Loggerhead sea turtle

Appearance

The loggerhead sea turtle is the world's largest hard-shelled turtle. Adult loggerheads have an average weight range of 80 to 200 kilograms and a length range of 70 to 95 centimeters. The maximum reported weight is 545 kilograms and the maximum carapace length is 213 centimeters. The head and carapace range from a yellow-orange to a reddish-brown, while the plastron is typically pale yellow. The turtle's neck and sides are brown on the tops and yellow on the sides and bottom.

The turtle's shell is divided into two sections: carapace and plastron. The carapace is further divided into large plates, or scutes. Typically, there are 11 or 12 pairs of marginal scutes which rim the carapace. Five vertebral scutes run down the carapace's midline, while five pairs of costal scutes border them. The nuchal scute is located at the base of the head. The carapace connects to the plastron by 3 pairs of inframarginal scutes forming the bridge of the shell. The plastron features paired gular, humeral, pectoral, abdominal, femoral, and anal scutes. The shell serves as external armor, although loggerhead sea turtles cannot retract their head or flippers into their shells.

Sexual dimorphism of the loggerhead sea turtle is only apparent in adults. Adult males have longer tails and claws than females. The males' plastrons are shorter than the females', presumably to accommodate the males' larger tails. The carapace of males are wider and less domed than the females, and males typically have wider heads than females. The sex of juveniles and subadults cannot be determined through external anatomy, but can be observed through dissection, laparoscopy, histological examination, and radioimmunological assays.

Lachrymal glands located behind each eye allow the loggerhead to maintain osmotic balance by eliminating the excess salt obtained from ingesting ocean water. On land, the excretion of excess salt gives the false impression that the turtle is crying.

Distribution

The loggerhead sea turtle has a cosmopolitan distribution, nesting over the broadest geographical range of any sea turtle. The loggerhead inhabits the Atlantic, Indian, and Pacific Oceans and the Mediterranean Sea.

Status

Loggerhead sea turtles are classified as endangered by the International Union for the Conservation of Nature and are listed under Appendix I of the Convention on International Trade in Endangered Species, making international trade illegal.

Behavior

Loggerhead sea turtles observed in captivity and in the wild are most active during the day. In captivity, the loggerheads' daily activities are divided between swimming and resting on the bottom. While resting, loggerheads spread their forelimbs to about mid-stroke swimming position. They remain motionless with eyes open or half-shut and are easily alerted during this state. At night, captive loggerheads sleep in the same position with their eyes tightly shut and are slow to react. Loggerheads spend up to 85% of their day submerged, with males being the more active divers than females. The average duration of dives is 15–30 minutes, but they can stay submerged for up to four hours. Juvenile loggerheads and adults differ in their swimming methods. Juveniles keep their forelimbs pressed to the side of their carapace and propel themselves by kicking with their hind limbs. As the juveniles mature, their swimming method is progressively replaced with the adults' alternating-limb method. The loggerheads depend entirely on this method of swimming by the time they are one year old.

Water temperature affects the sea turtle's metabolic rate. Lethargy is induced at temperatures between 13 and 15 °C. The loggerhead takes on a floating, cold-stunned posture when temperatures drop to approximately 10 °C. However, younger loggerheads are more resistant to cold and do not become stunned until temperatures drop below 9 °C. The loggerheads' migration helps to prevent instances of cold-stunning. Higher water temperatures cause an increase in metabolism and heart rate. A loggerhead's body temperature increases in warmer waters more quickly than it decreases in colder water. The loggerhead sea turtle's critical thermal maximum is currently unknown.

Female-female aggression, which is especially uncommon in marine vertebrates, is common among loggerheads. Ritualized aggression escalates from passive threat displays to combat. This conflict primarily occurs over access to feeding grounds. Escalation typically follows four steps. First, initial contact is stimulated by visual or tactile cues. Second, confrontation occurs, beginning with passive confrontations characterized by wide head-tail circling. They begin aggressive confrontation when one turtle ceases to circle and directly faces the other. Third, sparring occurs with turtles snapping at each other’s jaws. The final stage, separation, is either mutual, with both turtles swimming away in opposite directions, or involves chasing one out of the immediate vicinity. Escalation is determined by several factors, including: hormone levels, energy expenditure, expected outcome, and importance of location. At all stages, an upright tail shows willingness to escalate, while a curled tail shows willingness to submit. Because higher aggression is metabolically costly and potentially debilitating, contact is much more likely to escalate when the conflict is over access to good foraging grounds. Further aggression has also been reported in captive loggerheads. The turtles are seemingly territorial and will fight with other loggerheads and sea turtles of different species.

Reproduction

Female loggerheads first reproduce between the ages of 17 and 33, and their mating period lasts up to six weeks. They court their mates, however these behaviors have not been thoroughly examined. Male forms of courtship behavior include nuzzling, biting, and head and flipper movements. Studies suggest females produce cloacal pheromones to indicate reproductive ability. Before mating, the male approaches a female and attempts to mount her, while she resists. Next, the male and female begin to circle each other. If the male has competitors, the female may let the males struggle with each other. The winner then mounts the female; the male's curved claws usually damage the shoulders of the female's shell during this process. Other courting males bite the male while he is attempting to copulate, damaging his flippers and tail, possibly exposing bones. Such damage can cause the male to dismount and may require weeks to heal.
While nesting, females produce an average of 3.9 egg clutches, and then become quiescent, producing no eggs for an average of two to three years. Unlike other sea turtles, courtship and mating usually do not take place near the nesting beach, but rather along migration routes between feeding and breeding grounds. Recent evidence indicates ovulation in loggerheads is mating-induced. Through the act of mating, the female ovulates eggs which are fertilized by the male. This is unique, as mating-induced ovulation is rare outside of mammals. In the Mediterranean, loggerheads mate from late March to early June. The nesting season peaks in June and July, but varies by nesting beach.

Loggerheads may display multiple paternity. Multiple paternity is possible due to sperm storage. The female can store sperm from multiple males in her oviducts until ovulation. A single clutch may have as many as five fathers, each contributing sperm to a portion of the clutch. Multiple paternity and female size are positively correlated. Two hypotheses explain this correlation. One posits that males favor large females because of their perceived higher fecundity. The other states that because larger females are able to swim more quickly to mating grounds, they have a longer mating period.

All sea turtles have similar basic nesting behaviors. Females return to lay eggs at intervals of 12–17 days during the nesting season, on or near the beach where they hatched. They exit the water, climb the beach, and scrape away the surface sand to form a body pit. With their hind limbs, they excavate an egg chamber in which the eggs are deposited. The females then cover the egg chamber and body pit with sand, and finally return to the sea. This process takes one to two hours, and occurs in open sand areas or on top of sand dunes. The nesting area must be selected carefully because it affects characteristics such as fitness, emergence ratio, and vulnerability to nest predators. Loggerheads have an average clutch size of 70 eggs.

Food

The loggerhead sea turtle is omnivorous, feeding mainly on bottom-dwelling invertebrates, such as gastropods, bivalves, and decapods. The loggerhead has a greater list of known prey than any other sea turtle. Other food items include sponges, corals, sea pens, polychaete worms, sea anemones, cephalopods, barnacles, brachiopods, isopods, insects, bryozoans, sea urchins, sand dollars, sea cucumbers, starfish, fish, wrasses, hatchling turtles, algae, and vascular plants. During migration through the open sea, loggerheads eat jellyfish, floating molluscs, floating egg clusters, squid, and flying fish.

Loggerheads crush prey with their large and powerful jaws. Projecting scale points on the anterior margin of the forelimbs allow manipulation of the food. These points can be used as "pseudo-claws" to tear large pieces of food in the loggerhead's mouth. The loggerhead will turn its neck sideways to consume the torn food on the scale points. Inward pointing, mucosal papillae found in the foreregion of the loggerhead's esophagus filter out foreign bodies, such as fish hooks. The next region of the esophagus is nonpapillated, with numerous mucosal folds. The rate of digestion in loggerheads is temperature dependent; digestion rate increases as temperature increases.

Predators

Loggerheads have numerous predators, especially early in their lives. Egg and nestling predators include oligochaete worms, beetles, fly larvae, ants, parasitoid wasp larvae, flesh flies, crabs, snakes, gulls, corvids, opossums, bears, rats, armadillos, mustelids, skunks, canids, procyonids, cats, pigs, and humans. During their migration from their nest to the sea, hatchlings are preyed on by dipteran larvae, crabs, toads, lizards, snakes, seabirds such as frigatebirds and other assorted birds and mammals. In the ocean, predators of the loggerhead juveniles include other fish, such as parrotfish and moray eels, as well as portunid crabs. Adults are more rarely attacked due to their large size, but may be preyed on by large sharks, seals and killer whales. Nesting females are attacked by flesh flies, feral dogs, and humans. Salt marsh mosquitoes can also pester nesting females.

In Australia, the introduction of the red fox by English settlers in the 19th century led to significant reductions in loggerhead sea turtle populations. In one coastal section in eastern Australia during the 1970s, predation of turtle eggs destroyed up to 95% of all clutches laid. Aggressive efforts to destroy foxes in the 1980s and 1990s has reduced this impact; however, it is estimated that it will be the year 2020 before populations will experience complete recovery from such dramatic losses.

Along the southeastern coast of the United States, the raccoon is the most destructive predator of nesting sites. Mortality rates of nearly 100% of all clutches laid in a season have been recorded on some Florida beaches. This is attributed to an increase in raccoon populations, which have flourished in urban environments. Aggressive efforts to protect nesting sites by covering them with wire mesh has significantly reduced the impact of raccoon predation on loggerhead sea turtle eggs. On Bald Head Island, in North Carolina, wire mesh screens are used on every confirmed nest to prevent excavation by resident red foxes. A new concern with the steel cage technique is interference with the normal development of the nestlings' magnetic sense due to the use of ferrous wire, which may disrupt the turtles' ability to navigate properly. Efforts are underway to find a nonmagnetic material that will prevent predator's gnawing through the barrier.

Up to 40% of nesting females around the world have wounds believed to come from shark attacks.
Loggerhead sea turtles were once intensively hunted for their meat and eggs; consumption has decreased, however, due to worldwide legislation. Despite this, turtle meat and eggs are still consumed in countries where regulations are not strictly enforced. In Mexico, turtle eggs are a common meal; locals claim the egg is an aphrodisiac. Eating turtle eggs or meat can cause serious illness due to harmful bacteria, such as "Pseudomonas aeruginosa" and "Serratia marcescens", and high levels of toxic metals that build up through bioaccumulation.

The U.S. West Coast is a critical migratory corridor for the Pacific loggerheads, in which these turtles swim across the Pacific to California’s coast from breeding grounds in Japan. Important foraging habitats for juveniles in the central North Pacific have been revealed through telemetry studies. Along with these foraging habitats, high levels of bycatch from industrial-scale fisheries have been found to overlap; with drift gillnets in the past and longline fisheries presently. Many juvenile loggerheads aggregate off the coast of Baja California Sur, Mexico, where small coastal fisheries increase these turtle’s mortality risk; fishers have reported catching dozens of loggerheads with bottom-set gear per day per boat. The most common commercial fishery that accidentally take loggerheads are bottom trawls used for shrimp vessels in the Gulf of California; as well as gillnets, longlines, etc. In 2000, between 2,600 and 6,000 loggerheads were estimated to have been killed by pelagic longlining in the Pacific.

Fishing gear is the biggest threat to loggerheads in the open ocean. They often become entangled in longlines or gillnets. According to the 2009 status review of loggerheads by the Fisheries Service, drowning from entanglement in longline and gillnet fishing gear is the turtles’ primary threat in the North Pacific. They also become stuck in traps, pots, trawls, and dredges. Caught in this unattended equipment, loggerheads risk serious injury or drowning. Turtle excluder devices for nets and other traps reduce the number being accidentally caught.

Nearly 24,000 metric tons of plastic is dumped into the ocean each year. Turtles ingest a wide array of this floating debris, including plastic bags, plastic sheets, plastic pellets, balloons and abandoned fishing line. Loggerheads may mistake the floating plastic for jellyfish, a common food item. The ingested plastic causes numerous health concerns, including: intestinal blockage, reduced nutrient absorption, suffocation, ulcerations, malnutrition or starvation. Ingested plastics release toxic compounds, including polychlorinated biphenyls, which may accumulate in internal tissues. Such toxins may lead to a thinning of eggshells, tissue damage or deviation from natural behaviors.

Artificial lighting discourages nesting and interferes with the hatchlings' ability to navigate to the water's edge. Females prefer nesting on beaches free of artificial lighting. On developed beaches, nests are often clustered around tall buildings, perhaps because they block out the man-made light sources. Loggerhead hatchlings are drawn toward the brighter area over the water which is the consequence of the reflection of moon and star light. Confused by the brighter artificial light, they navigate inland, away from the protective waters, which exposes them to dehydration and predation as the sun rises. Artificial lighting causes tens of thousands of hatchling deaths per year.

Destruction and encroachment of habitat by humans is another threat to loggerhead sea turtles. Optimum nesting beaches are open sand beaches above the high tide line. However, beach development deprives them of suitable nesting areas, forcing them to nest closer to the surf. Urbanization often leads to the siltation of sandy beaches, decreasing their viability. Construction of docks and marinas can destroy near-shore habitats. Boat traffic and dredging degrades habitat and can also injure or kill turtles when boats collide with turtles at or near the surface.

Annual variations in climatic temperatures can affect sex ratios, since loggerheads have temperature-dependent sex determination. High sand temperatures may skew gender ratios in favor of females. In one study, nesting sites exposed to unseasonably warm temperatures over a three-year period produced 87–99% females. This raises concerns over the connection between rapid global temperature changes and the possibility of population extinction. A more localized effect on gender skewing comes from the construction of tall buildings, which reduce sun exposure, lowering the average sand temperature. This results in a shift in gender ratios that favor the emergence of male turtles.