legs and had been for quite some time.
Very few vertebrates can survive the loss of a limb, but finding terrestrial turtles missing a leg is actually quite common. Hatchling and juvenile turtles are eaten by pretty much every predator out there: crows, raccoons, foxes, herons, snapping turtles, etc. The shell of a hatchling turtle is soft and at small sizes they are little more than little turtle Oreos wandering the landscape waiting to be eaten. As they grow, however, their shells get harder and harder until they reach a size at which they are protected from most predators. The shell of an adult Wood Turtle can keep pretty much any native predator from killing them, but some dexterous predators, such as raccoons, are able to pry a leg out to gnaw on for a bit. Predators were long assumed to be the primary reason for limb loss in Wood Turtles, but more recent studies point to agricultural equipment as another possible cause. Indeed, over 10% of adult Wood Turtles from most populations are missing a limb. What is astonishing is that they are able to survive such losses and sometimes live for decades afterwards.
One site The Orianne Society recently visited was first put on our radar after the landowner reported finding a one-legged juvenile Wood Turtle in their field. Unsurprisingly, that turtle, which had very recent injuries, died shortly after being found. When we visited that property we found two adult turtles at least 40 years old, both of which were missing one of their limbs and had recovered from those injuries. While we later did find turtles at that site with all four legs firmly attached, visiting that particular property was an eye-opening experience for us in regard to the prevalence of limb loss in some Wood Turtle populations.
Wood Turtles spend substantial amounts of time in close proximity to river banks, which nocturnal mammalian predators often follow during their foraging expeditions, which puts Wood Turtles at heightened risk for predation attempts. It does appear as though three-legged turtles do live shorter lives on average than their four-legged counterparts, but there isn’t much data on the matter, and many do live long productive lives absent one limb and manage to travel great distances over land in the process. Two-legged Wood Turtles, on the other hand, have a much harder time and adopt a much more aquatic lifestyle.
What I find fascinating about the younger two-legged turtles I have seen that the turtle’s shells both had clues about when they sustained their injuries. Wood Turtles develop growth rings on the plates (scutes) of their shell which allow people to age them fairly accurately until they are between 15 and 20 years old. You can even tell how much relative growth they put on year to year by comparing the size of the growth rings. Female Wood Turtles usually have consistently good growth until about 10 years or so of age when they start reproducing and their energy is re-allocated to egg production. One of the two-legged turtles, however, had rapid growth the first three years of her life, then her growth rate slowed significantly over the following 7 years. While knowing the exact cause for this is difficult, a logical explanation is that she was mutilated by a predator at three and has spent most of her life with no hind limbs. Unfortunately, she will be unable to excavate a nest and will be put at increased risk of future predation, not to mention the possibility that a male turtle could drown her during copulation, which can even happen to four-legged turtles. Similarly, the other two-legged juvenile showed a similar slowing of growth at age four through its current age of six.
What we don’t really know, however, is what percent of turtles that lose limbs actually survive and how many die of trauma or resulting infection. Because the ones that do survive can live a long time, biologists usually just encounter the ones that made it, so it’s easy to jump to the conclusion that most make it, when in reality the survivors may be a minority. Most Wood Turtle populations are in sharp decline and such injuries are much more common in habitats close to areas with lots of people. Raccoons, for example, are more common in urban and agricultural settings than in remote wilderness, so turtles are at greater risk in places with more people. These sorts of sub-lethal injuries may not always kill turtles outright, but do impact populations and can exacerbate population decline over time.