ePostcard #116: Nightwatch: Sea Turtles Are Ecosystem Sentinels (Part 2)

Sea turtles are ecosystem sentinels—critical threads in the complex tapestry of global biodiversity. For sea turtle recovery efforts to succeed globally, we must ensure that the far-flung threads of a sea turtle’s existence are intact; when the oceans are healthy; and when nesting beaches, seagrass pastures, coral reefs, and migratory pathways remain safe and usable. To do this, we must fully understand each species’ unique natural history—the adaptations that attune their lives to the marine environment, their distributional patterns, migratory travels, their diet and key feeding areas, and their breeding and nesting behaviors. With the exception of the flatback sea turtle, whose distribution is largely restricted to the tropical waters of northern Australia, the other six species (leatherback, green, loggerhead, hawksbill, and the Kemp’s and olive ridleys) occur in the ice-free areas of the global ocean, including the temperate to tropical reaches of the Atlantic and Pacific Oceans, the Mediterranean, and the tropical and subtropical waters of the Indian Ocean. Although the lives of sea turtles remain mysterious in many ways, the advent of remote camera traps, environmental sensors, GPS loggers and other satellite-enabled tracking equipment have greatly expanded our understanding of sea turtle migrations, as well as their behavioral ecology and physiology.

 In 2003, I had the remarkable experience of paddling along a moonlit beach in Tortuguero National Park with a group of turtle protection volunteers from the village of Tortuguero. Tortuguero National Park, which lies on the northeastern Caribbean coast of Costa Rica, is the most important nesting site for the endangered green sea turtle (photo #2) in the Western Hemisphere and hosts one of the two largest green turtle populations on Earth. The region was protected as a nesting sanctuary in 1963 and declared a National Park in 1970, largely due to the efforts of legendary turtle biologist Archie Carr and his Caribbean Conservation Corporation. It was the beginning of the nesting season for green sea turtles and our task was to scan the beaches at Playa Tortuguero for the fresh, tractor-like trails made by nesting females as they emerge from the sea to lay a clutch of eggs. It was a clear night and I rejoiced in the light-gathering capability of my binoculars, but there were no turtles or tracks to be seen. After several hours floating silently along in the velvet darkness, we paddled to an area that was normally off-limits to check for nesting activity there. Suddenly, the lion-like roars of mantled howler monkeys reverberated across the water. Were they harassing a potential predator or responding to a territorial infringement by other howler monkeys? I never imagined that jaguars might kill and eat turtles. What I soon learned from our biologist guide was that the howlers were most likely harassing a jaguar, an increasingly common predator of sea turtles on the nesting beaches at Tortuguero (see photo #3).

 Wherever tropical forests come in contact with beaches in the Americas, jaguars have probably been preying on nesting sea turtles for millennia. This juxtaposition poses an unusual challenge for conservation biologists because sea turtles and jaguars are ecosystem sentinels and are also protected in their respective habitats. The first reports of jaguar predation on sea turtles in Central America came from Costa Rica in the 1980s, specifically in Tortuguero National Park and Pacuare Nature Reserve. During the breeding season for sea turtles, lone jaguars patrol the beaches at night in search of nesting sea turtles. When the jaguar discovers a nesting sea turtle, it usually attacks with a crushing bite to the head or neck that kills the turtle instantly.

To determine how much jaguar predation the sea turtles could sustain, SWOT (State of the World’s Sea Turtles) researchers Luis Fonseca, Ian Thomson and their colleagues conducted a 10-year study (2010–2020) using infrared camera traps to document the deadly nighttime interactions between these two ecosystem sentinels, at Tortuguero and at the Guanacaste Conservation Area, which is located on Costa Rica’s Pacific Northwest coast. Guanacaste includes a 68-mile stretch of coastline with nine sandy beaches, including Nancite Beach, one of only a handful of sites worldwide where olive ridleys come ashore in the mass nesting events known as arribadas (photo #4). Beginning in 2010, at Nancite Beach, which is less than 0.6 miles long, the researchers reported that jaguars kill between 20 and 50 sea turtles a year, and the project identified around 20 individual jaguars. This may seem a stunning number of jaguars, but the actual number of turtles killed represents less than 1 percent of the local nesting population. Between 2014 and 2019, an average of 37,000 sea turtles nested annually, but jaguars killed only 140. As you might imagine, high on my naturalist’s wish list is a visit to Playa Nancite and the chance to see thousands of female olive ridley turtles coming ashore in an arribada—and, yes, catching a glimpse of a jaguar on patrol is also on the list!

 With the majority of their lives spent underwater, sea turtles reveal their secrets slowly. These ocean nomads come ashore only to rest when necessary, bask in the sun, and most importantly, to nest. Unlike their freshwater relatives, the head and limbs of sea turtles are fixed outside the shell and cannot retract into the shell. This distinctive feature, along with a streamlined shell, makes them more hydrodynamic in the water than their land-based counterparts, and allows them to maneuver easily through their ocean realm (photos #’s 5 & 6). To help them efficiently power their bodies through water, sea turtles have long flippers instead of the webbed feet of their freshwater counterparts. The large and strong front flippers act like paddles to propel them through the water, while the smaller back flippers function as rudders to help them steer. In females, the hind flippers have another purpose as well—they are used with deft precision to dig an egg incubation chamber in the sand. Sea turtles mate at sea, usually quite close to where the females will go to shore to nest. Their flipper-powered gracefulness underwater, especially during mating, reveals a beauty denied them during their brief times on land.

 The largest living sea turtle, the leatherback (photos #’s 7 & 8), provides us with a stunning example of sea turtle diversity and adaptation. Paleontologists believe that the leatherback is the sole survivor of a primitive order that resembled the first sea turtles. Describing leatherback biology is couched in superlatives: they are the biggest reptiles in the world, with the largest measuring at least 8 feet in length and weighing in at more than a ton. Leatherbacks are also the deepest-diving sea turtles, capable of reaching depths of 3,000 feet, and are unique among reptiles in being able to regulate their body temperature. One of their most fascinating adaptations has to do with their favorite food—jellyfish. As you can imagine, jellyfish don’t go down the gullet without a fight (see photo #9). The leatherback’s esophagus is lined with cartilaginous papillae, sharp, keratinized prongs that grab the slippery jellyfish and force it down into the stomach and keep the turtle from being injured by the stinging cells of its prey. Jellyfish are primarily water, a little protein, some vitamins and minerals, and some fat. How does the leatherback fuel its migratory journey eating jellyfish? Another adaptation! They have an extremely long esophagus that leads from the mouth to the rear of the body, and then it loops up the side again until it reaches the stomach. This long esophagus acts as a holding pouch so that the leatherback can continually digest its food – as parts of its meal leaves the stomach digested, new jellyfish are being pushed into the stomach.

 When sea turtles are not nesting, they make incredibly long migrations between feeding and breeding areas, spending different parts of their long lives in various parts of the world. The multiyear, epic migrations of sea turtles involve some of the most remarkable feats of orientation and navigation in the animal kingdom. Adult turtles of several species migrate across hundreds or thousands of miles of open ocean to nest on or very near the beaches where they hatched, which are often located in some of the most remote places on Earth. Leatherbacks, for example, travel an average of 3,700 miles each way. Sea turtle migrations are all the more astonishing in view of the fact that they are accomplished in an open-ocean environment devoid of visual landmarks and by marine animals whose poor eyesight above water probably precludes the use of star patterns and other celestial cues. As hatchlings, turtles that have never been in the ocean before establish unerring courses towards the open sea as soon as they enter the water and then maintain their headings after swimming beyond sight of land. The “onboard” mechanisms that enable sea turtles to navigate the open ocean and to locate their natal beaches have mystified scientists for more than 50 years. While earlier studies showed that sea turtles use the Earth’s magnetic field as part of their navigation system in the open ocean, it remained unclear whether adult turtles also depended on magnetic features to recognize and return to the nesting sites where they were born.

 Scientists know that sea turtles use magnetic features and a variety of other cues to navigate the oceans. Depending on the specific location on earth, the magnetic field has a specific inclination and intensity. Sea turtles can perceive both the magnetic field’s intensity and its inclination angle, the angle that the field lines make with respect to the Earth’s surface, as a distinctive magnetic “signature.” Recent research has shown that sea turtles likely “imprint” on the magnetic signature of their natal beach, through a process called geomagnetic imprinting, at a very young age or even before they hatch. This magnetic signature acts like a compass for them, even as they travel hugely long distances from their natal beaches—returning a couple decades later, as breeding age adults, to lay their eggs on the same beaches where they hatched.

But how do adult sea turtles know when to migrate? Recent research reported in the Journal of Experimental Marine Biology and Ecology, suggests that leatherback sea turtles may be able to sense seasonal changes in sunlight by means of an un-pigmented spot on the crown of their head—known as the pink spot (see photo #12). Researchers examined the anatomical structures beneath the pink spot and found that the layers of bone and cartilage were remarkably thinner than in other areas of the skull. This thin region of the skull allows the passage of light through to an area of the brain, called the pineal gland, that acts as biological clock, regulating night-day cycles and seasonal patterns of behavior. The researchers suggest that the lack of pigment in the crowning pink spot and thin skull region underlying it act as a “skylight,” allowing the turtles to sense the subtle changes in sunlight that accompany changing seasons, signaling them to return south when autumn approaches.

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