How Old is That Sea Turtle?

By LARISA AVENS

Maybe folklore and popular culture have imbued turtles with the aspect of the eternal, or maybe sea turtles seem improbably and impossibly large (and presumably old) compared to most of their terrestrial and freshwater counterparts. Whatever the reason, one of the most common questions that sea turtle researchers and conservationists hear from the public is “How old is that turtle?”

From the moment they hatch and leave the beach to disperse in the marine environment, sea turtles make it very difficult for us to calculate how old they are. Their long migrations and their multiple habitat shifts often span entire ocean basins and thus impede our ability to follow wild individuals throughout their lives so we can directly monitor sea turtle age and growth.

Captive turtles have shown remarkable growth potential, but hard-won mark-recapture data for wild turtles have typically demonstrated slower overall growth rates. And because there is so much variability in growth rates across species, populations, and even individuals, it is impossible to accurately predict age on the basis of size alone. Moreover, unlike some turtle species, sea turtles do not retain lifelong records of annual growth increments in the scutes of the carapace or plastron.

In light of those challenges to directly quantify sea turtle age by size or appearance, researchers have explored a number of indirect approaches for studying growth and aging among live turtles. Unfortunately, attempts to relate rates of change in molecular or chemical “clocks” to age in wild sea turtles have been hampered by limited information about individual histories, such as influences of heredity, thermal environments, and stressors.

Another indirect approach to studying growth and aging is to examine growth increments in the bones of dead animals (similar to using tree rings to estimate a tree’s age), a practice known as skeletochronology. Given the large numbers of sea turtle strandings that occur worldwide, this method makes it possible to collect age and growth data relatively rapidly, as long as a number of considerations are recognized and addressed. These considerations include (a) finding the most optimal bone and processing method to measure skeletal growth marks, (b) verifying whether the marks are deposited annually to allow age estimation, and (c) defining the relationship between bone and body growth to permit somatic growth rate calculations from bone growth mark spacing.

Because early growth marks at the center of the bone are often absorbed in larger juvenile and adult sea turtles, it is also necessary to collect samples from all life stages so we can develop predictive models that account for any early marks that were lost. Finally, because this method is limited to studying stranded turtles whose cause of death is typically unknown, sample sizes must be large enough to ensure that the data are truly representative of the study population (i.e., finding the signal in the noise).

Over the past few decades, advances have been made in meeting such requirements, primarily for hard-shelled sea turtle species. Recent skeletochronological studies have generated size-at-age relationships and somatic growth rates for individuals and populations over periods spanning decades. Those studies provide valuable information regarding long-term, large-scale patterns in age and growth. One of the most important insights recently highlighted through correspondence among mark-recapture, captive-rearing, and skeletochronology data is that a spectrum of sizes at any given age is possible, depending on interactions among a suite of individual-specific influences and experiences. As a result, the ages and sizes at which wild sea turtles transition between life stages and mature will vary extensively.

Admittedly, this approach is not very helpful for answering the question of how old any particular live turtle might be. That being said, using the same standardized skeletochronological approach for wild loggerheads and Kemp’s ridleys in the western North Atlantic has provided a valuable opportunity for comparison between species. The time to maturation for loggerheads—as well as their reproductive longevity—appears to be two to three times longer than that for Kemp’s ridleys, thereby highlighting species-specific life history strategies and potentially providing a framework for evaluating relative influences of anthropogenic threats and management approaches.

Characterization of sea turtle age and growth using diverse approaches is ongoing, and additional comparisons among populations and species will be forthcoming. In addition, by our combining skeletochronology with recent advancements in stable isotope and trace element analyses, we can now integrate age, growth, foraging ecology, and habitat use data, which further increases our understanding. As new technologies are developed, refined, and applied, we will continue to make progress on solving the mystery of how old that turtle really is.

This article originally appeared in SWOT Report, vol. 14 (2019). Click here to download the complete article as a PDF.