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.
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.
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.
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.
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.
(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 second vertebral scute
(photo courtesy of Roberto Herrera-Pavon)
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
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?’“
(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.
Banana boxes? Check. Air support? Check. Volunteers? Check. After several years’ experience, conservation groups on Cape Cod know exactly what they need to rescue sea turtles that wash up on their shores every winter. But how do these turtles end up stranded in Massachusetts, USA, a place that seems so far from their usual tropical waters?
Sea turtles come and feed in Cape Cod bay during the warm summer months. Come the colder months, the turtles migrate back to warmer waters in the South. However, due to the geography of the Cape, some turtles cannot navigate their way out of the bay and get trapped. As the water cools these cold-blooded animals suffer from hypothermia, loose their ability to move, and wash up on the shores of the Cape.
Fortunately volunteers patrol the beaches to help the turtles. The cold-stunned turtles are taken to an animal care centre, such as the New England Aquarium, where they receive medical treatment. The turtles’ body temperatures are gradually raised every day and their progress is carefully monitored. Full recovery can take anywhere between a few days and several months, after which the turtles are placed in specially prepared banana boxes and are either flown or driven back South and released in warmer waters.
The cold-stunned turtles are cared for at rescue centres
(photo courtesy of Liz Maloney)
This year, for the first time since the start of the rescue activities, volunteers are recovering turtles from the beach as late as January. This may be due to rising temperatures, which causes the turtles to migrate south later in the season. Already over 500 cold-stunned turtles were reported this season, amongst which are Kemp’s ridley, loggerhead, and green turtles. In 2014 a record of 1200 turtles were recovered from the beaches around the Cape. Thanks to the efforts of the volunteers and rescue centres, the majority of these turtles made it back home safely.
Getting ready for the trip home!
(photo courtesy of Liz Maloney)
Sea turtles lay their eggs in underground nests on sandy beaches. The eggs are then left to incubate unattended and are subject to a suite of environmental conditions. The interplay of these environmental variables affects important factors such as the development rate and hatch success of the nest.
Since sea turtles exhibit temperature-dependent sex determination, the incubation temperature of the egg also determines the sex of the offspring. Warm incubation temperatures (typically above 29 °C) lead to a majority of females being born whereas cooler temperatures lead to males being born.
A new study published recently in Endangered Species Research shows the importance of rainfall throughout the incubation process. The researchers from Florida Atlantic University (USA) recorded rainfall and sand temperatures at a loggerhead turtle nesting beach in Florida. Ms Lolavar, the lead author of the study, tells me that the results of the study show that “heavy rainfall events can cool the sand and bring the incubation temperature of a nest in the male-producing range. However, if beach temperatures are too warm, which is often the case in Florida, most rainfall events don’t shift the nest temperature enough to produce males.”
This is not a phenomenon limited to Florida of course. A 2007 study of leatherbacks nesting in Grenada showed similar results. What sets the new research apart is that the Florida researchers also determined the sex of hatchling sea turtles laparoscopically. This enabled them to empirically show that during particularly rainy nesting seasons more male turtles hatched.
Ultimately, this study shows that understanding the effect of rainfall on incubation temperatures is key to anticipating the effects of the changing climate on sex ratios, and thus for the successful long-term conservation of sea turtles.
Loggerhead hatchlings: are they female or male?