More info about this fact

Did you know that the temperature of a turtle nest determines the sex of the turtle hatchlings? In mammals the sex of babies is determined during fertilization and depends on which combination of sex chromosomes they inherit from their parents, but not all species are the same. In the case of reptiles such as crocodiles, alligators, tuatara and turtles, the sex is determined after fertilization and depends on what temperature the eggs are incubated at. This is known as temperature-dependent sex determination (TSD).

Let’s have a closer look the green sea turtle, Chelonia mydas. These turtles live in tropical, and subtropical seas. (Fun side note, they are called ‘green’ turtles because the fat under their carapace/shell is green, not because of the color of their shell.) When females of this species reach sexual maturity, they return to breeding grounds near where they themselves hatched in a process called ‘natal homing’.

Following successful breeding the females clamber up the beach and begin to dig a hole in which to lay their eggs. This hole must be above the high tide line, or their eggs will drown, and deep enough to avoid being dug up by predators. There are typically between 100 and 200 eggs in each clutch laid by a female, and these eggs are covered up by their mother and left to incubate.

The key period for sex determination occurs between days 20 and 40 after being laid. At this time, the temperature of the nest will determine if the hatchlings will be male or female. If the temperature of the nest is less than 28.5°C the hatchlings will be male, whereas if the nest is 30.3°C or more the hatchlings will be primarily female. Nests where the temperature fluctuates between these two extremes result in a mix of male and female hatchlings. This process is so finely tuned that eggs in the middle of the clutch versus eggs on the edges of the clutch may be at different enough temperatures to result in male and female hatchlings.

As you can imagine, a warming climate is a serious concern for turtle conservationists. If turtles are no longer able to lay any eggs in nests that are cooler there may come a day where only female turtles exist. Already in some areas of the world females outnumber males by the staggering number of 116:1. Without local factors such as shady coastal trees and heavy sand-cooling rains turtles are at extreme risk of extinction.

You may like to read more about TSD in green sea turtles. Our Quick Snip about TSD in loggerhead turtles may be of interest to you too.