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Turtle

Testudines

Turtles are reptiles of the order Testudines with a characteristic bony shell. They are slow-moving, air-breathing animals that are opportunistic feeders, seeking almost sedentary creatures and plants for their diet. While turtles get their name from the French word tortue or tortre, the order has its roots in the Latin term testudo, which was coined by the German naturalist August Batsch in 1788. 

Around 360 living and recently extinct turtle species exist, including land-dwelling tortoises, freshwater terrapins, and marine sea turtles. Like other amniotes, including mammals, they do not lay eggs underwater, although they may live close to a water body.

Description

Size

The leatherback sea turtle (Dermochelys coriacea) is the world’s largest living turtle and the fourth-largest reptile. It measures over 2.7 m (8 ft 10 in) in length and weighs over 500 kg (1,100 lbs). 

However, overall, the largest known turtle is Archelon ischyros, a Late Cretaceous sea turtle that was up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide, and weighed over 2,200 kg (4,900 lbs).

On the contrary, Chersobius signatus of South Africa is the smallest living species with no more than 10 cm (3.9 inches) long and 172 g (6.1 oz) in weight.

Body Plan

Shell

A turtle’s shell is typically made of 50 to 60 bones arranged into a dorsal, domed carapace and a ventral, flat plastron. The latter is an assemblage of bones from the sternum, shoulder girdle, and abdominal ribs, while the carapace is fused with the vertebrae and the ribs.

The shell is externally covered with 54 scutes (epidermal scales) on the carapace and 16 on the plastron, totaling 38 scutes on the carapace and 16 on the plastron. The carapace scutes concentrated on the margins are called the marginals, while those on the vertebral column are the vertebrals. Similarly, the scutes on the neck are the cervicals, whereas those between the marginals and vertebrals are called pleurals. These scutes are typically arranged like mosaics but are found to overlap in the hawksbill sea turtle.

While terrestrial turtles have more dome-shaped shells, they are flatter, smoother, and more streamlined in aquatic dwellers to reduce underwater drag and enable smooth propulsion. In some turtles, such as the Alligator snapping turtle, the shells are spiked to provide protection from predators.

Head and Teeth

The turtle’s skull is unique among all living amniotes in having a rigid structure. There are no openings for muscle attachment or temporal fenestrae; instead, the muscles are attached to the grooves on the back of the skull.

Their skulls vary in shape from long and narrow, as in softshells, to broad and flattened in matamata. Conversely, some species have developed large and thick heads to create greater bite force.

All living turtles possess beaks characterized by keratin sheaths, which may either have sharp and serrated edges for cutting and clipping or broad plates for breaking through shells of prey.

Neck

All turtles have extremely flexible necks (to compensate for the high rigidity of their shells), with eight neck vertebrae. While sea turtles have short necks, snake-necked turtles have long necks. Some turtles have special sound-producing folds in their larynx or glottis, while others may have vocal cords made of elastin.

Limbs

Tortoises have elephant-like feet and short toes to enable terrestrial locomotion, whereas freshwater turtles have long toes with webbed feet. The front limbs in sea turtles have been modified into flippers, while the shorter hind limbs are rudder-like.

Tails

Unlike most reptiles, turtles usually have much-reduced tails, although snapping and big-headed turtles have longer tails. The cloaca is found at the base of the tail, while males have their penis inside it. In sea turtles, the tail is longer and adapted for grasping their mates during reproduction.

Male turtles generally have considerably longer tails than their female counterparts, thus making them easily distinguishable from the other sex.

Organ System

Circulatory

Turtles have three-chambered hearts comprised of two atria and only one ventricle, which is internally subdivided into three more chambers. The heart is partitioned by a muscular ridge that separates the blood flow so that it is directed either to the lungs via the pulmonary artery or to the entire body through the aorta.

Respiratory

The lungs of turtles are directly connected to the carapace above, while connective tissue anchors them to the organs below. The lungs also have multiple chambers, such as lateral and medial, and a single terminal chamber. The right lung is connected to the liver by a structure called the root, while the left lung is attached to the stomach by the mesentery. They are well-ventilated using a group of abdominal muscles attached to the organs that help to compress the lungs. 

While exhaling, the transversus abdominis muscle contracts, pushing the organs into the lungs and forcing air out of them. In contrast, while inhaling, the oblique abdominis muscle relaxes and flattens, pulling the transversus muscle down and letting air flow back into the lungs.

Turtles spend a significant amount of time underwater and can hold their breath for up to 45 minutes before surfacing in search of oxygen. Occasionally, they undergo long stretches of anaerobic respiration, in which sugars are partially broken down into lactic acid to produce energy.

Nervous

They have well-developed, sensitive eyes containing rod cells for low-light vision and cone cells with three different photopigments for clear vision in bright light. A fourth type of cone cell detects ultraviolet light and may be present in some species, like red-eared sliders. These cone cells contain oil droplets that shift the perception toward the red part of the visibility spectrum, thus enabling discrimination of different colors. Unlike terrestrial turtles, aquatic turtles possess a lens behind the cornea that helps focus light underwater.

Turtles have a scaly eardrum for auditory reception, which is covered by a bony capsule called the otic capsule (absent in other reptiles). These reptiles also possess olfactory and vomeronasal receptors along their nasal cavities for olfaction.

Excretory

All turtles possess bladders to excrete nitrogenous waste from their bodies. While sea turtles have single-unit bladders connected to two small accessory bladders, most freshwater turtles possess bilobed bladders. In tortoises that live in arid regions, the bladders act as reserves and store fluids up to 20% of their body weights.

Taxonomy

Although early phylogenetic studies have placed turtles closer to the superorder Lepidosauria (tuataras, lizards, and snakes) than to the clade Archosauria (crocodilians and birds), recent genomic-scale phylogenetic research states the converse. It is estimated that birds and turtles might have separated during the Permian Age, around 255 million years ago.

In 2013, Zhuo Wang and colleagues used genomic sequences of the green sea turtle and the Chinese softshell turtle to predict that turtles are most likely a sister group to crocodilians and birds. Currently, modern turtles and their extinct relatives with complete shells are kept under the clade Testudinata and are further divided into two living suborders: Cryptodira and Pleurodira. While Pleurodirans retract their neck sideways in front of the shoulder girdles, Cryptodirans do so only backward into the shell. They also differ in the attachment of their pelvis. In Pleurodira, the pelvis has sutures joined by bony connections, while in Cryptodira, the pelvis is free and attached to the shell solely by ligaments.

Turtles (Testudines)

Evolution

Distribution and Habitat

Turtles have a worldwide distribution, mostly inhabiting oceans or freshwater bodies, while others are terrestrial. For example, those in the suborder Pleurodira are only found in freshwater bodies of the Southern Hemisphere, while those belonging to Cryptodira are either terrestrial, freshwater, or marine dwellers. 

Again, turtles living in colder climates, such as the box turtles, live in altitudes as high as 2,000 m (6,600 ft) in New Mexico, whereas the leatherback sea turtle survives in depths as low as 1,200 m (3,900 ft). 

Diet

Although most turtles are opportunistic omnivores, terrestrial ones are mostly herbivorous, while aquatic dwellers prefer a carnivorous diet. They prefer slow-moving invertebrates, such as mollusks, worms, beetles, and insect larvae, as they are easy to catch. Nevertheless, the African helmeted turtle and snapping turtles consume fish, frogs, birds, and other turtles. Tortoises consume grasses, leaves, and fruits and sometimes supplement their meals with egg shells, animal droppings, and bones for added nutrition.

Their diet often varies with age, sex, and season. For example, the larger females of Barbour’s map turtles feed on mollusks, while the smaller males restrict feeding on tiny arthropods. Similarly, the European pond turtle, usually carnivorous, lives off water lilies in summer. Some species, such as the hawksbill, are specialists in consuming sponges, whereas the leatherback mostly feeds on jellyfish. 

Behavior

Feeding

Turtles use a very easy and uncomplicated technique for feeding. For example:

Locomotion

Burdened by their heavy shells, turtles are generally slow-moving reptiles. For example, a desert turtle moves at an average speed of 0.22 to 0.48 km/h (0.14 to 0.30 mph). However, sea turtles can accelerate up to 30 km/h (19 mph) underwater.

While freshwater turtles, such as snapping turtles and mud turtles, walk along the bottom of the sea, terrapins paddle with all four limbs, alternating between the opposing front and hind limbs to maintain directional stability.

Sea turtles, like the green sea turtle, rotate their front limb flippers like bird wings, generating propulsive force during the upstroke and downstroke. In contrast, the Caspian turtle uses its front flippers like the oars of a boat, creating a negative thrust with each recovery stroke.

Defense

Communication

Though turtles are considered rather mute animals, they often communicate through various sounds. Both marine and freshwater turtles vocalize using short, low-frequency calls to aggregate their group members during migration. They also bellow during courtship and while mating with their partners.

The oblong turtle (Chelodina oblonga) exhibits an extensive vocal range and produces sounds like clacks, clicks, squawks, hoots, various kinds of chirps, wails, growls, howls, and drum rolls.

Migration

Sea turtles are the only reptiles capable of migrating through remarkably long distances in search of breeding grounds. Sea turtles, such as the Olive Ridley, travel to beaches for nesting, where they lay their eggs. After hatching, the young turtles migrate far and wide, returning to the same site during mating and for laying eggs.

Some non-marine turtles, such as those belonging to the genera Geochelone, Chelydra, and Malaclemys, migrate over short distances (up to around 27 km) to lay eggs.

Lifespan

Turtles are generally long-lived animals. The oldest living turtle, a Seychelles giant tortoise named Jonathan, is currently 191 years old. Similarly, Harriet, a Galapagos tortoise collected by Charles Darwin, lived for an impressive 176 years and died in 2006.

However, such exceptional longevity is rare in the wild, where turtles usually survive 40 to 50 years.

Reproduction and Life Cycle

Turtles engage in various courtship behaviors before mating but do not particularly form pair bonds. A male tortoise usually bobs his head and overpowers its mate by butting or biting her. The males of mud turtles often bite the female’s tail or hind limbs, followed by mounting.

In some species, like green sea turtles, female sexual selection is the utmost priority. To avoid the male’s attempts at courtship, females have developed specific behaviors, such as swimming away or taking a refusal position by spreading their legs out and flashing their plastron towards the male. During this choice of mates, the males fight with each other and establish a dominance hierarchy for access to the females. Semi-aquatic and bottom-dwelling species sometimes take complete advantage of their large size and try to mate forcibly with their partners.

During copulation in aquatic turtles, the males mount from behind while the females support the mounting males while swimming. The male Eastern Box turtle hooks itself to the female’s plastron for ease of mounting and aligns his tail with hers to allow the entry of the penis into the cloaca. Some female turtles store the sperm inside their bodies and fertilize their eggs internally.

Most turtles lay eggs on land, though some lay near water. They usually build nests on nesting beaches to deposit these eggs; however, some species may deliver in vegetation or even inside crevices. The number of eggs laid varies between species (ranging from one to a hundred), and they are either hard or soft-shelled, oval, rounded, or elongated in shape. The sex of the young is mostly determined genetically, but in some, like sea turtles, it is determined by the incubation temperature of the mother (Temperature-dependent sex determination). While higher temperatures give rise to females in some species, similar conditions give rise to males in others. Depending on the species, the length of incubation varies from two to three months, after which they hatch. Though most mother turtles cover their eggs and leave immediately, some guard their nests for a few days or weeks.

The hatchlings use a temporary tooth on their upper beak called the egg tooth to crack open the egg and release themselves into the environment. They take refuge in water or vegetation and grow quickly over the years, slowing only after reaching maturity.

Predators

Turtles are preyed upon by carnivorous mammals, such as raccoons, opossums, skunks, and foxes. Adult turtles feed on turtle eggs and hatchlings, like monitor lizards and snakes. Similarly, birds of prey, like hawks and eagles, and other predatory waterbirds eat turtle hatchlings.

Adaptations

References Article last updated on 17th May 2024
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