The living tag experiments

At what age do sea turtles reach sexual maturity? A simple question, but like many others in sea turtle research it is harder to answer than it seems. The difficulty lies in that sea turtles spend the first several years of their lives out at sea where they cannot be easily observed. Since the late 1950s scientist have come up with different methods to measure the time it takes for a turtle to reach sexual maturity in the wild. These included implanting magnets, injecting rare metals and tattooing turtles, which would then allow to identify a turtle when it returns to nest and to deduce its age. However, none of these experiments proved feasible, successful or practical enough to implement in large-scale experiments.

In the early 1980s a new method of tagging sea turtles was developed: the “living tag“. Sea turtles were tagged using a surgical autografting procedure: a small sliver of tissue of a turtle’s carapace (the upper shell) was switched with a small sliver of tissue from the same turtle’s plastron (the lower shell). Since the turtle’s carapace is dark in colour and the plastron is light in colour, the result was a turtle that had a light spot on its carapace and a dark spot on its plastron.

Living tag in the second left costal of a Kemp's ridley sea turtle. photo courtesy of Michael Coyne

A Kemp’s ridley sea turtle with a living tag on its second left costal scute
(photo courtesy of Michael Coyne)

Approximately 950 turtles of three different species were marked in this manner at four different sites. The turtles were then released in the hope that they would be recaptured when they are sexually mature. To be able to recognize recaptured turtles, the particular scute on which the autograft was done coded for the year and location of release of each turtle.

In 2002 one of these turtles released as a hatchling in 1985 was seen nesting. Later that same year a male turtle released as a hatchling in 1983 was observed mating. More turtles were seen again in the following years. All living tag returns were made after 15 years or more.

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(photos courtesy of the Florida Coop Fish & Wildlife Research Unit)

The results of the living tag experiments provided interesting insight into many different aspects of turtle biology, including age-at-maturity and hatchling growth rates. However, it is important to keep in mind that the turtles with living tags were kept for months or even years before being released in the wild to increase their chances of survival. In that sense these results do not exactly mimic natural conditions and the true age-at-maturity of a wild sea turtle still eludes scientists. The use of living tags has never become systematic or widespread but you might still come across one of the turtles tagged in the 1980s with a funny spot on its shell.

A green sea turtle with a living tag on its vertebral scute (photo courtesy of sea

A green sea turtle with a living tag on its second vertebral scute
(photo courtesy of Roberto Herrera-Pavon)