Terrestrial basking and climate change

Terrestrial basking is a rare behavior observed in populations of green sea turtles in Hawai’i, Western Australia, and the Galápagos Archipelago. Being cold-blooded, the main reason why turtles bask on land is probably to regulate their body temperature but scientists speculate that terrestrial basking may also aid immune function, predator avoidance, and may even prevent unwanted courtship.

New research published in Biology Letters examined the relationship between terrestrial basking and climate. Having counted the number of turtles that bask on one Hawaiian beach every day for six years, the researchers found that terrestrial basking peaks in the year when the sea surface temperatures are lowest. Terrestrial basking generally happens when sea surface temperatures fall below 23°C. This suggests that terrestrial basking is a response to seasonally cool ocean temperatures.

Picture 2

Basking varies seasonally in concert with cool SST. Green circles are standardized anomalies of the number of turtles observed basking weekly at Laniakea, Oahu. Blue circles are weekly AVHRR SST data for this location. Thick dark lines are the Fourier series for each timeseries. (Source: Biology Letters)

However, since the sea surface temperatures at the sites where turtles bask on land is warming on average 0.04°C per year, the researchers predict that in the future the waters will be warm enough that the turtles will no longer come on land for warmth. The researchers estimate terrestrial basking may cease in Hawaii by 2039, in Australia by 2086, and in the Galápagos by 2102. Since other populations of marine turtles are successful without having to resort to terrestrial basking, this will probably not have drastic negative impacts on these green turtle population, but this does mean that beach goers of the future will not have the privilege of sharing the beach with napping turtles.


The riddle of the Ridley

In the 1940s much less was known about sea turtles than we know today. Dr Archie Carr, who has been called the father of sea turtle research, spent his life trying to understand sea turtle biology and ecology. One particular species puzzled him: Kemp’s ridley sea turtle. Of all the species of sea turtles it was the only one that had never been observed nesting. Understanding Kemp’s ridleys became one of Dr Carr’s lifelong quests.

Dr Archie Carr fixing an early radio tracking device on a green sea turtle (photo couretsy of the Sea Turtle Conservancy)

Dr Archie Carr fixing an early radio tracking device on a green sea turtle
(photo courtesy of the Sea Turtle Conservancy)

The fact that no scientist had ever seen a Kemp’s ridley nest gave rise to wild conjectures about the turtle. Some speculated that the species had lost its ability to reproduce and that the current ridleys were the last members of a line on its way to extinction. Some said that the species did not mate but that hatchlings would arise by spontaneous generation. Others thought that the species laid saltwater-resistant eggs at sea. Another hypothesis was that the species gave birth to live offspring as do mammals. Or perhaps all the turtles of the species nested on one beach that had not yet been discovered by scientists.

Dr Carr deemed the last theory most scientifically sound and so began a search for the nesting ground of Kemp’s ridley. He surveyed hundreds of beaches in the Gulf of Mexico and in the Caribbean where the turtle was most often sighted. He spoke to fishermen and turtlers to gather as much information he could on the elusive turtle. Over the years, he walked hundreds of kilometres of beaches looking for unfamiliar turtle tracks. However, more than a decade after he started his search the turtle still evaded him.

The year 1962 brought a breakthrough. Dr Carr was presented with home movie footage shot in 1947 by Mexican engineer Andrés Herrera. The short 16 mm film showed a beach with thousands of turtles nesting in bright daylight. Dr Carr identified the turtles to be the Kemp’s ridley that he had been searching for all these years. His search had not just led him to one specimen: it is estimated that 40,000 turtles nested on that beach in northern Mexico that day!

The Ridley riddle  is still not entirely solved, but locating its nesting grounds was definitely a significant milestone. To Dr Carr the footage was simply groundbreaking:

“The film was short. It was shaky in places, faded with time, and rainy with scratches. But it was the cinema of the year all the same, the picture of the decade. For me really, it was the movie of all time. [..] It made Andrés Herrera in my mind suddenly a cinematographer far finer than Fellini, Alfred Hithcock, or Walt Disney could ever aspire to be. […] To me Andrés Herrera is a man who ought to be knighted, or to get a Nobel Prize, or some kind of prize.”

Video courtesy of Sea Turtle Inc.

For a more complete narration of Dr Carr’s quest for Kemp’s ridley I highly recommend his books The Windward Road (Chapter 1: The Riddle of the ridley) and So Excellent a Fishe (Chapter 5: Arribada).