Polyandry and multiple paternity

Female sea turtles will often mate with more than one male during the breeding season. This type of mating system is termed polyandry. In addition, a female can store sperm in her oviducts for long periods of time and use the sperm to fertilize her eggs later. An interesting consequence of this is that within a single turtle nest, it is possible to find hatchlings that were sired by different fathers. So in the same turtle nest you might find not only siblings, but also half-siblings. But what are the potential benefits of polyandry and multiple paternity for sea turtles?


A pair of loggerhead sea turtles mating in the Mediterranean
(photo courtesy of Kostas Papafitsoros)

The suggested advantages of polyandry include fertilization assurance and genetic benefits. In other words, having more than one mate can decrease the chance of having one “bad” (for example infertile) mate while increasing the chance of having at least one “good” (for example exceptionally fit) mate. However, a study published in 2004 showed that multiple paternity did not correlate with any estimator of reproductive success in green turtles. Comparing single-fathered clutches to those with multiple fathers, no evidence for genetic benefits was detected with fitness indicators such as clutch size, hatching success or offspring quality. This research therefore suggested that there are no direct genetic advantages to polyandry for female sea turtles. So is polyandry simply a consequence of the incidence of male-female encounters?

“Initially, there appeared to be a simple correlation between population size and the frequency of multiple paternity in sea turtle populations,” comments Dr Patricia Lee, a lead author of the 2004 study. “However, exceptions kept cropping up. For example, multiple mating by female leatherback turtles was relatively infrequent even for moderately sized populations, whereas for a similar sized loggerhead sea turtle population in Greece, over 90% of the females was found to be polyandrous.”

In a new study published recently in Advances in Marine Biology, Dr Lee and her colleagues explored the idea that frequency of multiple paternity was linked to the local density of a nesting population. By examining data from rookeries around the world they found a tight relationship between how densely populated a rookery was and the occurrence of multiple mating within that rookery. For example, individuals that congregate in small areas and do not move very far are more likely to encounter other individuals more frequently.

In summary, multiple paternity occurs more often at densely populated rookeries. While the benefits of polyandry are still unclear, it appears that female turtles are only opportunistically polyandrous. Dr Lee concludes: “Although there may be many reasons as to why females would choose to mate more than once, they would first have to have the opportunity to meet more than one male before they are able to have this choice.”

A review of patterns of multiple paternity across sea turtle rookeries” was published by Advances in Marine Biology (2018). Authors: Patricia L.M. Lee, Gail Schofield, Rebecca I. Haughey, Antonios D. Mazaris and Graeme C. Hays.


Sea turtle conservation: a success story

A new study published recently in Science Advances shows that sea turtle numbers are increasing worldwide. The authors of this paper examined trends in annual sea turtle nesting numbers across the world and found that of the 299 datasets studied, 95 showed increases in abundances while 35 showed decreases. This is the first time such a comprehensive study was done on the global scale and for all seven species of sea turtles.

This is very encouraging news for sea turtle conservation and is testimony to effective practices put in place as far back as the 1950s. It shows that simple strategies like protecting turtles’ habitats, protecting nesting females and their eggs, and reducing turtle by-catch have positive effects in the long-term.

The study also showed that small populations of sea turtles have the ability to bounce back and grow. In ecology, smaller populations often have a higher risk of disappearing – a phenomenon called the Allee effect – but this does not seem to be the case with sea turtles. “The ability for small populations to bounce back, such as the green turtle population in Frigate Shoals, Hawaii, might be due to the fact that males and females aggregate at specific breeding areas, allowing encounters,” comments Dr Gail Schofield, a lead author of the new paper. “Plus, both males and females mate with multiple individuals, which probably reduces the risk of genetic bottlenecks. Immigration from nearby sites, particularly males frequenting more than one breeding ground, might also enhance genetic flow, allowing population recovery.” From a conservation perspective, this means that protecting even the smaller populations of sea turtles is well worth the efforts and is beneficial to the species as a whole.


Global trends in the nesting abundance of sea turtles. Colors reflect upward (green) or downward (red) trends. Abbreviations denote species: CC = Caretta caretta (loggerhead turtle); CM = Chelonia mydas (green turtle); DC = Dermochelys coriacea (leatherback sea turtle); EI = Eretmochelys imbricata (hawksbill turtle); LK = Lepidochelys kempii (Kemp’s ridley); LO = Lepidochelys olivacea (olive ridley); ND = Natator depressus (flatback turtle). (Source: Science Advances)

Even though this study shows that the overall trend is positive, it is key to continue conservation efforts. Some populations of sea turtles are declining. For example there is a high risk that leatherback sea turtles, the biggest of sea turtles, will disappear in the Pacific. On top of that, sea turtles are faced with relatively new threats including climate change and plastic pollution in the oceans.

“The key message of this paper is cautionary optimism,” underlines Dr Schofield. “Our findings demonstrate the success of ongoing efforts, and that these efforts are effective, but that we need to continue funding and support monitoring to safeguard future sea turtle populations.”

Global sea turtle conservation successes” was published by Science Advances (2017). Authors: Antonios D. Mazaris, Gail Schofield, Chrysoula Gkazinou, Vasiliki Almpanidou and Graeme C. Hays.

Estimating sea turtle population sizes

A study recently published in Proceedings of the Royal Society B demonstrates how numbers of nesting turtles may be overestimated by a factor of two. Does this mean that there may only be half as many turtles as previously thought?

Sea turtle population size estimates have traditionally depended on walking kilometres of beach to record turtle sightings, tracks and nests, night after night. Marking turtles with small metal flipper tags helps to identify individuals and determine how many times the average female lays eggs. However, since it is impossible to flipper tag and intercept every turtle every time it nests, there is a tendency to underestimate the number of egg clutches that a female lays. Based on existing data, scientists have assumed that green turtles lay on average 3.5 clutches in a nesting season. This means that if, for example, 210 egg clutches were recorded on one beach, then the local nesting population would consist of 60 individual females. Currently, most population size estimates around the world work with this assumption.

In this new study, researchers used satellite tags to track individual female green turtles in the Indian Ocean to assess how many times they nested during the breeding season. The high-accuracy GPS location data revealed that individual turtles laid on average six clutches of eggs – almost twice as many as previously thought. On the basis of these data, a recording of 210 egg clutches would result in a much smaller nesting population of 35 females.



Fastloc-GPS Argos SPLASH tags (a) were attached to nesting green sea turtles (b) to record how many clutches females lay in a breeding season. (Source: Proceedings of the Royal Society B)

This research confirms similar conclusions of studies on green turtles nesting in Ascension Island and loggerhead turtles nesting in Florida. This suggests that scientists and conservationists need to re-examine their assumptions about sea turtle nesting frequency and take into account the possibility that many sea turtle nesting population numbers are being over-estimated. Dr Jeanne A Mortimer, an author of the study, comments: “We are not saying that all sea turtle populations have been overestimated by a factor of two. But we demonstrate how easy it is to do so inadvertently.” So while the absolute number of sea turtles in the oceans has not changed, our understanding of their biology and our estimates of their population sizes have improved. The authors hope that this new research “will encourage more people to use satellite tracking technology to help solve the many remaining mysteries about sea turtles that are so important to enabling us to effectively assess and manage their populations.”

How numbers of nesting sea turtles can be overestimated by nearly a factor of two” was published by Proceedings of the Royal Society B (2017). Authors: Nicole Esteban, Jeanne A. Mortimer and Graeme C. Hays.


The bomb pulse

Answers concerning the life-history traits of sea turtles sometimes come from the most unlikely sources. Most recently, the nuclear tests during the mid-twentieth century proved key to determining the age and growth rates of sea turtles in Hawaii. To understand the improbable link between Hawaiian sea turtles and nuclear weapons, we have to go back in time by over half a century.

During the 1940s and early 1960s nuclear tests were being carried out by various nations across the globe. As a direct result of this, the concentration of carbon-14 (14C) in the atmosphere nearly doubled within a decade. In 1963 over 100 nations signed a treaty agreeing to ban nuclear weapon tests in the atmosphere, in outer space and under water. Since then the atmospheric concentration of 14C has been decreasing at a steady rate due to natural exchanges with the biosphere. As atmospheric 14C is assimilated in the biosphere it can be found in all plants and in the animals that eat them. Interestingly, the 14C concentrations inside an organism mirror those present in the atmosphere and because the temporal change in the concentrations of 14C is well-documented, scientists can accurately determine the age of an organism based on its 14C content. This technique, known as bomb-radiocarbon dating (or bomb-pulse dating), is similar to the more widely-known radiocarbon dating used to date fossils.


The levels of carbon-14 in the atmosphere have been relatively stable over long time periods, with the exception of a large addition of carbon-14 in 1955–1963 as a result of nuclear bomb tests. The boxed region in a is shown in more detail in b. (Source: Nature)

A team of researchers from NOAA and Duke University recently applied bomb-radiocarbon dating to the hard tissue of 36 hawksbill turtle shells collected since the 1950s. This allowed them to approximate growth rate and reproductive maturity of these turtles and gave them new insights into this hawksbill population. Their results, published in Proceedings of the Royal Society B, show that this sea turtle population starts breeding at an average age of 29 years (range from 23 to 36 years). This is much later than other populations of this species, and may be a reason why this population, one of the smallest in the world, is not rebounding. In addition, the research reveals that these turtles’ diet has changed over time: they were omnivores until the 1980s but are now mostly herbivores. This indicates a dramatic change in the turtles’ food supply, which could be a sign of long-term ecosystem changes occurring in Hawaii.


Researchers have generated new growth curves for the Hawaiian hawksbill sea turtles by studying the interior structure of the turtles’ posterior marginal scutes and using bomb-radiocarbon dating.  (Source: Proceedings of the Royal Society B)

Bomb-radiocarbon dating appears to be more accurate in assessing the development of sea turtles than previously-used methods. It also has many applications in other research fields, including investigating Nazi war crimes and accurately determining wine vintages. However, as the atmospheric 14C issued from the bomb pulse disappears, so does our ability to use it to accurately age organisms. It is estimated that the bomb pulse will die out within the next two decades. Until then, there is no doubt that the bomb pulse will continue to be of unexpected scientific significance.

The living tag experiments

At what age do sea turtles reach sexual maturity? A simple question, but like many others in sea turtle research it is harder to answer than it seems. The difficulty lies in that sea turtles spend the first several years of their lives out at sea where they cannot be easily observed. Since the late 1950s scientist have come up with different methods to measure the time it takes for a turtle to reach sexual maturity in the wild. These included implanting magnets, injecting rare metals and tattooing turtles, which would then allow to identify a turtle when it returns to nest and to deduce its age. However, none of these experiments proved feasible, successful or practical enough to implement in large-scale experiments.

In the early 1980s a new method of tagging sea turtles was developed: the “living tag“. Sea turtles were tagged using a surgical autografting procedure: a small sliver of tissue of a turtle’s carapace (the upper shell) was switched with a small sliver of tissue from the same turtle’s plastron (the lower shell). Since the turtle’s carapace is dark in colour and the plastron is light in colour, the result was a turtle that had a light spot on its carapace and a dark spot on its plastron.

Living tag in the second left costal of a Kemp's ridley sea turtle. photo courtesy of Michael Coyne

A Kemp’s ridley sea turtle with a living tag on its second left costal scute
(photo courtesy of Michael Coyne)

Approximately 950 turtles of three different species were marked in this manner at four different sites. The turtles were then released in the hope that they would be recaptured when they are sexually mature. To be able to recognize recaptured turtles, the particular scute on which the autograft was done coded for the year and location of release of each turtle.

In 2002 one of these turtles released as a hatchling in 1985 was seen nesting. Later that same year a male turtle released as a hatchling in 1983 was observed mating. More turtles were seen again in the following years. All living tag returns were made after 15 years or more.

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(photos courtesy of the Florida Coop Fish & Wildlife Research Unit)

The results of the living tag experiments provided interesting insight into many different aspects of turtle biology, including age-at-maturity and hatchling growth rates. However, it is important to keep in mind that the turtles with living tags were kept for months or even years before being released in the wild to increase their chances of survival. In that sense these results do not exactly mimic natural conditions and the true age-at-maturity of a wild sea turtle still eludes scientists. The use of living tags has never become systematic or widespread but you might still come across one of the turtles tagged in the 1980s with a funny spot on its shell.

A green sea turtle with a living tag on its vertebral scute (photo courtesy of sea turtle.org)

A green sea turtle with a living tag on its second vertebral scute
(photo courtesy of Roberto Herrera-Pavon)


Drones and sea turtle research

Unmanned Aerial Vehicles (UAVs – drones) are increasingly employed to monitor and protect wildlife. The new technology has proven to be particularly useful to survey species and habitats that are difficult to access. In sea turtle research drones are being used for various purposes. In Australia conservationists use them to count turtles at Raine Island, the world’s largest green sea turtle nesting site, and map the island’s topography. In Suriname the World Wide Fund for Nature (WWF) uses drones to survey beaches and collect evidence of the illegal poaching of turtle eggs.

Drones can also be used to answer questions about turtles’ lives in the oceans. A new study published recently in Herpetological Review  deployed UAVs to observe the behaviour of green sea turtles off the coast of Mexico. One of the benefits of using this technology is that there is little risk of disturbing the animals that are being observed. The study revealed interesting footage of the courtship and mating behaviours of turtles at sea. The authors of the study conclude that “UAV technology is useful for not only enhancing our understanding of sea turtle behaviors in the natural environment, but also in identifying the location of critical habitat for important life-history events, such as courtship and mating.” One of the other benefits is that drones will show a unique bird’s-eye view of turtles returning to the water after nesting!

Video courtesy of The Leatherback Trust

The importance of beach vegetation

Female-biased sea turtle populations are reported at important sea turtle rookeries globally. This heightens concerns for the conservation of sea turtles in the long-term. For this reason researchers are measuring temperatures at nesting beaches around the world to better understand the male-to-female ratio that these rookeries are producing.

I recently collaborated on a research project looking at the incubation temperatures of turtle nests in the Chagos Archipelago (Western Indian Ocean) where both hawksbill and green turtles breed. In this study, we recorded sand temperatures on Diego Garcia, the largest island of the archipelago. Temperature loggers were placed at nest depths in the different areas where the hawksbill and green turtles nest. The results showed relatively cool temperatures. The beaches of Diego Garcia have several characteristics that make for these relatively cool nest temperatures. Firstly, the island, which is in the world’s largest marine protected area, has intact natural vegetation that provides heavy shade where some turtles nest. Together with heavy rainfall and narrow beach platforms, which require sea turtles to nest close to the sea, this provides for cool sand temperatures. Consequently, we expect that hatchling sex ratios at this site are currently fairly balanced, producing 53% and 63% male hatchlings for hawksbill and green turtles respectively. The results of this study were published in Scientific Reports this week.

Dr Jeanne A Mortimer, one of the authors of the article, has studied Western Indian Ocean sea turtles since 1981. She states that “our study helps us to better understand why different species of sea turtles choose the nesting sites that they do. Our results demonstrate that in order to produce offspring with a relatively balanced sex ratio, these hawksbill turtles need to lay their eggs amongst vegetation on the upper beach crest.  Hawksbills are relatively small sea turtles with an average nest depth of only about 30-50 cm (compared to the larger green turtles whose nest depths average some 70-85 cm). Green turtles often lay their eggs on the open beach platform. Our results tell us, however, that hawksbill nests constructed in an area of open sand are more likely to produce female-biased offspring, and in some cases might even be too warm to produce viable offspring.  This highlights the importance for habitat managers to maintain the natural vegetation on the beach crest in order to provide optimal nesting habitat for hawksbill turtles – a species currently listed by IUCN as a Critically Endangered species. Our research also helps to answer the very basic question asked by just about anyone who has watched hawksbill turtles nest: ‘How come they always seem to go up into the bushes to lay their eggs?’“

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(photos courtesy of Daniel Barker, Nicole Esteban, Kip Evans and Graeme Hays)

Male hatchling production in sea turtles from one of the world’s largest marine protected areas, the Chagos Archipelago” was published by Scientific Reports (2016). Authors: Nicole Esteban, Jacques-Olivier Laloë, Jeanne A. Mortimer, Antenor Guzman and Graeme C. Hays.