Turtle Mating Habits - Planet Earth

Despite the ban on the international trade of its shell, numbers of the critically-endangered Hawksbill turtle are continuing to decline.  

Because these turtles spend most of their lives at sea, exact numbers are not known, but females come to lay their eggs in the Seychelles on the Island of Cousine.  And, by studying the genetic material of these turtles and their hatchlings, recently published research is helping conservationists plan for the future.   

Planet Earth podcast presenter Sue Nelson went to meet Karl Phillips from the University of East Anglia to find out more about the research...

Karl Phillips: Welcome to the lab. This is the business end of it, as it were.

Sue Nelson: You've got a large refrigerator here - open it up and lots of colourful plastic boxes. There's a blue one.

Karl Phillips: So inside each of these plastic boxes are 100 tubes of ethanol and each tube containing a very small piece of turtle tissue. If I can find the box I'm looking for, I can show you the kind of samples that we are collecting from these animals in the field.

Sue Nelson: Which part of the turtle is the sample actually from?

Karl Phillips: So from the adult females we collect the sample from the trailing edge of the front flippers and we try to do that when the adult is laying its nest, when an adult female turtle is laying her nest she goes into a kind of trance and she really becomes completely oblivious, so the procedure of collecting a tissue is going to be a bit uncomfortable for the animal but if its done at a time when she's in that kind of trance it's almost like she is under an anesthetic she really doesn't react at all.

Sue Nelson: She's too busy focusing on what she is actually doing.

Karl Phillips: Her eggs and her nest.

Sue Nelson: So the sample itself is incredibly small isn't it?

Karl Phillips: Yes. So here is a sample from an adult female.

Sue Nelson: Oh my goodness that is just a couple of millimetres square effectively isn't it?

Karl Phillips: Yeah, and from that I will then take a razor blade and take a very small slither of that and that will yield enough DNA to do a DNA profile of that female. The hatching ones are even smaller. These really are tiny and these are taken -

Sue Nelson: In goes the tweezers again - oh, yes, this is almost one millimetre square effectively.

Karl Phillips: Yes, that really is tiny. So that's taken from above the right back leg of the hatchling just when it's on its way to the sea. So the flesh of the shell is still soft at this time and there's good published evidence that this kind of sampling doesn't cause any longterm harm to the hatchlings. So the female turtles lay their nest, we pick 20 hatchlings at random, take a small flesh sample from each of them and then the hatchlings are released to the sea. They are allowed to finish their scampering down the beach.

Sue Nelson: And what have you found then from examining the DNA from these samples?

Karl Phillips: What we've found is that the typical female hawksbill in our population tends to mate with just a single male. Now within a nesting season a female hawksbill will lay four to five clutches of about 160 or so eggs about two weeks apart. Now if we look at the sequential clutches of a particular female, we find that it is the same male for each female who has fathered all of her offspring. She is mating once at the beginning of the season, storing the sperm from that single mating and then using it to fertilise all of the sequential eggs, and when you think of that that's quite an impressive feat. She's laying five clutches of 160 eggs, so 800 eggs all fertilised by a single mating. That's pretty special.

Sue Nelson: Gosh - so with gaps in between. So storing that sperm and then however many...it is weeks or months later?

Karl Phillips: It's typically about two weeks between clutches. So after five clutches, it could be five or six clutches, we're talking 75 days she might have stored that sperm. It has remained viable all that time. One other thing I should point out is that a very small number of our females, about 10%, had mated with a second male but what was particularly striking about this was that just as a female had been singly mated and we see the same one father across all five of her clutches, if she were mated twice it would be the same two males across all of her clutches and what's more those two males would have roughly the same proportions of paternity consistently across their nests.

Sue Nelson: Can this help with the conservation of these endangered turtles?

Karl Phillips: Yes it can. Without doing this kind of study you wouldn't know how many breeding males are contributing to your population, you might see 50 females nesting of your beach but what if they've all been mated by the same male. We don't see that. In fact every single female in our sample had been mated with a different individual male. That to us is highly indicative that there's a large number of males out there and that's very good from a conservation genetics perspective.

Sue Nelson: So for once this sounds like quite good news from a conservation point of view.

Karl Phillips: It's very good news and coupled with the fact that on some of these protected islands in the Seychelles the hawksbill breeding population is now going up and so as things stand based on this that the future looks quite promising.