Agreed James Estes of the University of California at Santa Cruz, "The length of the method -- because of the isotopic analysis method -- is really unprecedented."
Vander Zanden used a small biopsy punch tool to gather pencil-eraser sized shell samples from adult female turtles while they were nesting at Cape Canaveral National Seashore in Florida. Removing the samples, which cut away only the dead tissue of the shell, is harmless and painless to the turtles.
She ground the samples into thin layers and analyzed them using a mass spectrometer, a machine that separates stable isotopes according to charge and mass.
The higher an animal on the food chain, the more heavy stable isotopes it accumulates, the greater the ratio of heavy to light isotopes in its tissue. Different ocean latitudes, meanwhile, have different ratios of light and heavy isotopes, ratios also incorporated into shells or other tissues.
So while the analyses revealed that the turtles were surprisingly different in their individual diet or travels -- and that they maintained these differences over the dozen years of growth reflected in the shell samples -- it did not specify discrete food items or locations.
"The problem with stable isotopes is that diet and habitat are kind of confounded," Vander Zanden said. "So we can't necessarily parse out what is causing these differences. Whether this turtle is eating just blue crabs or is eating whelks. Whether this turtle is eating in New Jersey or in the Bahamas."
She said she will seek to sort out that question in the remainder of her dissertation research -- with luck filling in major gaps about a species once celebrated as
|Contact: Hannah Vander Zanden|
University of Florida