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Episode 345: Leatherback Sea Turtles Encore

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A leatherback turtle resting on the beach (image from www.nwf.org)

These unique turtles have existed pretty much unchanged for 110 million years, determined by a very well-preserved skeleton fossil. They are imaginatively named leatherback turtles for their leathery skin that covers their body, making them unique amongst turtles as they lack the traditional shell. Instead, their soft skin covers the osteoderm, a set of bony plates connected by collagen fibres. This structure allows for the contraction of the body so that their lungs can contract and expand so they can withstand dives as deep as 1200 metres. Their carapace structure has evolved over millions of years to have the perfect hydrodynamic design, making them fast swimmers. Unusually for reptiles, they also maintain a pretty high core body temperature, at times keeping their temperature as high as 18 degrees Celsius above the surrounding waters. Being highly active animals helps them with this; they spend most of their time swimming, constantly creating muscle derived heat. Laboratory based studies have also found that leatherbacks have a resting metabolism up to 3 times higher than other reptiles of an equivalent size. Leatherbacks have no teeth, but a sharp cutting edge on their beak, and their oesophagus has backwards facing spines to help them swallow their food and stop it escaping.

The somewhat scary looking spines in the Leatherbacks’ throat

Leatherbacks are essential for the healthy functioning of the oceans, as they feed almost entirely on jellyfish. With their simple life histories and resilience, jellyfish can very easily multiply and swamp the oceans. This is particularly important for fisheries, as jellyfish species frequently prey on the larval stages of many common commercial fish species. Without Leatherbacks there to keep them in check, the food web will easily unbalance.

Mysterious Marine Migrations

The mammoth migrations of the different Leatherback populations (image from www.grida.no)

Leatherback turtles embark on the longest migration of any reptile on earth, travelling as far as 11,000 km per year, reaching as far north as Alaska and as far south as Chile and New Zealand. This is why their ability to regulate their own body temperature is so vital. They also have some other incredible adaptations for their cold-water jaunts, including a thick layer of fat to insulate, control over their blood flow to reduce heat loss from their extremities and high levels of brown adipose tissue, which is common in the animal kingdom across animals that need to thermoregulate. Typically, in other turtles and vertebrates in general, as temperatures increase, muscle function also increases. However, studies comparing pectoral muscle tissue of green turtles and leatherback turtles found that leatherback muscles have temperature independent muscle function. The functional ability of their muscles didn’t change between 5 and 38 degrees Celsius. This adaptation gives them the ability to swim and forage effectively in tropical, temperate and cold oceans.

On their migrations, leatherbacks travel across whole ocean basins, covering the Atlantic, Pacific and Indian oceans. Adult females return to their natal beaches in tropical climes, lay their eggs and return to the oceans for feeding. After nesting turtles need cooler water to hunt their jellyfish prey. With the threats of climate change, tagged turtles have been found travelling even further north more often to forage, being located off the coast of Nova Scotia.

Their migrations are not the only thing affected by changing climates. Like many reptiles, the sex-based development of their eggs is dictated by temperature. The temperature of the nest will determine the ratio of the sex of the offspring. There is a pivotal, ideal temperature at which there will be a 50:50 ration of males to females. Above and below this temperature the sex ratios will skew. For leatherbacks, temperatures below 27.7 degrees Celsius will produce a nest almost entirely consisting of males, and above 31 degrees Celsius, the nest will be almost entirely females. Mother turtles do their best to buffer the effects of temperatures by building deeper nests. But the weather can still impact these deeper nests, particularly excess rainfall or drier weather. This has huge implications for the future of the species as higher temperatures lead to nests full of females, with a dwindling pool of males for them to breed with. Research has also shown that temperature can affect the morphology of hatchlings, with very high nest temperatures leading to turtles with a lower body mass, smaller flippers and shorter carapaces.

Hatchlings on their mad dash from their nests towards the ocean (image from www.smithsonianmag.com)

Once the eggs hatch, the baby turtles are thrust straight into danger as they dash across the beach to the waves of the sea. Many off these hatchlings will be picked off by shoreline predators as they struggle towards the ocean. Once they make it to the sea, juvenile turtles are at considerable risk from predators like cephalopods, sharks and other large fish.

Their migrations are certainly an impressive feat, but how exactly do leatherbacks figure out where they are heading? The open ocean is notoriously lacking in any landmarks by which to navigate. To this day, the secret of how turtles find their way from point A to point B in the big blue hasn’t been cracked. Tagging studies in turtles revealed that they swim straight across open oceans, not along coastlines where they could follow the land mass or topography of the ocean floor. Tracked turtles all followed parallel, almost entirely straight lines across oceans. At those depths in the open ocean, the sea floor would not be visible for navigation, there are no stationary reference points, and any odour trails would disperse very quickly on the seas. On top of all this, turtles have pretty poor eyesight in the ocean anyway. Loggerhead turtles have been proved to have a kind of solar compass, navigating based off the position of the sun; could leatherbacks have this same system?

Some research has suggested that turtles can orient themselves to the earth’s magnetic fields. Magnetic orientation has been demonstrated in several species spanning the animal kingdom, including Monarch butterflies, yellowfin tuna, sockeye salmon and many birds. In lab-based experiments, turtle hatchlings were placed in a dark tank and exposed to reverse magnetic field. The hatchlings oriented themselves in the opposite direction, suggesting they possess some kind of light independent magnetic compass. Despite experimentation, scientists still haven’t cracked the secret behind these incredible migrations yet.

Danger on the high seas: threats facing Leatherbacks

Although they are currently listed as vulnerable on the IUCN red list, leatherbacks are facing significant threats globally, and some populations are locally classed as critically endangered. Leatherbacks were largely missed out of conservation discussions, until a scientific paper in 2000 highlighted the imminent collapse of the Leatherbacks global populations. Annual counts of nesting turtles on beaches in Mexico and Central America have declined by more than 90% since the 1980s. the exploitation of eggs for human consumption is considered one of the biggest threats to the species. In Asian societies, the eggs are considered a delicacy, and the Asian market is the main source of demand. Nests are not just at risk from poaching; unaware beach goers can very easily disturb or crush nests without meaning to, and dogs will easily sniff out and dig up a nest before their owners can stop them. Invasive species like feral cats, dogs and pigs may also predate on nests. Coastal development is also threatening nesting beaches, leaving no space for the leatherbacks to lay their eggs. Once nests are successfully laid and the turtles hatch, there are major risks to the hatchlings, as they are drawn to manmade lights, leading them away from the oceans and towards roads, where they are impossible to see and easily hit by cars.

Eggs for sale on the black market in Asia. This trade is devastating to Leatherback populations (image from www.newvision.co.ug)

Leatherbacks suffer greatly at the hands of industrial fishing fleets, getting tangled or hooked in fishing lines and drowning as fisheries bycatch. As fishing efforts expand into more areas, leatherbacks face increasing chances of being caught in nets and other fishing equipment. As well as dangers from the detritus of fishing, leatherbacks are especially at risk from the scourge of the oceans, plastic pollution. With a leatherback’s poor eyesight, a plastic bag floating in the water is almost indistinguishable from a jellyfish and are easily ingested, where they block their throats and stomach, leading to an eventual death. Chemical pollution can also cause issues for leatherbacks, with oil spills killing turtles and toxins from agricultural run off causing increased rates of fibropapillomatosis, a condition that leads to the development of multiple tumours at once.

A diver attempting to rescue an unfortunate leatherback that has become massively entangled in discarded fishing gear (image from the WWF)

Conservation optimism

Fortunately, leatherbacks are the subject of a lot of conservation research and attention. An analysis of hundreds of leatherback research papers identified the following areas to focus on for conservation of this species:

  • Improve the techniques for the safe handling and release of bycaught turtles in large fisheries.
  • Trial bycatch mitigation measures like modified fishing gear to reduce bycatch of turtles.
  • Increase the monitoring effort of bycatch in fisheries.
  • Introduce some initiatives for fisherman to avoid bycatch, such as putting increased value on those fish that were determined to have been caught with bycatch mitigation measures.
  • Protect nesting sites from erosion and human interference.
  • Improve nest relocation practices in areas of high temperatures

Scientists working to tag and measure a female Leatherback after she has laid her eggs on the beach in Costa Rica (image from oceanographicmagazine.com)

To tackle issues with nests heating up too much, putting the sex ratios of the hatchlings at risk, scientists have tested simple and low-cost nest shading techniques, like stretching cotton sheets or palm trees over the nests. This had some success in reducing temperatures in the nests and don’t require a lot of time or resources to do. In addition to these measures, sometimes turtle scientists will relocate nests to cooler windward beaches, although there are some risks with this that the disruption can lead to a nest failure, so different nest relocation techniques are being studied. In places like Florida, beaches are monitored for the arrival of nesting turtles, and the nests located and fenced off to protect them, with heavy fines for anyone caught messing with them. There is also a great initiative in Florida and other parts of the North American coasts, where there is a requirement to turn lights off along the beach or use turtle safe, red lights, to stop them being attracted away from the ocean towards human lights.

The Leatherback conservation trust is a Costa Rica based not for profit that focuses entirely on the Leatherback Turtle and its conservation. They work hard to protect the vitally important beaches of Costa Rica for nesting turtles as well as their migration routes, and often tag turtles for research and monitoring. They are also huge proponents of education and its importance in highlighting the plight of this species and do a lot of advocacy work, pushing for political and social change. Read all about their amazing work in Costa Rica here: The Leatherback Trust

Awesome videos!



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July 19, 2023
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