TurtleChron: Developing a scute increment pattern chronology

As nearly all reptiles and amphibians, turtles and tortoises are ectothermic (cold-blooded), which means that they cannot regulate their body temperature, and are thus depended on the environmental conditions. As a result, their growth speeds up in the warmer months of spring and summer, and slows down in winter. Much like tree rings in the cross-section of the trunk of a tree, this seasonally varied growth shows up a distinct line on the carapace (shell) of a turtle, which can also be used to assess the age of the turtle. Depending on the climatic fluctuations over time, these growth lines form a pattern which is shared by individuals which lived in the same conditions. As with tree-rings, this shell-growth pattern can be matched between different individuals, if their lifespan overlapped sufficiently. This allows to build a master sequence, starting with a living turtle, and going back in time with turtle shells found in herpetological collections and archaeological contexts. The shell of a turtle consists predominantly of keratin, which contains (as all proteins) carbon. This carbon derives from the atmosphere, and is bound by plants, which then get eaten by the turtle, and eventually deposited in the keratin in its shell. Thus, the growth rings in a turtle shell thus preserved the isotopic environment in the year that it formed. This isotopic signal of each individual ring can be measured using radiocarbon dating, resulting in a curve representing the changes over time. This project aims to examine this isotopic archive in the turtle shell for the first time. TurtleChron could allow us to identify regional variations in radiocarbon and thus provide regional, which is not only important for reconstructing past climates, but would also allow to build a regional and thus more accurate calibration curves for radiocarbon dating.

Turtle shells are made primarily of keratin, a protein that, like all proteins, contains carbon. This carbon originates from the atmosphere, is absorbed by plants, and then enters the turtle's body through its diet, eventually becoming part of its shell. The TurtleChron project is based on the idea that the growth rings in a turtle's shell preserve information about the environmental conditions in the year they formed. By measuring the radiocarbon in each individual ring, scientists can potentially reconstruct changes in the environment over time. TurtleChron aimed to investigate this isotopic "archive" in turtle shells for the first time. One of the main goals was to explore whether this method could reveal regional differences in radiocarbon levels, which could help reconstruct past climates. To do this, we examined the growth rings of a population of tortoises living in the Balkans from the 1950s onward. They recorded the rings using high-resolution digital microscopes and applied methods from dendrochronology, the study of tree rings, to analyse the sequences. The project faced several challenges. Initial attempts to document growth rings using macro photography and 3D imaging did not provide sufficient detail. Switching to digital microscopy allowed for better resolution, but only a few individuals had long enough growth sequences to allow meaningful analysis. Despite these limitations, we were able to record compatible growth patterns within individual tortoises. Comparisons between different individuals, however, proved more difficult. These results revealed several surprising findings. First, growth patterns in turtle shells are more individual than previously thought. Second, tortoises often form more than one growth ring per year. Third, scutes (the plates along the shell) show slightly different growth patterns depending on their position, with spine and side scutes varying. Although the method did not yield the expected information for reconstructing past climates, it provided valuable insights into tortoise biology. TurtleChron shows how individual tortoises respond differently to environmental changes, offering a unique perspective on population growth variability and adaptation strategies. TurtleChron demonstrates that even everyday animals like tortoises hold hidden stories in their bodies. By studying something as simple as shell growth rings, scientists can uncover how individual animals cope with environmental change - a reminder that nature's archives are all around us, waiting to be explored.