I would wager that the oldest terrestrial animal in almost every state in the lower 48 is some sort of turtle, but knowing exactly how long they can live is a fairly tricky matter to resolve. Wood Turtles (Glyptemys insculpta), for example, have been confirmed up to at least 55 years of age in the wild, but experts agree that is likely nowhere near their actual potential longevity. To determine that, a biologist first needs to find a turtle when it is young enough to age with confidence. Because Wood Turtles develop annual growth rings (annuli) on their shells, you can do that for about the first 20 years of their life, after which point they stop growing*. Then the turtle needs to be marked in a way that will allow a future biologist to identify that same turtle (usually through notches filed into the edge of the shell). Finally, that future biologist needs to capture that same turtle 50 or more years later, recognize the shell notches, look up the original data (if it still exists), and then look for a matching notch code with a corresponding age. That does not happen very often, but that is how we know Wood Turtles can survive at least into their 50s (turtles marked as adults in the 1970s were recaptured in 2012-2013).
Through those long-term mark-recapture events, however, we know that closely-related species live much longer. Blanding’s Turtles (Emydoidea blandingii) recently had their longevity record bumped into the late 80s, and Eastern Box Turtles (Terrapene carolina) have been confirmed over 100. While it is no stretch of the imagination to think that Wood Turtles can also crack 100, there is additional evidence to support the theory that their longevity may far exceed five decades.
After Wood Turtles stop growing, their shells begin to wear down and, over time, the annuli become smooth and polished, eventually to a point where they vanish entirely. While the rate of smoothing can vary from turtle to turtle based on their habitat use, you can roughly estimate a Wood Turtle’s age based on the percent wear on the bottom of their shell (their plastron). Wood Turtles in their 50s still have quite a bit of texture on their shell, but it is common to find them with completely worn down shells that look more like polished stone on close examination. Based on photographs comparing the change in shell wear over time, it is estimated that complete wear of the top of the shell (carapace) should take about 80 years. How long can they survive beyond that remains a matter of speculation. When I give presentations on the subject of Wood Turtle conservation, the number I throw out there is that if nothing kills them first, they can probably live into their 90s – a statement that often draws audible gasps from the audience.
Exactly how long Wood Turtles can survive in the wild probably doesn’t matter other than to satisfy our natural curiosity. The bottom line with Wood Turtles and other similar species is that the survival of their populations are absolutely dependent on the longevity of adults. Wood Turtles might not lay eggs until reaching 14 or more years of age, but once they start laying they will continue to do so every spring until they die, no matter how old they get. Those eggs have very low survival rates, as do the hatchlings (pretty much everything eats baby turtles, even chipmunks and Blue Jays). Their odds of surviving another year increase as they grow, however, and adults in healthy populations can have survival rates just barely below 100%. If nothing kills them first, a female Wood Turtle might lay eggs 50 or more years in a row, and eventually some of those young will survive long enough to join the adult population. Unfortunately, the river valleys where Wood Turtles spend much of the summer are heavily impacted by human activity, and these turtles now encounter many threats that their populations are unequipped to deal with.
The same river valleys where turtles forage in the summer tend to have fertile soils and are the easiest places to farm, so much of the turtle’s habitat is now dominated by agricultural land use. Furthermore, river valleys are also the easiest places to build roads, so many of the best Wood Turtle streams are paralleled on one or both sides by major highways. With Wood Turtles roaming over a thousand feet from water to forage, sometimes spending months on end in the river valleys, their odds of surviving to old age can be quite low. Eventually, they get hit by a car, run over with mowing equipment, or removed from their habitat to be illegally kept in captivity. With expected lifespans in such areas cut drastically short, not enough of these turtles are able to replace themselves through reproduction before being killed and their populations are shrinking rapidly.
Thankfully, there is growing momentum in the Northeastern United States to conserve habitat of Wood Turtles and other species of conservation need, and both state and federal programs can sometimes pay farmers to set aside land along rivers to restore water quality, reduce erosion, and provide critical foraging habitat for the turtles. Doing this piecemeal across the entire species’ range might not result in any single population being spared an eventual collapse, but by targeting conservation efforts to the populations deemed most critical to the species as a whole, I am optimistic the species can be saved in many of the rivers they reside in today.
Note: For a thorough review of existing data on Wood Turtle longevity and survivorship, head to page 51 of Status and Conservation of the Wood Turtle in the Northeastern United States. Many of the numbers I mentioned above were compiled in that document, which pooled information from a large number of data sources.
* Counting annuli is not a reliable means of determining age for many other species, but is considered fairly accurate for Wood Turtles for their first 15-20 years of life.