The green sea turtle (Chelonia mydas)
The impacts of a warming planet can be seen in virtually every ecosystem. Increased temperatures can result in critical changes to a species life-history traits, which can have longer term effects on the greater population. For many reptilians, including crocodiles, some freshwater turtles and all species of sea turtles, sex is determined by the incubation temperature during embryonic development (as temperature-dependent sex-determination, TSD). For sea turtles, the TSD phenomenon produces more males under cooler temperatures, while warmer temperatures produce more female hatchlings. The pivotal temperature, being the incubation temperature that produces a 50/50 ratio of each sex, can vary among species and populations. However, the difference in temperature that produces either %100 males or %100 females is only a couple of degrees Celsius. Furthermore, elevated incubation temperature not only produces feminized clutches, but also causes high mortality clutch mortality.
Michael Jensen, a marine biologist with the National Oceanic and Atmospheric Administration in La Jolla, California and his colleagues have used novel methods to examine the sex ratios of Green sea turtles, Chelonia mydas, in Australia’s Great Barrier Reef (GBR). Turtles are in the GBR are divided into two genetically distinct populations found at opposite ends of the GBR, the northern stock (nGBR) and southern stock (sGBR). Work recently published in Current Biology describes using genetic markers and a mixed-stock analysis (MSA), combined with sex determination through laparoscopy and endocrinology, to link male and female green turtles foraging in the Great Barrier Reef (GBR) to the nesting beach from which they hatched.
Their findings when looking the adult population as a whole were 1:4.2 Males:females, which for seas turtles is not out of the ordinary according to Jensen. However, when teasing the data into the two genetically separate populations the results were more concerning. In the cooler southern Great Barrier Reef, there was a moderate female sex-bias comprising about 67 percent. While, the warmer sand-soaked Northern nesting beaches were extremely female biased (99.1% of juvenile, 99.8% of subadult, and 86.8% of adult-sized turtles)— with one male for every 116 females. Combining their results with temperature data they suggest the nGBR population has been producing primarily females for the last 20 years and that complete feminization is very possible in the future if climate change continues in its current rate.
Most sea turtles tend to nest during the warmest parts of the year, suggesting that the female bias is an adaptive trait, varying the pivotal temperature and threshold, by adjusting the nesting ground choice. However, based on strong natal homing and slow sexual maturity sea turtles are likely very vulnerable to the repercussions of climate change. Although females can store sperm and polygynous male behavior, males have been shown to exhibit philopatry to courtship areas near their natal beaches. Therefore, the future of the nGBR population, one of the largest sea turtle populations in the World, is in the balance.
To read this fascinating article access the full pdf text here
http://www.cell.com/current-biology/pdf/S0960-9822(17)31539-7.pdf
Jensen MP, Allen CD, Eguchi T, Bell IP, LaCaseela EL, Hilton WA, Hof CAM, Dutton PH (2018) Environmental Warming and Feminization of One of the Largest Sea Turtles Populations in the World. Current Biology. E4: 154-159. http://dx.doi.org/10.1016/j.cub.2017.11.057
