Sea Turtles: History, Characteristics, Behavior, Reproduction

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loggerhead turtle

Turtles are reptiles. They are cold blooded and lay eggs. There are 250 species of turtles. They live on land and in salt water and fresh water. Seven species live in the sea. Males can be distinguished from females by their longer tails. Some can live to be over a 100 years old. [Source: Anne and Jack Rudloe, National Geographic, February 1994]

Craig Welch wrote in National Geographic: “Spend enough time watching sea turtles and it’s hard to escape how astonishing they are. They soar through oceans with winglike front flippers, dig nests using back appendages that scoop and toss sand almost like hands, and squeeze salt water, like tears, from glands near their eyes. Their mouths are similar to bird beaks, perhaps because turtles share a common ancestor with chickens. All but leatherbacks, with their layer of thick skin, have bony external skeletons covered in scutes of keratin, the material found in rhinoceros horns and our own fingernails. But each species is different. Hawksbills help reefs by eating sponges that can smother coral. Loggerheads use powerful jaws to crush horseshoe crabs. Leatherbacks feed on jellyfish and sea squirts and can easily migrate from Japan to California. [Source: Craig Welch, National Geographic, October 2019]

Seas turtles can not breath underwater. Like dolphins and whales they have to surface from time to time to breath.
They are preyed up by large sharks. Some sea turtles have scars on their shells and chunks from limbs missing as a result of shark attacks. The populations of sea turtles are estimated by counting the number of turtles at hatching sights.

Turtles and tortoises are reptiles with shells. Turtles live mostly in the water and tortoises live mostly on land. According to the official Red List by the World Conservation Union almost half the turtles and tortoises are threatened. The males and females of many turtles species bob their heads up and own, sometimes for hours, before mating. The males of some species bite the females' head and suck on their feet to get the females in the mood. During copulation males often bite into the shell of the female to steady himself. Some males have to prop themselves up in an almost vertical position on the female to achieve penetration.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio; Turtle Restoration Project

History of Sea Turtles

Archelon skeleton. This extinct marine turtle from the Late Cretaceous ( 100.5 million years ago to 66 million years ago) is the largest turtle ever to have been documented, with the biggest specimen measuring 4.6 meters (15 feet) from head to tail and weighed 2.2–3.2 tonnes (2.4–3.5 U.S. tons)

Turtles have been the around for around 200 million years. Sea turtles date back to the time of the dinosaurs and have not changed that much since then. They evolved from air-breathing land-dwelling tortoises. Craig Welch wrote in National Geographic: “Marine turtles split from their terrestrial relatives more than 100 million years ago. They survived the asteroid that killed the dinosaurs and squeaked past a marine extinction two million years ago that cut their numbers almost in half. Today sea turtles are found on the beaches of every continent except Antarctica, and they swim in all tropical and temperate waters. [Source: Craig Welch, National Geographic, October 2019]

“Perhaps their ubiquity explains the many roles they’ve played for people. They tell our stories: In Chinese mythology, sea turtle legs hold up the sky. We turn to them for healing: Turtle meat in West Africa was once believed to fight leprosy, and bathing in a broth of loggerhead plastron, the bony undershell, was considered a tonic for lung ailments. Even today, bones and scutes are sold as medicine from China to Mexico. “Through most of this shared history, turtles haven’t just survived — they’ve thrived. “The sea was all thick with them, and they were of the very largest, so numerous that it seemed that the ships would run aground on them,” a Spanish priest wrote of Christopher Columbus’s view of Cuba’s sea turtles in 1494, during his second voyage.

“Some scientists today believe the pre-Columbian Caribbean alone may have been home to 91 million adult green turtles. That’s roughly 10 times as many as all the adult sea turtles of every species believed to be alive today. So many occupied the Cayman Islands in the 1700s that English settlers used them to supply Jamaica with meat. It wasn’t long before West Indies turtles were being served in London pubs and John Adams was slurping sea turtle soup during the First Continental Congress. Within a century, though, Caribbean turtle populations had crashed, sending turtle hunters to new coastlines, foreshadowing a great transition.

Origin of Turtles

Oliver Rieppel, curator of fossil reptiles at the Field Museum in Chicago, told the Chicago Tribune, “The origin of the turtle shell has been a big debate in paleontology for a long time. The turtle shell is such a specialized, unique feature. No other vertebrate animal group has this kind of body plan that has always been a big mystery.”

Notochelones and Protostega are extinct genuses of sea turtle, which existed about 100 million years ago.

It appears that turtles first had shells only on their bellies. In an article in Nature in November 2008, Rieppel described a fossil — found in what used to be a shallow sea in Guizhou Province of southwestern China — of a turtle that lived 220 million years ago, a time when dinosaurs first appeared. The toothy aquatic creature — named “Odontochelys semitestacea”Latin for “half-shelled turtle with teeth”) was about 40 centimeters long and had a shell on its belly (a plastron) but lacked one on its back (the carapace). Its ribs and backbones were beginning to expand and grow together in such a way that millions of year larger would yield a carapace.

Some scientist dispute the conclusion. Robert Reisz, an evolutionary biologist at the University of Toronto, said he thinks Odontochelys once had a carapace and lost as an adaption to its environment the same ways some modern sea turtles have. The oldest previously known turtle fossil has a full shell, beaked mouth and no teeth like modern turtles. It was would found in Germany and dated to about 206 million years ago.

Car-Sized, Dinosaur-Era Sea Turtles

The largest turtle on record —Archelon — lived roughly 70 million years during the Late Cretaceous Period, when dinosaurs were still alive. The biggest specimen measuring 4.6 meters (15 feet) from head to tail weighed 2.2–3.2 tons. It is known only from the Pierre Shale, a geologic formation east of the Rocky Mountains in the U.S. Great Plains,. The leatherback sea turtle was once thought to be its closest living relative, but now, Archelon is thought to belong to a completely separate lineage from any living sea turtle. [Source: Wikipedia]

Archelon was a sea turtle with a leathery carapace instead of the hard shell and had an especially hooked beak and jaws designed for crushing, so it probably ate hard-shelled crustaceans, mollusks, and possibly even sponges. With its large and strong foreflippers, Archelon was likely a string swimmer. It inhabited the northern Western Interior Seaway, a mild to cool temperate area, dominated by plesiosaurs, hesperornithiform seabirds, and mosasaurs.

In 2022, scientists announced that had found fossils of giant turtle the size of a small car that lived 83 million years ago in northeastern Spain. Reuters reported: Researchers described remains of a turtle named Leviathanochelys aenigmatica that was about 3.7 meters (12 feet) long, weighed a bit under two tons and lived during the Cretaceous Period. It dwarfed today's largest turtle — the leatherback, which can reach 2 meters (7 feet) long and is known for marathon marine migrations. Leviathanochelys nearly matched the largest turtle on record — Archelon."Leviathanochelys was as long as a Mini Cooper while Archelon was the same size as a Toyota Corolla," said paleontologist and study co-author Albert Sellés of the Institut Català de Paleontologia (ICP), a research center affiliated with Universitat Autònoma de Barcelona. [Source: Will Dunham, Reuters, November 18, 2022]

recreation of an archelon

“It was good to be the size of a car, considering the hazardous traffic in the ancient Tethys Sea in which Leviathanochelys swam. Huge marine reptiles with powerful jaws called mosasaurs were the largest predators — some exceeding 50 feet (15 meters) in length. Various sharks and rays as well as long-necked fish-eating marine reptiles called plesiosaurs also lurked. "Attacking an animal of the size of Leviathanochelys possibly only could have been done by large predators in the marine context. At that time, the large marine predators in the European zone were mainly sharks and mosasaurs," said Oscar Castillo, a master's degree student in paleontology at Universitat Autònoma de Barcelona and lead author of the study published in the journal Scientific Reports. "During the Cretaceous, there was a tendency in marine turtles to increase their body size. Leviathanochelys and Archelon might represent the apex of this process. The reason for this increase in body size has been hypothesized to be predatory pressures, but there might be other factors," Castillo added.

Leviathanochelys aenigmatica means "enigmatic leviathan turtle" owing to its large size and the curious shape of its pelvis that the researchers suspect was related to its respiratory system. "Some pelagic (living in the open ocean) animals show a modification in their respiratory system to maximize their breathing capacity at great depths," Sellés said. Other large turtles from Earth's past include Protostega and Stupendemys, both reaching about 13 feet (4 meters) long. Protostega was a Cretaceous sea turtle that lived about 85 million years ago and, like its later cousin Archelon, inhabited the large inland sea that at the time split North America in two. Stupendemys prowled the lakes and rivers of northern South America about 7-13 million years ago during the Miocene Epoch.

Scientists unearthed the Leviathanochelys remains near the village of Coll de Nargó in Catalonia's Alt Urgell county after fossils protruding from the ground were spotted by a hiker in the Southern Pyrenees mountains. To date, they have found parts of the posterior portion of its carapace, or shell, and most of the pelvic girdle, but no skull, tail or limbs. The fossils indicated that it possessed a smooth carapace similar to leatherback turtles, with the shell itself about 7.7 feet (2.35 meters) long and 7.2 feet (2.2 meters) wide. Leviathanochelys appears built for the open ocean, returning to land only rarely — for instance to lay eggs. The presence of a couple of bony bulges on the front side of the pelvis differs from any other known sea turtle, indicating that Leviathanochelys represents a newly discovered lineage. It shows that gigantism in marine turtles developed independently in separate Cretaceous lineages in North America and Europe.

Reptile Characteristics

green turtle
Turtles are reptiles. Reptiles have a distinctive hearts with three chambers — two rear chambers (auricles) and one front chamber (ventricle) — with a partition that divides the heart almost completely in two. By contrast mammals have a four-chambered heart and amphibians have a three-chambered heart without a partition.

Reptiles swallow rather than chew their food. Many have jaw bones that bend and pivot and even come unhinged to allow the reptile to manipulate large prey. These developments freed the tongue from manipulating prey and allowed it become a sense organ . The forked tongue that many snakes and reptiles possess dates back to around 65 million years ago. It picks up chemical clues in “stereo” which allows reptiles to locate things.

Reptiles are ectothermic (cold blooded), which means they can not regulate their body temperature internally and can not create heat with their bodies like mammals can and are at the mercy of the sun and their surroundings for heat. This explains why reptiles are less common in cold and temperate areas. Sea turtles are (ectothermic (“cold blooded” and heterothermic (have a body temperature that fluctuates with the surrounding environment)., have bilateral symmetry (both sides of the animal are the same).

Reptiles were the first creatures to develop eggs. They passed on the ability to birds. Today, most reptiles lay eggs but not all of them. Boa constrictors, rattlesnakes and chameleons give birth to live young while pythons, cobras and iguanas lay eggs. Many reptiles have no sex chromosomes. Instead gender is determined by temperature. In crocodiles for example males are hot: eggs incubated in sand above a certain “pivotal temperature” almost always hatch males. This could spell trouble if global warming takes hold and female crocodiles — and reptiles — become scarce.

Sea Turtle Characteristics and Behavior

All sea turtles are marine reptiles that must come to the surface to breathe. Turtles can not breath like human by expanding and contracting the rid cage. Instead they rely on muscles around their lungs to do the job. Unlike freshwater turtles and tortoises, sea turtles cannot withdraw their head or flippers into their shells. Most turtles are able to withdraw into their shell as a means of escaping predator. No sea turtle is able to do this

Turtles don't have teeth. They chew and grasp things with horny jaws that contain sharp, almost knifelike cutting edges. Some turtles are completely vegetarian. Others are completely carnivorous. Many are omnivorous, eating whatever they can find. Turtles can not breath like human by expanding and contracting the rid cage. Instead they rely on muscles around their lungs to do the job.

green turtle breathing

Turtle shells consist of two parts: 1) the carapace, the upper, arched part; and 2) the plastron, the flat lower part. The carapace is attached to the backbone and ribs. The palstron is fused to the breastbone. In hard shelled turtles the bone is covered by a shield made of a horn-like material. Soft shelled turtles have a covering of tough skin over the bony shell. The carapace consists of two layers: an inner core of bony plates that are fused together, and an outer layers of shields made of hornlike keratin, called scutes. The shape and patterns of the scutes is a useful clue in identifying different species. A turtle's shell grows along with the animal and, unlike the skin of lizards and snakes, is not periodically shed. Most turtles when flipped on their back can turn themselves back over.

Sea turtles have some special adaptations for their marine environment. They have salt glands near their eyes, which allow them to drink sea water and excrete salt in high concentrations. Many people have seen nesting females supposedly "crying" for their young, but they are simply excreting excess salt.

Sea turtles are strongly adapted for swimming, spending nearly all of their life in the water. They are migratory (make seasonal movements between regions, such as between breeding and wintering grounds) and sense using vision, touch, sound and chemicals usually detected with smell. Sea turtles mostly cruise at a leisurely pace but are capable of moving a great speeds with legs that have been modified into long, broad flippers. Some sea turtles can dive to a depth of more than 1,000 meters (3,300 feet). Sea turtles often rest under ledges at night. Sometimes they become disoriented trying to return to the surface and drown. Adult female sea turtles return to land to lay their eggs in the sand — they are remarkable navigators and usually return to a beach in the general area where they hatched decades earlier. [Source: NOAA]

Craig Welch wrote in National Geographic: “Turtles didn’t survive 100 million years without developing strategies to weather hard times. They can slow their metabolism and go months without eating. Some females have skipped nesting seasons for years, only to show up again a decade later. New research suggests males may mate with many females when populations are stretched thin. And sea turtles may switch nesting beaches in times of stress. [Source: Craig Welch, National Geographic, October 2019]

Sea Turtle Shells

hawksbill sea turtle with its beautiful shell, greatly desired by some fashion designers and eyeglasses makers

Turtle shells consist of two parts: 1) the carapace, the upper, arched part; and 2) the plastron, the flat lower part. The carapace is attached to the backbone and ribs. The palstron is fusedto the breastbone. In hard shelled turtles the bone is covered by a shield made of a horn-like material. Soft shelled turtles have a covering of tough skin over the bony shell. The leatherback turtle differs from all other turtles in that ribs and vertebrae are fused to the outer shell. The covering consisted of hundred of bony plates imbedded in the skin.

The carapace consists of two layers: an inner core of bony plates that are fused together, and an outer layers of shields made of hornlike keratin, called scutes. The shape and patterns of the scutes is a useful clue in identifying different species. A turtle's shell grows along with the animal and, unlike the skin of lizards and snakes, is not periodically shed.

Sea turtles are unable to retreat completely into their shells. They can feel objects moving across their shells. Some enjoy having their shells scratched.

Feeding Sea Turtles

Turtles don't have teeth. They chew and grasp things in their beak-like mouths with horny jaws that contain sharp, almost knifelike cutting edges. Some turtles are completely vegetarian. Others are completely carnivorous. Many are omnivorous, (eat a variety of things, including plants and animals) eating whatever they can find.

Most Sea turtles are primarily carnivores and molluscivores (mainly eat mollusks). They eat non-insect arthropods, other marine invertebrates, pelagic crustaceans, algae,insects blown from the shore.aquatic crustaceans echinoderms (starfish and sea cucumbers) and cnidarians (jellyfish).

Jellyfish is a favorite of some species such as latherbacks. Loggerhead turtles like to feed on Portuguese man-of-war. Turtles in Australia like to feed on deadly box jellyfish. Sometimes the turtles become so intoxicated with poison their eyes swell shut. Like humans, turtles eyes can water. When they being butchered it looks as if the are crying.

Sea Turtle Reproduction

olive ridley sea turtle covering her eggs
Sea turtles are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups, multiple times in successive seasonal cycles). Most male sea turtles spend their lives in the open sea, approaching the shore only during the mating season. Females stay closer to shore and climb on to the beach several times every two or three years to nest.

Sea turtles produce reptilian eggs that can only develop and hatch in the air. The developing embryos need gaseous oxygen. Without it they will suffocate and die. Every year after mating in the sea, females seek dry land to lay the eggs. Some turtle species have slow rates of reproduction. They can live to be 100 but don’t reach sexual maturity for 20 to 30 years and then lay eggs only once every two to four years.

Turtles have few opportunities to find mates in the open ocean and generally mate near sandy beaches or around reefs. During mating the male sea turtle clasps on to the back of a female's shell with his flippers and stays there sometimes for hours. Turtles mate year round. Sometimes two males will pile on a single female.

Turtle eggs have tough leathery shells. Females lay several dozen to several hundred eggs in a nest which they dig out with their hind legs. The buried eggs hatch in less than two months. Sand temperature play a part in sex determination. More females are born when it is warm and more males are born when it is cool.

Temperatures and the Sex Determination of Sea Turtle Hatchlings

All sea turtle eggs can develop into males or females hatchlings, with gender determined by the temperature of the sand in which the eggs are buried to incubate. Cooler sand temperatures produce more males and warmer sand produces more females. The phenomenon is called Temperature-Dependent Sex Determination, or TSD, and governs the genders of other reptiles, too, including alligators and crocodiles.

The pivotal temperature for hawksbill turtles is 29.2̊°Celsius (84.6°Fahrenheit). When temperatures are below that more males are born. When temperatures are above that more females are born. For other sea turtles research shows that if a turtle's eggs incubate below 27.7°C (81.86°F ), the turtle hatchlings will be male. If the eggs incubate above 31°C (88.8°F), however, the hatchlings will be female. Temperatures that fluctuate between the two extremes will produce a mix of male and female baby turtles. See the different sea turtle species. [Source: NOAA]

Sea turtle hatchling
Researchers have also noted that the warmer the sand, the higher the ratio of female turtles. As the Earth experiences climate change, increased temperatures could result in skewed and even lethal incubation conditions, which would impact turtle species and other reptiles. There are some concerns that global warming could cause mass extinction of sea turtles by causing sand temperatures to rise so much that only females would be born.Current NOAA research suggests that warming trends due to climate change may cause a higher ratio of female sea turtles, potentially affecting genetic diversity. Research by Stephanie Jill Karmel of the University of Toronto has found that in places where there is a lot of human development and global warming is on the rise more females are being born as result of hotter sand associated with the development. Crocodiles also produce young whose sex is determined by the temperature of the eggs and some species are having the same problem.

Sea Turtle Nesting Sites

In summertime when the weather is warm, pregnant female sea turtles return to the beaches where they themselves hatched years before. They swim through the crashing surf and crawl up the beach searching for a nesting spot above the high water mark. Using their back flippers, they dig their nests in the sand. Digging the nest and laying eggs usually takes from one to three hours, after which the female turtle slowly drags themselves back to the ocean. Generally, they lay around 100 eggs, which incubate in the warm sand for about 60 days.

Female turtles sometimes make several nests, each with 80 to 100 eggs. It takes about two hours for a female to dig her nest, deposit the eggs, and cover them up in the sand. Like other reptilian females she has no interest in her eggs and offspring after the eggs are laid. When the job is finished, she strains to drag herself back into the sea.

Artificial lights can confuse nesting turtles who use reflections on the ocean surface to orient themselves when they are on land. Noise doesn't bother them so much. They have difficulty hearing sound in the air. It was long believed that newly hatched turtles crawled toward the sun, but recent research seems to show may actually trying to get away from the shadows of the dunes.

Sea Turtle Hatchlings

young turtle in Hawaii

Baby sea turtles hatch from their nest en masse and then rush to the sea all together to increase their chances of surviving waiting predators. When the tiny turtles are ready to hatch out, they do so virtually in unison, creating a scene in the sandy nest that is reminiscent of a pot of boiling water. In some areas, these events go by the colloquial term "turtle boils." [Source: NOAA]

Once hatched, the turtles find their way to the ocean via the downward slope of the beach and the reflections of the moon and stars on the water. Hatching and moving to the sea all at the same time help the little critters overwhelm waiting predators, which include sea birds, foxes, raccoons, and wild dogs. Those that make it through the gauntlet swim to offshore sargassum floats where they will spend their early years mostly hiding and growing.

A young sea turtle has less than a one in a thousand chance of living until maturity. Turtles eggs are eaten by fly larvae, fungi, crabs, feral dogs, racoons, and birds. When the hatchling emerge from their shells they are snatched up by birds that line the beach huge numbers, waiting to feast on them, and crabs that pull the hatchlings into their holes and eat only the tender flesh — the eyes, the necks and soft underbellies. Those the reach the open seas fall prey to sharks, groupers and fishnets.

Sea Turtle Migration

Sea turtles return to the same nesting sites en masse after drifting in the open sea. Many sea turtles migrate thousands of miles between feeding and nesting sites. Sometimes they travel on migration corridors. Some sea turtles circumnavigate the Pacific Ocean, paddling from Japan and Borneo to Mexico and Baja California.

No one knows for sure how sea turtles navigate their way to these nesting sights or how they know when it is time to go. Scientist are trying lean more about their migration routes and how they navigate. Some theorize they have an internal compass that picks up magnetic signals from the earth. Traces of magnetic substances have been found in their brains.

Sea turtles appear to rely on geomagnetic cues when they migrate thousands of kilometers to lay eggs on the same beaches where they hatched. Natal homing is an animal's ability to return to its birthplace in order to reproduce. Natal homing is found in all species of sea turtles and in other animals such as salmon.

sea turtle lifecycle land and sea migreations

How these turtles are able to return to their birthplace is an interesting phenomenon. Many researchers believe that sea turtles use a process called imprinting, which is a special type of learning that occurs when turtles first hatch that allows them to recognize their native beach.

There are two types of imprinting that are thought to be the reason turtles can find these beaches. The first is the chemical imprinting hypothesis. This hypothesis states that much like salmon, sea turtles are able to use olfactory cues and senses to smell their way home. However, a problem with this hypothesis is that some turtles travel thousands of miles to return to their native beaches, and the scents from that area aren't likely to travel and be distinguishable from that distance. The second hypothesis is the geomagnetic. This hypothesis states that as it hatches, a young turtle will imprint on the magnetic field of the beach they are born on. This hypothesis strongly correlates to the method which sea turtles use to navigate the earth.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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