Because of the difficulty in studying marine turtles in the open ocean, there are a great many things still unknown about their behavior. Decades of research, however, including observations at sea, have produced useful insights into daily activities and behaviors such as courtship, mating and nesting.
Sea turtles are known to feed and rest off and on during a typical day. During nesting season, research conducted in the southeast United States helped discovered that loggerheads follow regular patterns between the nesting beach itself and offshore reefs and other rocky structures. It is presumed that mating and/or feeding occurs at these offshore areas. When it is not nesting season, sea turtles may migrate hundreds or even thousands of miles. Sea turtles can sleep at the surface while in deep water or on the bottom wedged under rocks in nearshore waters. Many divers have seen green turtles sleeping under ledges in reefs and rocks. Hatchlings typically sleep floating on the surface, and they usually have their front flippers folded back over the top of their backs.
Courtship and mating for most sea turtles are believed to occur during a limited “receptive” period prior to the female’s first nesting emergence. Afterwards, only females come ashore to nest; males almost never return to land once they leave the sand of their natal beach. During mating season, males may court a female by nuzzling her head or by gently biting the back of her neck and rear flippers. If the female does not flee, the male attaches himself to the back of the female’s shell by gripping her top shell with claws in his front flippers. He then folds his long tail under her shell to copulate.
Females observed on the nesting beach after recently mating often have scratched shells and may be bleeding from where the males’ were hooked to their shells. Copulation can take place either on the surface or under water. Sometimes several males will compete for females and may even fight each other. Observers of sea turtle mating have reported very aggressive behavior by both the males and females. Females may mate with several males just prior to nesting season and store the sperm for several months. When she finally lays her eggs, they will have been fertilized by a variety of males. This behavior may help keep genetic diversity high in the population.
Very little is known about why sea turtles nest on some beaches and not on others. In Florida, loggerheads nest by the thousands on the central east coast, while identical looking beaches to the north see far fewer loggerheads. This nesting distribution may reflect conditions that existed centuries ago, when temperature, beach profiles or the lack of predation made some areas preferable to sea turtles. Today, humans are affecting the places where sea turtles nest. Beach erosion caused by coastal armoring and navigational inlets, artificial lighting and beach renourishment are all impacting once pristine beaches. These changes will likely have lasting effects on future nesting patterns. The more we understand about how, where and when sea turtles nest, the better we will be able to protect their nesting habitat.
Most females return faithfully to the same beach each time they are ready to nest. Not only do they appear on the same beach, they often emerge within a few hundred yards of where they last nested.
Only the females nest, and it occurs most often at night. The female crawls out of the ocean, pausing frequently as if carefully scoping out her spot. Sometimes she will crawl out of the ocean, but for unknown reasons decide not to nest. This is a “false crawl,” and it can happen naturally or be caused by artificial lighting or the presence of people on the beach. Most females nest at least twice during the nesting season, although individuals of some species may nest only once and others more than ten times. Sea turtles are generally slow and awkward on land, and nesting is exhausting work.
The female turtle crawls to a dry part of the beach and begins to fling away loose sand with her flippers. She then constructs a “body pit” by digging with her flippers and rotating her body. After the body pit is complete, she digs an egg cavity using her cupped rear flippers as shovels. The egg cavity is shaped roughly like a tear drop and is usually tilted slightly.
When the turtle has finished digging the egg chamber, she begins to lay eggs. Two or three eggs drop out at a time, with mucus being secreted throughout egg-laying. The average size of a clutch ranges from about 80 to 120 eggs, depending on the species. Because the eggs are flexible, they do not break as they fall into the chamber. This flexibility also allows both the female and the nest to hold more eggs. Nesting sea turtles appear to shed tears, but the turtle is just secreting salt that accumulates in her body. Many people believe that while laying her eggs a sea turtles goes into a trance from which she can not be disturbed.
This is not entirely true. A sea turtle is least likely to abandon nesting when she is laying her eggs, but some turtles will abort the process if they are harassed or feel they are in danger. For this reason, it is important that sea turtles are never disturbed during nesting. Once all the eggs are in the chamber, the mother turtle uses her rear flippers to push sand over the top of the egg cavity. Gradually, she packs the sand down over the top and then begins using her front flippers to refill the body pit and disguise the nest. By throwing sand in all directions, it is much harder for predators to find the eggs. After the nest is thoroughly concealed, the female crawls back to the sea to rest before nesting again later that season or before beginning her migration back to her feeding ground. Once a female has left her nest, she never returns to tend it.
Incubation takes about 60 days, but since the temperature of the sand governs the speed at which the embryos develop, the hatching period can cover a broad range. Essentially, the hotter the sand surrounding the nest, the faster the embryos will develop. Cooler sand has a tendency to produce more males, with warmer sand producing a higher ratio of females.
Unlike baby alligators, which are liberated from their nest by their mother, sea turtle hatchlings must do it all themselves. To break open their shells, hatchlings use a temporary, sharp egg-tooth, called a “caruncle.” The caruncle is an extension of the upper jaw that falls off soon after birth. Digging out of the nest is a group effort that can take several days. Hatchlings usually emerge from their nest at night or during a rainstorm when temperatures are cooler. Once they decide to burst out, they erupt from the nest cavity as a group. The little turtles orient themselves to the brightest horizon, and then dash toward the sea.
If they don’t make it to the ocean quickly, many hatchlings will die of dehydration in the sun or be caught by predators like birds and crabs. Once in the water, they typically swim several miles off shore, where they are caught in currents and seaweed that may carry them for years before returning to nearshore waters. There are many obstacles for hatchlings in the open ocean. Sharks, big fish and circling birds all eat baby turtles, and they die after accidentally eating tar balls and plastic garbage. The obstacles are so numerous for baby turtles that only about one in 1,000 survives to adulthood.
The ability of a sea turtle to migrate hundreds (and occasionally thousands) of miles from its feeding ground to its nesting beach is one of the most remarkable acts in the animal kingdom. That adult females return faithfully to nest on the very beach where they were born makes the feat even more amazing. Research into where and how sea turtles migrate has been a focus of scientists for decades. The information collected is vital to the development of conservation strategies for the species. We now know that sea turtles undergo migration throughout their lives, beginning with the first frenzied swim as a hatchling.
During its first critical 48 hours, a hatchling must travel from the beach to a place in the ocean where it is relatively safe from predators and where it can find food. Many hatchlings in the Atlantic and Caribbean make their way into Gulf stream currents, which are filled with floating sargassum weed. There the young turtles find an ample food supply and few predators. After several years of floating around the Atlantic, these young turtles are big enough to venture back into nearshore waters.
“Although all Florida loggerheads appear to spend a period of years within the North Atlantic gyre, different turtles probably do not follow precisely the same migratory route. In the diagram at left, the red lines indicate some possible migratory paths that different individuals may take. In addition, whereas most turtles appear to circle the gyre only once, some individuals may make more than one circuit, others may spend time in the Sargasso Sea, and a few have been captured in the Mediterranean.” Reprinted from a study of the Orientation and Navigation of Hatchling Loggerhead Sea Turtles.
Sea turtles typically spend their juvenile years eating and growing in nearshore habitats. Once they reach adulthood and sexual maturity, it is believed that they migrate to a new feeding ground. It is in this primary feeding area where adult turtles probably remain throughout their lives, except during breeding season. When it is their time to mate and nest, both males and females leave their feeding grounds and migrate to the nesting beach. This periodic migration will continue throughout their lives.
In the open ocean, sea turtles encounter strong currents; they have only modest vision, they can only raise their heads several inches out of the water, and there are often no visible landmarks. Even with these limitations, sea turtles regularly navigate long distances to find the same tiny stretch of nesting beach. How they do it is one of the greatest mysteries in the animal kingdom, and finding an answer has been the focus of generations of researchers. One promising new hypotheses on how sea turtles navigate suggests that they can detect both the angle and intensity of the earth’s magnetic field. Using these two characteristics, a sea turtle may be able to determine its latitude and longitude, enabling it to navigate virtually anywhere. Early experiments seem to suggest that sea turtles have the ability to detect magnetic fields. Whether they actually use this ability to navigate is the next hypotheses being investigated.
The migratory nature of sea turtles creates a number of challenges for those working to fully understand and protect these creatures. In particular, to adequately protect sea turtles in all their habitats, we must know where these habitats are, how the turtles behave while there, and routes the turtles take to migrate back and forth. Most sea turtle research has been carried out on nesting beaches – and for very logical reasons. These areas are easier for researchers to access, and what occurs on the nesting beach (production of new sea turtles) is extremely important to the species’ survival. Conservation efforts are also most easily directed at nesting beaches. However, of all the places where sea turtles travel throughout their life cycle, the least amount of time is spent on the nesting beach. Well over 90% of a sea turtle’s life is spent in the water – feeding, mating, migrating and doing whatever else a sea turtle does when no one is watching. Consequently, the threats zaced by sea turtles in the ocean present the greatest challenges to conservationists.
To fully protect sea turtles throughout their range, more must be known about their migratory patterns and their behavior in the water. Several methods are used by researchers to determine where sea turtles move. One of the simplest methods involves placing a small, harmless metal tag on one of the turtle’s flippers when she comes ashore to nest. Each tag includes a coded number and a message asking people to return the tag to a certain address if it is found. When people return a tag, they get a small reward and are asked where the turtle was encountered. In this way, researchers gradually learn about the many places to which turtles migrate. In the case of turtles nesting at Tortuguero, Costa Rica, tag returns make it clear that turtles nesting there disperse to feeding areas throughout the Caribbean. A large portion of them go to the Miskito Coast of Nicaragua. Efforts are now focused on limiting the number of turtles killed there for meat. The use of flipper tags has provided vital information, but it still leaves many questions unanswered.
Researchers have recently begun utilizing satellites to track sea turtles in the open ocean, see Sea Turtle Migration Tracking Education Program. First, a Sony Walkman-sized transmitter is attached to the back of an adult or juvenile sea turtle. The transmitter is glued directly to the turtle’s carapace, behind the head, where the unit’s small flexible antenna can break the surface to transmit when the turtle comes up to breathe. A passing Argos satellite receives the information and transmits it back to researchers on earth. After 8-10 months, the transmitter quits working and eventually falls safely off the turtle.
Using computer mapping programs, researchers can then see where the turtles migrate, what routes they travel and how fast they generally swim. If the map one is using has enough detail, it is also possible to determine the habitat characteristics at the turtle’s location. After monitoring a number of turtles in a specific population, researchers gradually learn where that population’s major feeding grounds are located and what threats they may be facing at sea. This information, of course, allows conservationists to focus efforts on the most important areas.