Sea turtles use temperature-dependent sex determination instead of the commonly used sex chromosome, but climate change have shifted some populations to hatching as 99.8 % females!

Most animals have their sex determined by their chromosomes, but far from all! There are actually several different systems for determining the sex of the offspring, and in this post we are going to be looking closer at the system found in sea turtles, as well as discussing some problems with this in light of the global increase in average temperature.

A Green sea turtle. Image by Brocken Inaglory, posted with the Creative Commons Attribution-Share Alike 3.0 Unported license..

Sea turtles uses a system called temperature-dependent sex-determination (shortened TSD from here on out) in order to determine if the offspring will be male or female. This is very different from the commonly used chromosomal determination, because it allows for the environment to make the final call, not chromosomes. This means that it can be manipulated much more easily than other systems, and sea turtle (and other reptile) breeders can easily choose which sex they want to produce.

TSD is only found in reptiles, and fish from the infraclass Teleostei, so you won’t ever find this system in mammals or birds. The embryo will begin to develop its sexual system at around one-third of the complete embryonic development, so it is at this thermosensitive period that the temperature is determining the sex of the embryo inside the egg. However, the exact time of the thermosensitive period is different in the different species.

The two different patterns of temperature-dependent sex determination

There are two (or kind of three) different patterns for the TSD. The pattern IA has a single cutoff point where offspring produced below the threshold are males, and above it females. This is the system used by most turtles, including the sea turtles, so this is the one will we look a bit more closely at in the next section of the post.

The Pattern IB the exactly the same, but gives females below the transition zone, and males above it. This IB pattern only occurs in tarantulas.

The final Pattern II has two transition zones, and will produce females if the temperature is on the extreme side of either ends. This means that only a certain temperature will yield males, with the rest of them giving females. However, this system also tend to give a mix of both sexes from the same batch of eggs. This system is found in some lizards, crocodilians and turtles.

Here is a nice diagram to show you what I wrote. Diagram by Peteruetz, posted with the Creative Commons Attribution 3.0 Unported license.

You would not really need more than a 1-2 degree change in temperature to change the majority of the offspring’s sex, so they are really sensitive to changes in temperature. At this point you can probably see where I’m going with this

A recent paper suggests that over 99 % of all sea turtle offspring at the Great Barrier Reef is female because of a warmer climate

A group of researchers have been looking into how the increased global temperature has affected the organisms that use temperature-dependent sex-determination instead of chromosomal determined, specifically by looking at sea turtles’ genetics and matching these with their nesting beach. This allowed them to put both the males and the females they sample on a map to see if there was certain sites that produces more females then others.

The reefs are pretty much divided into northern and southern Great Barrier Reef nesting sites, where the northern one is a lot warmer because of climate change. The southern nesting site is not that badly affected (yet), and the results reflect this very clearly. In the northern (warm) nesting sites, 99.1 % of all juveniles, 99.8 % of all sub-adults and 86.8 % of all adults were female. On the colder southern nesting site, the results were much more normal, ranging between 65 % and 69 %.

This shows that the warmer nesting sites have a huge production of females, and the same paper suggests that this site has produced primarily females for over 20 years.

Many people believe that climate change might not be that bad because the temperature only rises a few degrees, but that is more than enough for the turtles. (And it will completely change a lot of other stuff in a very bad way as well, so don’t ever try to think that global warming won’t affect us humans). As of right now there are still some nesting sites that have not been as affected by the global warming yet, but that will most likely change as we keep burning hydrocarbons.

A hatchling. Image by Manuel Heinrich Emha, posted with the Creative Commons Attribution-Share Alike 2.5 Generic license.

This might not destroy the population all that quickly

At first glace we would expect this to completely destroy the population of sea turtles in this area since there are so few males, but this is not a certain scenario. Sea turtles do not need a 1:1 ratio like many monogamous species do, and a single male can fertilize the eggs of many females. Some scientists speculate that this might increase the population number, at least for the short-term. No one can tell much about the long-term effect of this extremely skewed sex ratio, since we don’t really have anything to compare it to, so we will just have to wait and see.

Thanks for reading

Thanks for reading about temperature-dependent sex determination as well as how this affect the sea turtles that nest close to Great Barrier Reef. I hope you learned something new, or at least found it to be an interesting read! Please leave me a comment below if you got anything you want to share, got a question, or just want to say hello.