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 riddle of the Ridley

In the 1940s much less was known about sea turtles than we know today. Dr Archie Carr, who has been called the father of sea turtle research, spent his life trying to understand sea turtle biology and ecology. One particular species puzzled him: Kemp’s ridley sea turtle. Of all the species of sea turtles it was the only one that had never been observed nesting. Understanding Kemp’s ridleys became one of Dr Carr’s lifelong quests.

Dr Archie Carr fixing an early radio tracking device on a green sea turtle (photo couretsy of the Sea Turtle Conservancy)

Dr Archie Carr fixing an early radio tracking device on a green sea turtle
(photo courtesy of the Sea Turtle Conservancy)

The fact that no scientist had ever seen a Kemp’s ridley nest gave rise to wild conjectures about the turtle. Some speculated that the species had lost its ability to reproduce and that the current ridleys were the last members of a line on its way to extinction. Some said that the species did not mate but that hatchlings would arise by spontaneous generation. Others thought that the species laid saltwater-resistant eggs at sea. Another hypothesis was that the species gave birth to live offspring as do mammals. Or perhaps all the turtles of the species nested on one beach that had not yet been discovered by scientists.

Dr Carr deemed the last theory most scientifically sound and so began a search for the nesting ground of Kemp’s ridley. He surveyed hundreds of beaches in the Gulf of Mexico and in the Caribbean where the turtle was most often sighted. He spoke to fishermen and turtlers to gather as much information he could on the elusive turtle. Over the years, he walked hundreds of kilometres of beaches looking for unfamiliar turtle tracks. However, more than a decade after he started his search the turtle still evaded him.

The year 1962 brought a breakthrough. Dr Carr was presented with home movie footage shot in 1947 by Mexican engineer Andrés Herrera. The short 16 mm film showed a beach with thousands of turtles nesting in bright daylight. Dr Carr identified the turtles to be the Kemp’s ridley that he had been searching for all these years. His search had not just led him to one specimen: it is estimated that 40,000 turtles nested on that beach in northern Mexico that day!

The Ridley riddle  is still not entirely solved, but locating its nesting grounds was definitely a significant milestone. To Dr Carr the footage was simply groundbreaking:

“The film was short. It was shaky in places, faded with time, and rainy with scratches. But it was the cinema of the year all the same, the picture of the decade. For me really, it was the movie of all time. [..] It made Andrés Herrera in my mind suddenly a cinematographer far finer than Fellini, Alfred Hithcock, or Walt Disney could ever aspire to be. […] To me Andrés Herrera is a man who ought to be knighted, or to get a Nobel Prize, or some kind of prize.”

Video courtesy of Sea Turtle Inc.

For a more complete narration of Dr Carr’s quest for Kemp’s ridley I highly recommend his books The Windward Road (Chapter 1: The Riddle of the ridley) and So Excellent a Fishe (Chapter 5: Arribada).

Terrestrial basking

Turtles usually only come ashore to nest. It is rare to see a marine turtle on land otherwise. But it does happen: at certain beaches turtles come ashore to bask in the sun during the day. The only species known to exhibit this unique behaviour is the green turtle. It regularly comes to bask on the beaches of Hawai’i, and there have also been reports of turtles basking in the Galápagos Archipelago and in Western Australia. While basking the turtles will seemingly close their eyes and fall asleep. This creates the unique situation for beach goers to have to share the beach with napping turtles!

The obvious benefit of basking for reptiles is thermoregulation: raising body temperature accelerates metabolic processes like digestion and growth. Being cold-blooded, this is probably the main reason why these green turtle bask and nap on land. A possible added benefit of being on land is the reduction in exposure to marine predators such as tiger sharks. Finally, it could be that staying on land is also more energy-efficient as the turtle does not have to periodically swim to the surface for air. There are turtles that seem to prefer to bask at the same spot every day. On some beaches, tape is placed around the turtles’ favourite sleeping spots to ensure the turtles can rest undisturbed. It is also reported that certain groups of turtles always bask together, like friends. Occasionally Hawaiian monk seals also join the turtles for their nap, like in the pictures below!

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A big thank you to Mark Sullivan for all the great pictures of the turtles with the seals. Check out his great work with Hawaiian monk seal! I also thank Rebecca Scott for the original post idea.

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!

Avoiding predation

A sea turtle’s shell offers great protection to its owner. However, there are predators with razor sharp teeth that are not deterred by the hardness of the carapace and still present themselves as a threat to the sea turtles. The bites are not always deadly, and I have seen more than one turtle with a cookie bite-shaped chunk of its shell missing. The culprit was of course a shark.

An Olive Ridley with a small cookie bite in its shell

An Olive Ridley with a small cookie bite in its shell (above the hind left flipper)

Interesting footage recently published, shows that turtles also have behavioral strategies to avoid predation by sharks.

A first strategy is to roll sideways when a shark attacks. By doing so, the turtle presents the shark with a wide and hard surface that the shark has not angle to attack. Effectively, the turtle is using its shell as a shield that prevents it from fitting into the predator’s mouth.

A second strategy is to swim in tight circles near the shark. The larger body size of the shark does not allow it to turn as sharply and follow the turtle. By doing this, the turtle prevents the shark from closing in on the turtle and catching it. Clever!

Video courtesy of AnimalBytesTV and PRETOMA

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