Off the East Coast of Australia lies one of the most vibrant and important coral reefs on Earth– The Great Barrier Reef. This oceanic hotspot is home to thousands of different species and provides Austrailia with up to £6 billion in revenue every year. However, one particular member of the reef system is on the verge of extinction– The Pacific Green Sea Turtle. Green Sea Turtles are a favorite of many tourists that travel to visit the reef every year. Their majestic movements and gentle nature are characteristics that make them extremely exciting to see underwater. However, when a team of scientists from the United States’ National Oceanic and Atmospheric Administration in La Jolla, California (NOAA) traveled to Austrailia to study the species, they noticed something shocking: there were no male turtles. As the team swam around the reef examining the turtles while they fed on sea grass and rested amongst the corals, they noticed that all the animals at the site were female. Although it is not uncommon to see a large amount of female turtles at this site because it also serves as a nesting ground, the scientists were baffled that there were no males at all. So, where did all the male turtles go? The group’s recently published study in the journal Current Biologysheds some light.
The study revealed that climate change is likely to cause the extinction of these elegant and important sea creatures as increased temperatures transform the species into an all-female population. How could this happen? Sea-turtles, along with other reptiles, are temperature-dependent sex determinant (TSD) animals– meaning that the sex of their offspring is actually determined by the temperature at which their eggs are incubated. According to the NOAA study, unlike other species where the sex of the offspring is genetically predetermined before fertilization, research on turtles shows that, ‘if a turtle’s eggs incubate below 81.86° Fahrenheit [27.7°C], the turtle hatchlings will be male. If the eggs incubate above 87.8° Fahrenheit [31 °C], however, the hatchlings will be female. Temperatures that fluctuate between the two extremes will produce a mix of male and female baby turtles.’ When sea-turtles are ready to have offspring, they come to shore, make a nest on the beach, and bury their eggs in the sand for an incubation period of fifty-five days. Climate change is causing the temperature of the sand where these turtles lay their eggs to rise above the 31 °C temperature that determines their gender to be female. As the number of male turtles diminishes, the increasingly female population has fewer males to mate with causing the overall number of new turtles born each year to shrink, and eventually disappear.
In order for the research team to collect the data for this study, they had to travel to multiple Green Turtle nesting sites around Australia. Green Turtles commonly nest on both the Northern and Southern shores of the Great Barrier Reef, but nowhere in-between. The turtle population in each of these regions is genetically distinct– meaning each region hosts a unique species of Green Turtle, but the two species have identical nesting habits. During mating season, Green Turtles return to the same nesting site where they were born and mate near the waters off of their nesting beach. Because female Green Turtles can store sperm for an entire mating season, they will lay multiple clutches of about 100 eggs throughout the season. Once the eggs are laid, they will incubate for fifty-five-days before they hatch. Once the baby turtles are born, they scurry into the ocean and live their adult lives in open waters until they reach sexual maturity at the age of 25 and return to their nesting sites to have their own children.
Once the scientists conducting the study arrived at each nesting ground, they determined the sex of each adult turtle in the current population in order to determine the sex ratio. To do this, the team would jump in the water behind a group of swimming turtles and would carefully lure them to shore. Once the turtles were safely on sand, they took note of their gender and conducted a quick blood and DNA test before releasing the turtles back in the water. Once they had gathered and analysed all their data, they found that in the Southern nesting sites, where water and sand temperatures are cooler, female turtles only amounted to 65%-69% of the total population– which is only slightly higher than the number of documented males. However, in the North, where water and sand temperatures are much hotter, they discovered that female turtles accounted for 99.1% of the juvenile population, 99.8% of the subadult population, and 86.8% of adult population– drastically outnumbering the males. This data clearly shows that when sand temperatures become hotter in warmer climates, the population of female turtles outnumber male turtles by a ratio of more than 116 to 1. Thus, they concluded that as climate change inevitably causes global temperatures to rise, it will in turn cause the mass extinction of sea-turtles.
Map of turtle nesting sites courtesy of Jensen et al: https://www.cell.com/current-biology/fulltext/S0960-9822(17)31539-7
Why should people care? Not only are sea-turtles some of the most-loveable and harmless species on earth, but they also play an important role in maintaining the health of many marine eco-systems around the world. Green Sea Turtles are one of the only species of oceanic herbivores. Without these turtles to maintain the ocean’s many sea-grass beds, the sea-grass will overgrow, disturb currents, grow harmful algae, and destroy the environment around them. This will, in turn, lead to the destruction of other ocean-dwelling species that live in the surrounding environment. Sea-turtle nesting sites are also vital for beach and dune health. After the baby turtles hatch, their decomposing eggshells provide important nutrients to ground soil that give life to many plant and animal species in the surrounding dunes. Finally, sea-turtles provide the only source of food for many fish species. As turtles cruise around reef systems, healthy algae grow on their shells which become the perfect meal for many types of fish. Without the turtles to feast off of, these fish will die as well. While sea-turtles may just seem like cute creatures that are fun to snorkel with, they actually play a vital role in maintaining the delicate balance of earth’s oceanic eco-systems. Without making a drastic effort to protect these magnificent creatures and reverse the effects of climate change, sea-turtles will surely go extinct.
Banner Image: Image Courtesy of National Marine Sanctuaries via Flickr © 2016, public domain