Is global climate change already having an impact on sea turtles? The answer is a tentative yes. The warming trend in our shining seas and sands may well be altering the fate of sea turtles. A recent interview with Kartik Shanker, in his office at the Indian Institute of Science in Bangalore, revealed important clues.
Pondering over global climate change predictions, the images that immediately fill the mind are those of swelling seas, melting icecaps, changing oceanic currents and storms of greater ferocity. Rising sea levels could cause “coastal squeeze”— where inundated beaches (now used by nesting turtles) can no longer migrate landward because of developments along coastlines.
What affects oceans and coastlines will very likely affect sea turtles, as recently reviewed by Lucy Hawkes, her colleagues and many other scientists. And perhaps the most ominous of recent links to global climate change begins at the very beginning— inside the sea turtle nest, inside the eggs.
“Our initial results are showing…most of the temperatures that the nests experience is above the pivotal temperature,” Kartik Shanker summarized his long-term studies based in Rushikulya, Orissa, on the east coast of India— one of the world’s arribada (mass nesting) sites for olive ridley sea turtles.
“Reptiles have temperature-dependent sex determination, which means that that they don’t have sex chromosomes. The temperature of incubation determines the sex of the offspring. In turtles, there is a pivotal temperature where both females are males are produced. At higher temperatures females are produced and lower temperatures males are produced.”
For olive ridleys, the pivotal temperature hangs just above 29 degrees Celsius.
These days, egg incubation temperatures in the sand remain higher than pivotal. Does this mean that most hatchlings emerging from the famous mass nesting site are born female? Laboratory examinations are pointing in that direction. And in the long term, a female-dominated population could be devastating to the Bay of Bengal olive ridleys.
In mid-April, 2012, a waning crescent moon heralded the end of egg incubation in Orissa. On April 17, mass hatching began. Wave after wave of newly emerged hatchlings rippled down the watery sands.
Forest Department staff, researchers from the Indian Institute of Science and their assistants guided many of the disoriented young to sea with flashlights (when they were drawn towards town and factory lights). Days after the mass hatching events were over and most of the crowds gone, Kartik’s students, researchers and assistants began to assess the nests and inspect the hatchlings that failed to emerge. Those samples came back to the laboratory with Ema Fatima, a research assistant.
In Kartik’s laboratory, next door, Ema and Amrit Kumar Mishra stained the histology slides a fuchsia-pink. We crowded around the slides, which have been prepared every year for the past 5 years. “Our work in the lab is beginning to show our prediction…that most of the hatchlings would be female…”
Where are the cooler sands or cooler nesting seasons that might add balance to this stark reality? Certainly in the past, that is how sea turtles “regulated” their sex ratios. For one thing, there used to be more than one arribada. Back in the 70s and 80s, mass nesting at Orissa’s other arribada site, Gahirmata, used to happen in December-January, a cooler time of year. But that arribada no longer happens. The spring arribada now contributes to most of the olive ridley population in the Bay of Bengal.
Climate change is a concern not only because mean temperatures are rising; it is also a concern because that warming trend is now happening at a quicker pace than ever recorded. And most of the warmth is absorbed by the oceans—about 80% of it.
Without a balance of males, the future of olive ridleys is worse than uncertain. And at even higher sand temperatures, the eggs do not survive at all. “We’ve seen this in our work with hatcheries where nests laid very late in the season have extremely low hatching success…”
Can sea turtles—and indeed other marine life—keep up with the changes? It is an unknown shadowed by global unknowns.