Sea Turtle Stories – Episode 7

Renato: Hi Minnie, thanks very much for having me.

Minnie: Thank you so much for joining us.

So in this episode, we are going to be talking to you a little bit about a not very extensively researched or perhaps understood aspect of the sea turtle world, and that is all about male sea turtles specifically.

I’m going to tell all the listeners a little bit about you before we get into the conversation.

Renato’s journey into sea turtle research began with Project Tamar, a Brazilian sea turtle conservation project, where he worked with local fishermen in a feeding area for sea turtles.
He then traveled to Costa Rica in 2013 for an assistantship with the Sea Turtle Conservancy at Tortuguero National Park, and he has returned to Costa Rica’s Caribbean coast every year since. Renato went on to acquire a master’s degree, where he investigated the fluctuation of sex hormones throughout the reproductive cycle of northwest Atlantic green sea turtles. Today, as part of his PhD research, Renato continues to study the reproductive status, body condition, and nutritional ecology of green sea turtles at the Archie Carr Center for Sea Turtle Research.

Apart from being a NatGeo explorer, Renato also serves as the President and Scientific Director of Turtle Love, a non-profit conservation organization based in Costa Rica.

We are so happy to have you here with us today, Renato.

So if I can start us off, what are some of the main challenges that researchers face when studying male sea turtles?

Renato: Yeah, so males are an interesting segment of the population, because unlike the females, they don’t come ashore to lay eggs. So, you know, like males will come ashore maybe to bask in some places where water gets cold, such as Hawaii and Galapagos. But the fact that they don’t come ashore as often makes it hard for us to study them. So basically, logistics are more difficult, it’s more expensive, it’s more dangerous.

There are several obstacles to studying sea turtles in water, and most of the work with males has to be done in water. So I think that’s the main challenge that we face when we want to know more about male sea turtles.

Minnie: Yeah, they’re just not as easily accessible, really, are they? We cannot get hold of them. It is quite hard for us to track turtles and to understand, especially male turtles, because they’re not coming on to shore, like you said.

So is satellite tagging then a possible way of better understanding them? Can that help us sort of fill in these data gaps we have about what they’re up to?

Renato: Yes, satellite tagging is, you know, a great way to understand the movements and distribution of those turtles. I’ve personally done my first few tags this year, and I’m pretty stoked of always seeing my turtles and see where they are in the world.

Minnie: That’s really good.

Renato: But in general, you know, for you to satellite tag a turtle, you still need to catch it, right? Because it’s a complex process where you put like all sorts of layers, all things to attach those tags to the turtle’s shells. You still have to hold the turtles, which keeps, you know, the logistical challenges of getting to the males. But yeah, it is a great way of understanding several aspects of their biology.

Minnie: So we have to actually catch these guys in order to actually obtain any research about them. At the Oliver Ridley Project, we’ve just been able to tag one of our first male sea turtles with our project called ORP Track. And so most of the ones we’ve tagged have been juveniles or females. So that’s something that we’re quite excited about just because of how difficult it is.

But yeah, why is it so important to study male sea turtles in the first place?

Renato: So, you know, like one of my main reasons for studying sea turtles in general is that we can have more knowledge in order to base our conservation strategies on. We need to know the resource so we make the right management decisions.

And we don’t know much about males because of those logistical challenges that we talked about. Most of the knowledge that we currently have for sea turtles are available for females. And at that, females are the nesting ground where they’re coming ashore repeatedly to lay eggs.

So for us to have a more complete picture of what the threats to sea turtle populations and what the populations look like, really, then we have to study more males. I think that’s the most important part of it is the lack of knowledge that we have on males nowadays.

Minnie: Yeah, we can’t protect them if we don’t know where they are or what they’re doing.

We’re picking your brains on male turtles because you are participating as a researcher in the Global Male Sea Turtle Initiative, which is looking specifically into this topic. Could you tell us a little bit about the project and what your role has been working with them so far?

Renato: Yes, so the Global Male Sea Turtle Initiative is a project of the ProOceans organisation, which was founded by my friend and colleague, Dr. Marco Garcia Cruz from Venezuela. He was studying male sea turtles off Aves Island, the tiny island of Venezuela. And, you know, like in his literature search, he started figuring out that we don’t know much about males. So he started gathering, you know, people and researchers all over the world to direct the research with male sea turtles. So, yeah, the main idea is to promote and improve the study of the biology and ecology of male sea turtles worldwide.

And for that, we’ve been organising for four years now a workshop in the International Sea Turtle Symposium on male sea turtles, where researchers have been gathering and discussing what are the main areas of knowledge that’s important to develop when thinking about male sea turtles. And my main role so far has been organising and hosting those workshops, which I was lucky enough to do this year in the Thailand symposium as well. And we’re hoping to keep doing the same for the next symposium in Ghana.

So we invite everybody to come and join us to discuss about male sea turtles.

Minnie: Well, with all of that expertise then that has been gathered in order to figure out the most important kind of data gaps, is there an area that has been identified amongst all of you is the most important thing to figure out?

Renato: So in the first symposium we gathered in groups and, you know, there were genetics, population dynamics, like several different main areas of knowledge. And each of those discussion tables generated a list of five different questions that are the most important to understand about male sea turtles. They’re basically – what are sex ratios at different life stages, in terms of the whole population, how many males and how many females. Age of maturity for males at what time they become sexually mature within their lives, even though for females the understanding that we have nowadays is kind of hazy as well. Migratory patterns between males and females, how they differ. And if there are unique courtship and mating zones for each of the sea turtle populations that should receive special attention and designation for protection.

I think there was over 60 people that decided on these questions. And, you know, by people, I mean the sea turtle experts, those questions were ranked as five of the most important to answer about males.

Minnie: That’s really interesting then. Do we actually know at this stage if there is significant differences between like the maturity rates of males and females, or is that really something that we just don’t yet have the information for?

Renato: Yeah, the maturity information is kind of hazy and it’s very variable, you know, per life pattern of the populations. Are they like leatherbacks that live in the middle of the ocean in those formative years or they’re more, you know, hard-shelled species that become more coastal after a certain while.

What we do know is that, you know, for most populations, males are a bit smaller and that potentially tells you that they can mature at an early age, but that’s very uncertain. So, yeah, the thing about age of maturation, I think, is an area that needs development for both males and females.

Minnie: Absolutely.

And there’s a sort of aspect of reptile biology, which can be quite like temperature and nutrition dependent as well. I guess we know that some populations can mature faster or grow bigger, at least quicker. Whether they’re maturing, I guess, is another question, but they are growing larger at a faster rate compared to some that are maybe growing in areas with poor nutrition or cooler temperatures.

So there’s also those aspects that complicate these kinds of questions about these growth rates and age and maturity.

Renato: Absolutely.

Minnie: So, Renato, are male sea turtles always smaller than female sea turtles? And if so, do we know why?

Renato: Yeah, that’s a great question. So, you know, my personal experience with green turtles, males tend to be smaller. We attribute those differences in sizes because females need to carry sometimes a thousand egg yolks inside their bodies at once. They need more space to execute the reproductive functions, whereas males don’t have that size constraint to it.

However, a paper on sea turtle size dimorphism by Chris Figner, also from 2023, and she says that the sexual dimorphism in the size of sea turtles is actually less pronounced than what we expect. And, you know, the sample size for females is huge and for males is very small for most species. And although we see that females are slightly bigger, larger, that difference is not as great as we assumed it to be in the past.

And it’s also interesting that if you compare that size dimorphism with freshwater turtles, for example, the difference in size between males and females is way greater than what it is in sea turtles. And that one I cannot explain very well. So it’s just an interesting fact that within the order of testudines with other turtles, you have this difference.

Minnie: So maybe the more we establish that data set, the more measurements we can take, maybe the difference will be more minimal.

Renato: And it’s interesting that, you know, some of the basic dimorphism between males and females include, you know, at least included size, but also the conspicuous characteristics that males need for mating, right? Such as the big nails, the front nails of males are usually way larger than that of females because they need those nails to hook onto the female when mating. And also the tail is way larger because that is where the penis is contained. It needs to kind of go under the female’s tail so, you know, insertion can occur and fertilisation. So those are the usual ways that we differentiate between males and females.

Minnie: Yeah. And the last one, sometimes mature males tend to have like a bit of a dip in their plastron, so like their under shell, so that they can rest more easily on top of the female’s carapace, which is curved. But yeah, that’s really the only three features that we’ve associated with sexual dimorphism amongst sea turtles, big claws, big tail, and sometimes a bit of a curve in their plastron. But apart from that, size has not historically and has not been a reliable indicator and it is not a way of us telling them apart.

Do you have much information then regarding things like migratory routes? We’ve learned from previous episodes that females will migrate great distances across the ocean back to where they were born and they will kind of return to that area. Do the males do sort of exactly the same thing or the males that are residing in those foraging grounds mate with the females who are arriving or do they actually migrate themselves to their own area of birth?

Renato: Yeah, again, this may be very specific to each population, but the ones that I have knowledge firsthand, like the Tortuguero green turtle population, the turtles are foraging off Nicaragua and both males and females are migrating from north to south whenever time comes to breed. But what Marco did show in his studies with genetics, is that males at the Avis Island population do have natal homing or they go back to the general region where they were produced to then reproduce again.

Minnie: So they’re all migrating back really, as far as we know. They’re all eating in one place, to put it simply, and breeding in another. And often these can be thousands of kilometres apart.
So the boys are also doing that. They’re not just hanging around, waiting for females to arrive from other areas. They are actually also moving to their natal beaches in order to breed.

Renato: I think most studies have seen that male and female turtles take the same routes from point A to point B. However, there is hypothesis that some males don’t even complete the reproductive migration because female sea turtles can store sperm in their oviducts and then fertilise the oocytes as they come out during ovulation. So, you know, it may be that some males are just migrating either to a midpoint in the migratory route to mate.

There is an interesting aspect of this in Nicaragua and Costa Rica. So two places that I have worked, the Miskito Keys, north Nicaragua, which is the foraging grounds for the green turtles, and then in Tortuguero, Costa Rica, which is the nesting ground for that population. The continental shelf of Tortuguero is very narrow, and it starts to widen towards north Nicaragua. Based on data from Dr. Cynthia Lagou and Kathy Campbell here from the University of Florida as well, when we look at the sex ratios of the turtles caught in the fisheries in the north, you usually have a one-to-one, and then as you start moving south, and I think that’s also based on the paper by Dr. Jeanne Mortimer, that the sex ratios in the south of Nicaragua can be skewed towards males.

And, you know, that leads us to think that maybe those males are just waiting for the females en route to Tortuguero to pass there, where the continental shelf started to get a bit thinner, so they’re just mating with them as they’re moving towards Tortuguero. And that’s a theory that kind of resonates with me because, you know, Tortuguero is the most important Atlantic nesting beach for green turtles. And you have about a hundred thousand nests recorded in one year there, and whenever I look from the beach or I go out in a boat off the Tortuguero coast, I do not see enough mating happening to justify all that huge number of nests.

So I always think that those males may not even be getting to the nesting beach and some maybe just waiting at strategic points. So I think satellite tagging would help us elucidate all of that with males specifically because there’s a lot of work done with satellite tagging females in Tortuguero.

Minnie: That’s really interesting because I guess that’s going to be potentially quite environmentally based as well.

So you’ve explained there’s an actual geographical reason why these guys might not be kind of going the whole way, whereas I know there are some places in the world where we do find all of the males kind of offshore of the nesting beaches as well. Another fun thing about sea turtles is it’s quite hard to extrapolate from one subpopulation to the next because they will have quite unique challenges and potentially unique ways of handling that. So it’s not actually something we could say they’re all doing, but it’s interesting to know that there are areas where that’s happening.

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Minnie: Interesting. Do we know much about what the mate selection process is? When they’re actually going through the mating season, are they actively selecting females to mate with or is there quite an opportunistic aspect to it?

Renato: Yeah, we’re not sure about males being able to select or what characteristics would make a female more attractive to a male. But, you know, seeing the mating process where you have several males trying to mate with one female, it’s really aggressive, the process. And, you know, there’s even anecdotes from other places where male turtles would capsize a little boat trying to mate with it. So, you know, if they find something that looks like a female or whenever they find a receptive female, they would try to mate.
Minnie: Interesting. I suppose in general across the animal kingdom, usually it is the females who are selective towards the males anyway, and they make that decision. But, yeah, listeners at home may not know that actually you can also get a hybrid sea turtle. So, for example, like a green turtle mating with a hawksbill turtle or a loggerhead turtle. We don’t know a huge amount about this, but they’re not too choosy. So I suppose, yeah, maybe that the females preference and the female selection is something actually that may be more relevant than the males.

In your experience working with turtles across the board, do we see any sort of significant behavioural differences between males and females in terms of how they interact with one another or interact with their environment, perhaps? Or are they quite similar?

Renato: Yeah, so I never experienced like difference in behaviours to say, you know, one individual of the same species would act like this being a male or being a female. But again, the mating process can be very scary. And males, you know, kind of, I guess, similar to what you would experience in a mating round for whales, right? Whenever the whales, the male whales are courting the females, they’re just jumping and making a spectacle out of it.

And, you know, that’s when all the whale watching boats have to be really careful because they’re just reaching and slapping the water. So, I guess, in that phase of their lives, at least, the aggressiveness that males show, likely because of all the testosterone that’s running through their bloods during the mating-courting process. I think that’s the main difference that I can relate to when males and females see turtles in terms of behaviour.

You know, as we talked about, that’s not much behaviour, more like a life history pattern. But males will reproduce more often than females in most populations. So, you know, a male could potentially reproduce every year or every other year when females would take two, three, maybe four year intervals between the breeding seasons. So, I think that’s an important difference, too, between males and females.

Minnie: Yeah, that’s really interesting. Is that something that we’ve been able to establish, that the males can actually do it every year? Because, yeah, as the listeners probably already know, females tend to have like between two to four years between their breeding season.
And we’ve put that down to things like energy recuperation because of the distances travelled and the huge amount of metabolic pressure that they go under to lay all of those eggs and the toll on their body. There’s a few reasons why we think they take that long and it can differ between species. But is that the case, then, that males can do this annually without too much problem?

Renato: It seems to be the case that they can do it more often, not necessarily annually, but taking shorter intervals. In the populations I study in Nicaragua, you see about 50% of the adult males are reproductively active during the season, meaning that you may have some guys that go and breed every year, but most of them are probably breeding every other year. So that’s my experience with the green turtles there. And as you said, that’s all because the female has to produce, you know, a thousand eggs, migrate the distances back and forth, be able to cope with all that rough love that males will show them in the mating areas, whereas males’ sperm production is relatively cheap in comparison to yolk production. And that’s why they’re likely to do it more often.

Minnie: Yeah, females end up losing a lot of weight and have to put a lot of energy into it, so they need that recovery time.

So do we think then, you know, are male and female sea turtles affected differently by these threats from humans and environmental changes?

Renato: Yeah, that’s a very good question. You know, I think overall, when it comes to sea turtles, females have always the harder life, right? And the fact that they have to come ashore to lay eggs makes them more vulnerable to certain threats, right? Like exploitation by humans, predation by wild animals such as jaguars, the attack by feral dogs and that kind of stuff, which males don’t experience because they’re in water. The threats in water are pretty similar for males and females.

Where I work in Tortuguero has a very specific threat, which is turtle harpooning. So during the mating season, some boats will come to the places where the turtles are mating and harpoon them. And the couple of turtles are bobbing in the surface and they’re very vulnerable to being attacked at that time. The male is usually the one on top, so he’s the one that gets the first blow whenever harpooners hit.

So, you know, every once in a while we’ll see a male washing up ashore with the harpoon on their backs and, you know, their lungs are very close on the back of their shells, so usually gets puncture. And unfortunately, when we find them, they’re usually dead. So that’s very, you know, local specific, but that’s a threat that affects males differently there.

Minnie: I hadn’t thought of that. Yeah. I suppose as well, like you said, when because of the way that they mate and the male will be on top of the female and they’ll both be quite close to the surface to breathe, they may also be more vulnerable to boat strikes perhaps at that point as well.

You mentioned before that the global male sea turtle initiative is trying to answer about sex ratios. And do we know then what is a normal healthy sex ratio of males to females? Is it literally 50-50 or is there some like nuance in that?

Renato: Yeah. So a normal sex ratio for, you know, any sexually reproducing animals would be a one to one, you know, one female to one male.

There are other things, though, that must be considered, such as the operational sex ratios. That would be, you know, the proportion of adult males that are reproductively active in a year compared to the number of adult females that are reproductively active in a year. So what I’ve been seeing in Nicaragua is that about, you know, as I said, 50 percent of the males will become reproductively active during the reproductive season.
But that proportion varies a lot more for females. The difference from year to year may vary between 10 and 30 percent of the adult females that are preparing to breed. And we saw that for the Tortuguero population as likely influenced by climatic phenomena such as the El Niño-Southern Oscillation that, you know, provides climatic conditions that are good for the growth of seagrass, which is the main food for those green turtles.

And, you know, in good years, more females may be able to reach the energetic condition that’s necessary for them to enter the reproductive cycle. So the operational sex ratio is something that we don’t really know what’s ideal, but that’s important when considering what the population should be doing, right? Because we’ve been talking about feminization of sea turtle populations because of enhanced sand temperatures. Hotter temperatures will produce more females and cooler temperatures will produce more male hatchling sea turtles.

And, you know, there may be some aspects of the biology of the males that buffer that effect on the population, such as, you know, being able to breed more often. And then the females, I think, promiscuity to the fact that they will mate, you know, with several females, all of that sum to the idea that a small part of the population contains most of the genetics. I think that buffers a bit of the effect that the scarcity of males is causing in the whole population. And in this way, operational sex ratios are also important to be considered in this picture.

Minnie: Yeah. So not just that we have one male, one female, because there’s no benefit of having one female if the male that we have is juvenile and can’t reproduce yet.
So it’s how those are balanced. And if we have enough adult breeding males to reproduce with enough adult breeding females, as opposed to non-reproductively active members of the species.

I was actually going to ask you then, do we know how many times a male can mate? Like, how many females can he mate with? Is he mating with three, four, five? Is it 50, 60? How reproductively active can they be during a breeding season?

Renato: Yeah. It takes one sperm to fertilise one oocyte. And the oocyte of the female, you know, is like this 30 millimetres pack of yolk, full of nutrients, a lot of energy. Whereas the sperm is really just the genetic material and a little tail that makes them swim upstream. And so, you know, like males are producing millions and millions and millions of sperm.
It’s a good question. I’m not aware of the numbers, but we know that males can definitely fertilise several females throughout the nesting season.

And as you all know, the hatchlings in the same nest can be from a bunch different fathers. So that shows they’re definitely not faithful to one female. They’re just going around and trying to spread their seed as much as possible.

Minnie: Just doing what they got to do. The mating process for the female and the male can take sometimes hours, where the male will have to like hang on for extended periods of time in order to maintain his position there and actually successfully inseminate the female. So I suppose the limiting factor is how long can they fight for and how successful are they in actually staying on.

I’ve heard, you know, thrown out there that males with the larger tail or with the thicker, you know, girthier tail are more likely to succeed in breeding. But I’m not sure, you know, that’s been done in terms of like what characteristics or measurements make males better breeders. Yeah, interesting.

Minnie: There really is a lot for us to find out about the boys, isn’t there? Lots of interesting aspects. You mentioned there about um the feminization, which is something that we’ve discussed in previous podcasts. And that being that there are some areas of the world where the temperatures are heating up to such a point that the majority or even almost all of the eggs in a nest are actually being born female due to the sex determination, which is temperature based.

Are we seeing the effects of these imbalanced sex ratios in turtle populations yet or not so much?

Renato: So I was looking for some information on that regard and I found this cool paper by Graeme C. Hays, and the collaborators from 2023 and they’re talking about, you know, in some places where this hatchling production is skewed towards females, then what you’re seeing in those populations is a lower incidence of multiple paternity within the nests, you know, having several males fathering hatchlings in one nest.
So there are more females being produced. It’s likely that they’re already getting past puberty and into breeding age. And the scarcity of males then would be shown by the lower incidence of multiple paternity.

Minnie: Interesting. Then regarding this kind of feminization of the sea turtle population, and then we’ve said there’s a decline then in the multiple paternities of some of these nests.
Is that a concern? And if so, why? Why would that be of concern to us?

Renato: That’s a good question. We talked about ways that male biology, reproductive biology can mitigate the, you know, this effect of male scarcity in the populations. However, you know, the fact that there are fewer males in the population may potentially lead to a decline in genetic diversity, right? Because there is fewer males contributing to the population’s gene pool.

Minnie:I suppose maybe for people who aren’t aware, what actually are the ramifications of a lack of genetic diversity amongst the population?

Renato: Yeah. So I think that the main idea there is because when the genetic diversity of a population declines, you start having more inbreeding, right? So you have more related turtles reproducing to each other. And then that can cause the pairing of some undesirable genes.

And I think that can lead to decline in fitness of the individuals and of the populations as a whole. I think that’s why it’s of concern.

Minnie: Generally makes things more susceptible to certain diseases or congenital or slightly squiffy genes being overrepresented in a population where they otherwise wouldn’t usually be present. It takes kind of multiple generations of inbreeding, but certainly it is a big concern because then lots of the population can be susceptible to very small triggers and that kind of wipe them out much faster.

I think we’ve covered a lot of really interesting things about the boys there. I have one, maybe a final finishing question for you there. Is there an area that you’re particularly interested in when it comes to males? Is there something that you are kind of thinking is most critical for us to focus on in those next few years?

Renato: Yeah, so for my line of research now, for my PhD, I have two chapters that tackle research with males. We’re trying to understand what characteristics tells us whether males are preparing to breed or not. So we can define those operational sex ratios that I mentioned before. Then we collected, you know, measurements of the body, including tail length, tail girth, carapace length. And as we’re working with legally harvested green turtles from the Nicaraguan fishery, I was there when they butchered the turtles, which broke my heart. But allowed me to collect some important samples of the testicles and epididymis, so the whole gonads of those males. So we’re trying to put this together and compile a way to identify if a male is reproductively active or not. And then using that information to then establish the operational sex ratios, which was highlighted as one of the important areas by all the experts that got together in the Global Male Sea Turtle Initiative workshops.

Minnie: Really cool. Very interesting. Thank you so much, Renata.

That’s been really, really fascinating.

I think that’s really helped our listeners to fill in a bit of a gap that we have in our turtle life cycle as we’re sort of helping people to understand the various different points that are important in a sea turtle’s life. So this has been really insightful to help us get that last kind of piece of the puzzle, although there are still many pieces of that puzzle overall to answer.

So thank you so much for joining us. Don’t worry, though. We’re very excited we’ve got you back for the next episode. So we’ve got plenty more to discuss.

But thank you so much for joining us to talk about this topic. We really appreciate your knowledge and your insight, and we really look forward to speaking to you next episode.

Renato: Thank you all very much.

Minnie: Thanks very much.

Thank you all to everyone who’s listening. We would love to hear your thoughts. So please do leave us a review and let us know. If you’d like to learn more about sea turtles and the Olive Ridley Project’s work, please visit our website where you can also support it by naming and adopting sea turtles or adopting one of our sea turtle patients. And lastly, you can follow us on Instagram, Facebook, Twitter, and even TikTok and stay up to date with the world of sea turtles.

So we will see you for our next episode. And until then, I hope you have a turtelly awesome time.