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.


Female-biased sea turtle populations

The temperature at which a sea turtle egg incubates determines the sex of the hatchling. Below a pivotal incubation temperature of about 29.0°C the majority of individuals are born male and above that temperature the majority of individuals are born female. This unique process, known as Temperature-Dependent Sex Determination (TSD), is widespread among reptiles.

What will happen to sea turtle populations when air temperatures rise due to climate change? This is a central question of my latest research, the results of which were published in the Journal of Experimental Marine Biology and Ecology this week.

The study was a collaboration between researchers from Swansea University (United Kingdom) and Deakin University (Australia), and conservationists working on the Dutch Caribbean island of Saint Eustatius. We recorded sand temperatures at a turtle nesting beach where leatherbacks, hawksbill and green sea turtles nest. These data were combined  with temperature projections from the Intergovernmental Panel on Climate Change (IPCC) to model how sand temperatures will change in the next 100 years.

The results showed that sand temperatures are relatively high (ranging from 29.1-33.3°C) at this nesting site and that all three species of sea turtles are female-biased: we estimate that for the current populations of hawksbill, leatherback and green turtles the percentage of turtles born male are 36%, 24%, and 16%, respectively. Projecting into the future, it is likely that the female-skew will be intensified due to warming air temperatures. For example, projections indicate that only 2.4% of green turtle hatchlings will be males by 2030, 1.0% by 2060, and 0.4% by 2090.

Nicole Esteban, a Swansea University researcher and former St Eustatius National Parks (STENAPA) manager, who is a lead author of the paper says that “there is a real concern that there will not be enough males born on Saint Eustatius in the future. If there are too few males, the local population is at risk of collapsing. Another concern is that turtle eggs do not develop above a certain temperature. The study highlights the extinction risks of climate change to species whose biology is closely linked to temperature.”

This research underlines that there is real need for effective conservation measures to be put in place to prevent the localised extinction of these turtle populations. Potential conservation strategies include shading turtle nests on the beach or moving nests to a cooler section of a beach such as a protected hatchery.

fdfa http://www.frogfishphotography.com/index.html

A hawksbill turtle swimming near a wreck in Saint Eustatius
(photo courtesy of Frogfish Photography)

Sand temperatures for nesting sea turtles in the Caribbean: implications for hatchling sex ratios in the face of climate change” was published by the Journal of Experimental Marine Biology and Ecology (2016). Authors: Jacques-Olivier Laloë, Nicole Esteban, Jessica Berkel and Graeme C Hays.

Long-distance migrations

Most sea turtles come back to nest on the beach where they were born. How they do this, is still a mystery.

In their breeding years turtles will migrate from their feeding grounds to their nesting grounds. Before modern technology was available, scientists could only guess how far or where the turtles would go. Nowadays, GPS tags allow us to study the migrating behaviour of sea turtles.

The most common practice is to tag a nesting female turtle by placing a satellite tag on her shell. Out in the ocean, the GPS calculates the geographic position of the turtle every time she surfaces to breath. This information is relayed to the researcher’s computer via satellite. The turtle can be tracked for as long as the tag works, which can be weeks, months, and sometimes even years.

A green sea turtle returning to the sea with a satellite tag on her carapace (photo couretesy of Nicole Esteban)

A green sea turtle returning to the sea with a satellite tag on her carapace
(photo courtesy of Nicole Esteban)

Thanks to satellite telemetry studies, we now know where nesting turtles have their feeding grounds. For example, we now know that some of the turtles nesting on Ascension Island migrate to their feeding grounds in Brazil. This migration is quite a feat, considering that the trip from Brazil to Ascension Island is over 2000 kilometres and the turtles’ destination is a remote island less than twenty kilometres across.

Science has yet to find out how turtles navigate such vast distances. One hypothesis is that they use Earth’s magnetic field for navigation. In fact, a turtle’s ability to navigate the oceans of the world is one that amazed Charles Darwin himself, as shown in his Letter to Nature in 1873:

“Even if we grant to animals a sense of the points of the compass, of which there is no evidence, how can we account, for instance, for the turtles which formerly congregated in multitudes, only at one season of the year, on the shores of the Isle of Ascension, finding their way to that speck of land in the midst of the great Atlantic Ocean?”

Recently, the longest ever published migration for an adult cheloniid (a hard-shelled marine turtle) was recorded: a green turtle swam almost 4000 kilometres from its breeding grounds in Diego Garcia to its feeding grounds off the coast of Somalia!

Sea level rise

Climate change challenges conservation efforts worldwide. Different aspects of climate change are affecting sea turtle populations across the globe. Rising temperatures are an obvious problem for a species with temperature-dependent sex determination. Similar to crocodiles, the incubation temperature of a sea turtle egg determines the sex of the hatchling. The pivotal temperature is close to 29°C: nests that incubate below this temperature produce a majority of males and nests that incubate above it produce a majority of females. Therefore, a warming world would cause a female-bias in sea turtle populations.

Another dangerous aspect of climate change for sea turtles is rising sea levels. A risk of rising sea levels is that turtle nesting beaches will be lost and this could push local turtle populations over the brink unless new suitable nesting beaches are found. A new study that will be published in the December issue of the Journal of Experimental Marine Biology and Ecology looks at the predicted effect of sea level rise on the hatching success of leatherback turtles nesting in Colombia.

The study was conducted on La Playona Beach and looks at the effect of water content of sand on the hatching success of turtle nests. The results of the study show that high sand water content was correlated with reduced hatching success: the wetter a nest, the fewer hatchlings survive from that nest. Projected climate change may therefore be detrimental to the breeding success of sea turtles as nests will have higher exposure to water due to sea level rise and an increase of hurricane events (another predicted consequence of climate change).

Scatterplot showing the relationship between percent sand water content and hatching success (the proportion of eggs that produced live hatchlings that emerged from the nest at the sand surface) in leatherback turtle nests A) in-situ (r = − 0.73, F1,28 = 32.48, P < 0.0001) and B) in experimental clutches in five different sand water content treatments (r = − 0.84, F1,19 = 45.59, P < 0.0001). Black lines shows linear fitted lines. (Source: The Journal of Experimental Marine Biology and Ecology)

The authors of the study conclude that “projected climate change and sea level rise may thus negatively affect leatherback turtle nesting success, although there are several avenues for adaptation to such future change.” They also recommend studying whether or not similar results are found with other species of sea turtles.

Another interesting result of the study is that hatchlings born in wetter nests were smaller in size. They did not appear to have less vigour, though. This last part of the study was measured by “racing” different-sized baby turtles on the beach!

The lost years

Sea turtles hatchlings go straight to the ocean after hatching. They will return to coastal waters as juveniles to forage. Not much is known about the years in between these two events, which is why this period has been dubbed the “lost years”. One thing that is known is that when the hatchlings first reach the surf they swim out to open waters for several days on end during what is called the juvenile frenzy. One of the ways this was demonstrated in the 1970s was by having good swimmers follow the hatchlings out at sea. It is thought that after a certain amount of time, the great oceanic currents catch the hatchlings. New research carried out recently in Boa Vista, Cape Verde, confirms this hypothesis. The researchers followed hatchlings with a boat for up to eight hours and up to 15 km out at sea using acoustic nano-tags. Dr Rebecca Scott, the lead author of the study reports that “for years, people have always spoken about hatchlings being swept away in the currents, but this is really the first good, direct evidence for that happening.” The results of this study are valuable to help better understand the biology of sea turtles and to get more information about the “lost years”. This, in turn, will be vital to develop more efficient conservation policies to protect this endangered species.

Video courtesy of the New York Times – http://nyti.ms/1tyjsF1