Reptile sex ratio turns turtle owing to global warming

Rising global temperatures are distorting the sex ratio among some reptiles and may bring the populations of some reptiles, such as sea turtles, to the brink of extinction in the not-so-distant future.

Unlike in humans and other mammals, it is not sex chromosomes that decide whether a green sea turtle (and some other reptiles too) will be male or female; it is the ambient temperature that the egg is exposed to that decides the gender of the hatchling.

A team of scientists from the United States and Australia studying populations of green sea turtles at the Great Barrier Reef (GBR), off the coast of Queensland in north-eastern Australia, has found that the warming world is turning most young turtles into females. Many reptile species, including some turtles, have what scientists call temperature-dependent sex determination, where the sex of an individual is determined by the incubation temperature during embryonic development. In other words, depending on the sand temperature that they are exposed to, a clutch of eggs could hatch to all males or all females, or a mix of both. The temperature at which there is more or less an equal mix of males and females is called pivot temperature. The pivot temperature for green sea turtles is 29.3 degrees Celsius. If the sand temperature is a couple of notches above the pivot temperature, which is the case due to rising global temperature, the newly hatched turtles will predominantly be females.

The scientists, led by Michael P. Jensen at the National Oceanic and Atmospheric Administration at La Jolla in California, in a paper that appeared in the journal Current Biology on January 8, reported a palpable difference in the gender mix of green sea turtle populations found on either ends of the 2,300-kilometre-long coral ecosystem. While the green sea turtle rookeries in the relatively cooler southern fringes of the GBR had 65 to 69 per cent female turtles, up to 99 per cent of young turtles found in the northern end, where temperatures are relatively high, were females.

This unusually skewed sex ratio caused by a warmer world could pose a great danger to these reptiles unless these key rookeries are protected from heat.

T.V. Jayan