Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 29, Number 5;   May, 2002

Life History of the Sea Turtle

by Brian Clagett
Undergraduate Student – Case Western Reserve University

Other than the female coming ashore to nest, sea turtles spend their entire lives in the open sea. After hatching, the males migrate to the sea, never to return to land. All seven species of sea turtles share a common life cycle. They migrate from foraging areas to mating areas, and after mating, the males return to the foraging areas while the females return to their native beach to nest (Lutz & Musick, 52). Males and females meet in mating areas which are not too far from the nesting areas in order to copulate (Lutz & Musick, 58). Very few accounts of sea turtle mating are known, but it is believed that courtship rituals have a greater significance for the females than the males because of the willingness of males to mount objects that are of the "right size and general shape" (Lutz & Musick, 57). The courtship and mating process are believed to be very aggressive. Females come ashore with open sores from bites to the neck, head, and flippers, as well as damage to the shell from the males' claws (Lute & Musick, 57). The aggressiveness of courtship is further evidenced in male to male interactions. Males are prone to injury from other males who are courting the same female, and who bite each other in attempt to dismount their rivals (Lutz & Musick, 57).

After copulation, the females return to their native beach to nest, while males return to the foraging grounds. Nest site selection is a frequently studied event. Researchers attempt to analyze the nest site selections of different turtles, in order to understand the qualities that sea turtles look for in a nest site and the characteristics that are beneficial to the survival of the hatchlings. Studies have shown that in coarse or dry sand, the turtles will dig several holes termed "body pits," and often emerge onto the beach several nights in a row before they find a spot that fits their "criteria" (Mortimer, 1990). It was found that turtles who nest on beaches with fine-grained sand often emerge only once and make only one egg chamber before depositing their eggs (Mortimer, 1990). These discoveries are most likely linked to the fact that the survivorship of the eggs is lower in dry, coarse sand than it is in fine-grained moist sand (Mortimer, 1990). This finding may also explain why studies have found that turtles are more likely to nest in the monsoon season than in the dry season (Seabrook, 1989). Other studies have shown that sand softness, distance from human settlements, the presence of lagoons, beach length, and beach height are all important factors in the nest site selection of sea turtles (Kikukawa et al., 1999). The beach height specification is thought to be due to the selection of sites that are high above sea level to keep the eggs from being swallowed by the tide (Kikukawa et al., 1999).

The careful selection of the nest site by female sea turtles helps to insure a clutch with high survivorship. This is important for the hatchlings during the first stage of their life, the dispersal to sea. Perhaps the most important factor in the survivorship of the turtles during this frenzied scamper to the sea is the number of other hatchling sea turtles making the pilgrimage at the same time (Pough, 2001). The turtles are subject to high levels of predation in their unprotected state. That is why the hatchlings work together to dig out of the nest cavity and then wait until nightfall to make their sprint to the sea (Pough, 2001). If the hatchlings dispersed one by one, the predation rate would be much greater.

The single most important decision a sea turtle must make in its life most likely comes as it emerges from the nest. Sea-finding behavior has been studied in order to find the factors that are responsible for the hatchlings' remarkable seaward journey. The dependence on visual cues for sea-finding has been demonstrated by the fact that hatchling sea turtles cannot find the sea when their eyes are covered (Lutz & Musick, 110). It was also found that loggerhead hatchlings orient towards violet, green, and red light, and show an aversion to yellow and yellow-orange light (Witherington, 1991). Studies have also shown that hatchling green turtles are more strongly attracted to blue light than red (Lutz & Musick, 111). The aversion to colors like yellow and yellow-orange may function in preventing the turtles from leaving their nest in the light of day. The preference for shorter wavelengths like blue and green most likely aid in detection of the sea. Form vision was also shown to be important, with studies showing that sea turtles avoid darkened silhouettes and move towards the horizon (Lutz & Musick, 115).

Once the hatchling reaches the sea, it begins to swim. Studies have shown that the turtles will align themselves in respect to the direction of the oceanic waves and swim away from land (Goff, Salmon, & Lohmann, 1998). Once a turtle reaches deep waters, the contact with the coast is lost, and lost with it is the sensation of the waves. Therefore it is important for the turtles to use some other mechanism of orientation. The sea turtles will then transfer their orientation from one induced by waves to one of a magnetic compass (Goff, Salmon, & Lohmann, 1998). A study that placed hatchling turtles in a tank with a wave oscillator surrounded by Helmholtz coils showed that turtles who have swum into waves for 30 minutes will continue their direction even after the waves have stopped (Goff, Salmon, & Lohmann, 1998). So, the turtles begin their voyages by aligning themselves with waves and then after a short time, make a transition to a magnetic compass.

Once hatchling turtles reach the open sea, they will often remain there for 5 to 20 years during a period coined "the lost years" (Pough, 2001). The "lost years" are still not well understood, but recent evidence indicates that they spend their lives drifting in currents and feeding on food that accumulates in drift lines (Pough, 2001). At some point in their lives, the juveniles must return to the foraging areas and begin their lives as adults. The adult life consists of a cycle of migrations that lead to and from common "meeting" areas, such as feeding grounds, mating areas, and nesting beaches.

Satellite tracking of sea turtles has become very popular in the last two decades. Satellite tracking is accomplished by attaching a clock radio-sized transmitter atop the turtle's carapace. There are four polar orbiting satellites (the same satellites used to monitor weather patterns) that are equipped with instruments that detect animal tracking transmitters (satintro.htm). Signals sent from the transmitter include such data as approximate latitude and longitude, number of dives within the last 24 hours, the duration of the most recent dive, the water temperature, and the average speed (satintro.htm). The satellites circle the earth about every 101 minutes and therefore are only over one place on the planet for about 10 minutes. It takes about 3-5 minutes for a transmitter to be detected. Furthermore, the transmitter can only send a signal if it is very close to the surface and therefore it is uncommon to receive a location from a turtle each day (satintro.htm).

Satellite transmitters are often fitted to females as they come ashore to lay their eggs; therefore nearly all of the satellite-tracked turtles are females. Studies of a female green turtle tracked by a satellite show that the foraging grounds for this particular turtle were located over 600 km from the nesting grounds (Papi, et al., 1995). This turtle followed a slightly curved path to reach its destination and traveled 669 km to the feeding grounds that were 607 km away (straight-path) with an average speed of 2.23 km/h (Papi, et al., 1995). In a related study, loggerhead turtles were tracked by satellite and found to migrate from 545 to 1000 km from their nesting grounds to the feeding grounds (Papi, et al., 1997). Another study utilized a captive-raised loggerhead and found that it migrated a distance of 11,500 km across the Pacific Ocean from Baja California to Sendai Bay, Japan (Nichols, et al., 2000). The results from this study are surprising, since the animal spent ten years in captivity.

The life of a sea turtle seems to be governed by its instinctual behavior. It begins with the synchronous emergence of hatchlings frantically scurrying, empowered by cues of light intensity, color, and form that aid them in their quest to the sea. Then, the rhythmic movements of the ocean's waves guide them on their trek across the vast ocean. Soon they sense the earth's magnetic field and let their internal compass direct them where to go. Years later, they somehow find their way to the group feeding grounds, and later still, to the mating grounds where they will eventually begin to create young that will follow the same journey as their parents.

Works Cited:
Kikukaw, A., Kamezaki, N., and Ota, H. (1999) Factors affecting nesting beach selection by loggerhead turtles (Carretta caretta): A multiple regression approach. J. Zool., Lond. 249:447-454.
Mortimer, Jeanne A. (1990) The Influence of Beach Sand Characteristics on the Nesting Behavior and Clutch Survival of Green Turtles (Chelonia mydas). Copeia 1990:447-451.
Musick, John A. and Peter L. Lutz, eds. The Biology of Sea Turtles. Boca Raton: CRC Press, 1997.
Goff, Matthew, Salmon, Michael, & Lohmann, Kenneth J. (1998) Hatchling sea turtles use surface waves to establish a magnetic compass direction. Animal Behaviour, 55:69-77.
Nichols, W.J., Resendiz, A., Seminoff, J. A., & Resendiz, B. (2000) Transpacific migration of a logger turtle monitored by satellite telemetry. Bulletin of Marine Science, 67(3), 937-47.
Papi, P., Liew, H. C., Luschi, P., & Chan, E. H. (1995) Long range migratory travel of a green turtle tracked by satellite: Evidence for navigational ability in the open sea. Marine Biology, 122:171-75
Papi, F., Luschi, P., Crosio, E., & Hughes, G. R. (1997) Satellite tracking experiments on the navigational ability and migratory behaviour of the loggerhead turtle Caretta caretta. Marine Biology, 129:215-220.
Pough, F. Harvey, et al. Herpetology. 2nd ed. Upper Saddle River: Prentice Hall, 2001.
Seabrook, Wenday. (1989) The seasonal pattern and distribution of green turtles (Chelonia mydas) nesting activity on Aldabra Atoll, Indian Ocean. J. Zool., Lond, 219: 71-81.
Witherington, Blair E. & Bjorndal, Karen A. (1991) Influences of Wavelengths and Intensity on Hatchling Sea Turtles Phototaxis: Implications for Sea-Finding Behavior. Copeia 1991:1060-69.
<<http://www.cccturtle.org/satintro.htm> "Sea Turtle Migration-Tracking Education Program: How Satellite Tracking Works"