Turtles are an order of reptiles known as Testudines, characterized by a shell developed mainly from their ribs. Modern turtles are divided into two major groups, the side-necked turtles and hidden neck turtles, which differ in the way the head retracts. There are 360 living and recently extinct species of turtles, including land-dwelling tortoises and freshwater terrapins.
In this blog, Pritish Kumar explained Testudines, and their different properties.
They are found on most continents, some islands and, in the case of sea turtles, much of the ocean. Like other reptiles, birds, and mammals, they breathe air and do not lay eggs underwater, although many species live in or around water. Genetic evidence typically places them in close relation to crocodilians and birds.
Shells are made mostly of bone; the upper part is the domed carapace, while the underside is the flatter plastron or belly-plate. Its outer surface is covered in scales made of keratin, the material of hair, horns, and claws. The carapace bones develop from ribs that grow sideways and develop into broad flat plates that join up to cover the body.
This are ectotherms or “cold-blooded”, meaning that their internal temperature varies with their direct environment. They are generally opportunistic omnivores and mainly feed on plants and animals with limited movements. Many turtles migrate short distances seasonally. Sea turtles are the only reptiles that migrate long distances to lay their eggs on a favored beach.
The largest living species of turtle (and fourth-largest reptile) is the leatherback turtle, which can reach over 2.7 m (8 ft 10 in) in length and weigh over 500 kg (1,100 lb). Largest known turtle was Archelon isochrons, a Late Cretaceous Sea turtle up to 4.5 m (15 ft) long, 5.25 m (17 ft) wide between the tips of the front flippers, and estimated to have weighed over 2,200 kg (4,900 lb). The smallest living turtle is Chaetopods signatus of South Africa, measuring no more than 10 cm (3.9 in) in length and weighing 172 g (6.1 oz).
The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements. It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) carapace and the flatter, ventral (belly) plastron. They are connected by lateral (side) extensions of the plastron.
The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs). During development, the ribs grow sideways into a carapace ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace.
Development is signalled locally by proteins known as fibroblast growth factors that include FGF10. The shoulder girdle in turtles is made up of two bones, the scapula and the coracoid. Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.
Head and neck
The turtle’s skull is unique among living amniotes (which includes reptiles, birds and mammals), it is solid and rigid with no openings for muscle attachment (temporal fenestrae). Muscles instead attach to recesses in the back of the skull. Turtle skulls vary in shape, from the long and narrow skulls of softshell to the broad and flattened skull of the mate mata. Some turtle species have developed large and thick heads, allowing for greater muscle mass and stronger bites.
Necks of turtles are highly flexible, possibly to compensate for their rigid shells. Some species, like sea turtles, have short necks while others, such as snake-necked turtles, have long ones. Despite this, all turtle species have eight neck vertebrae, a consistency not found in other reptiles but similar to mammals. Some snake-necked turtles have both long necks and large heads, limiting their ability to lift them when not in water. Some turtles have folded structures in the larynx or glottis that vibrate to produce sound. Other species have elastin-rich vocal cords.
Turtles make use of vision to find food and mates, avoid predators, and orient themselves. The retina’s light-sensitive cells include both rods for vision in low light, and cones with three different photopigments for bright light, where they have full-colur vision. There is possibly a fourth type of cone that detects ultraviolet, as hatchling sea turtles respond experimentally to ultraviolet light, but it is unknown if they can distinguish this from longer wavelengths. A freshwater turtle, the red-eared slider, has an exceptional seven types of cone cell.
Sea turtles orient themselves on land by night, using visual features detected in dim light. They can use their eyes in clear surface water, muddy coasts, the darkness of the deep ocean, and also above water. Unlike in terrestrial turtles, the cornea, the curved surface that lets light into the eye, does not help to focus light on the retina, so focusing underwater is handled entirely by the lens, behind the cornea.
Turtles have no ear openings; the eardrum is covered with scales and encircled by a bony otic capsule, which is absent in other reptiles. Their hearing thresholds are high in comparison to other reptiles, reaching up to 500 Hz in air, but underwater they are more attuned to lower frequencies.
They have olfactory (smell) and vomeronasal receptors along the nasal cavity, the latter of which are used to detect chemical signals.
The rigid shell of turtles is not capable of expanding and making room for the lungs, as in other amniotes, so they have had to evolve special adaptations for respiration. The lungs of turtles are attached directly to the carapace above while below, connective tissue attaches them to the organs. They have multiple lateral (side) and medial (middle) chambers (the numbers of which vary between species) and one terminal (end) chamber.
Diet and feeding
Most turtle species are opportunistic omnivores; land-dwelling species are more herbivorous and aquatic ones more carnivorous. Generally lacking speed and agility, most turtles feed either on plant material or on animals with limited movements like molluscs, worms, and insect larvae.
Turtles generally eat their food in a straightforward way, though some species have special feeding techniques. The yellow-spotted river turtle and the painted turtle may filter feed by skimming the water surface with their mouth and throat open to collect particles of food. When the mouth closes, the throat constricts and water is pushed out through the nostrils and the gap in between the jaws. Some species employ a “gape-and-suck method” where the turtle opens its jaws and expands its throat widely, sucking the prey in.
The diet of an individual within a species may change with age, sex, and season, and may also differ between populations. In many species, juveniles are generally carnivorous but become more herbivorous as adults. With Barbour’s map turtle, the larger female mainly eats molluscs while the male usually eats arthropods. Blanding’s turtle may feed mainly on snails or crayfish depending on the population.
The European Pond turtle has been recorded as being mostly carnivorous much of the year but switching to water lilies during the summer. Some species have developed specialized diets such as the hawksbill, which eats sponges, the leatherback, which feeds on jellyfish, and the Mekong snail-eating turtle.
When sensing danger, a turtle may flee, freeze or withdraw into its shell. Freshwater turtles flee into the water, though the Sonora mud turtle may take refuge on land as the shallow temporary ponds they inhabit make them vulnerable. When startled, a softshell turtle may dive underwater and bury itself under the sea floor. If a predator persists, the turtle may bite or discharge from its cloaca. Several species produce foul-smelling chemicals from musk glands. Other tactics include threat displays and Bell’s hinge-back tortoise can play dead. When attacked, big-headed turtle hatchlings squeal, possibly startling the predator.
Turtles, including sea turtles, lay their eggs on land, although some lay eggs close near water that rises and falls in level, submerging the eggs. While most species build nests and lay eggs where they forage, some travel miles. The common snapping turtle walks 5 km (3 mi) on land, while sea turtles travel even further; the leatherback swims some 12,000 km (7,500 mi) to its nesting beaches. Most turtles create a nest for their eggs.
Females usually dig a flask-like chamber in the substrate. Other species lay their eggs in vegetation or crevices. Females choose nesting locations based on environmental factors such as temperature and humidity, which are important for developing embryos. Depending on the species, the number of eggs laid varies from one to over 100. Larger females can lay eggs that are greater in number or bigger in size. Compared to freshwater turtles, tortoises deposit fewer but larger eggs. Females can lay multiple clutches throughout a season, particularly in species that experience unpredictable monsoons.