This home has a turtle in the front yard. It is the only décor of any kind in the middle of the grass and trees. The residents of this home must really love turtles, think the turtle will bring them good luck or enhanced prosperity — according to feng shui or found this turtle in the bargain bin. It does look a little unusual; its legs are longer than those of a normal turtle; it stands a lot taller. Perhaps instead of a watchdog, these homeowners have a watch turtle. It does seem very alert. Regardless, turtles are fascinating creatures. As a child, I had one of those tiny turtles in a clear plastic bowl. It was fun to watch it swim around. When I went to Singapore as an adult, I saw a woman sitting by a small pond selling live turtles. They are considered symbols of good luck there. And, of course, we all know the famous story of the close cousin of the turtle “The Tortoise and the Hare.” Some of us may be slower than others, but we will win the race. Let’s learn more about turtles.
According to Wikipedia, turtles are reptiles of the order Chelonia or Testudines. They are characterized by a special bony or cartilaginous shell developed from their ribs that acts as a shield. Colloquially, the word "turtle" is generally restricted to fresh-water and sea-dwelling Testudines. Testudines includes both extant or living and extinct species. Its earliest known members date from the Middle Jurassic. Turtles are one of the oldest reptile groups, more ancient than snakes or crocodilians. Of the 360 known extant species, some are highly endangered.
Turtles are ectotherms — commonly called cold-blooded — meaning that their internal temperature varies according to the ambient environment. However, because of their high metabolic rate, leatherback sea turtles have a body temperature that is noticeably higher than that of the surrounding water. Turtles are classified as aminotes, along with other reptiles, birds and mammals. Like other amniotes, turtles breathe air and do not lay eggs underwater, although many species live in or around water.
Anatomy and morphology
The largest living chelonian is the leatherback sea turtle, which reaches a shell length of 6.6 feet and can reach a weight of over 2,000 lbs. Freshwater turtles are generally smaller. The largest terrestrial species, the Asian softshell turtle, produced a few individuals up to 6.6 feet. This dwarfs the better-known alligator snapping turtle, the largest chelonian in North America, which attains a shell length of up to 2.6 feet and weighs as much as 250 lbs. Giant tortoises of the genera Geochelone, Meiolania and others were relatively widely distributed around the world into prehistoric times and are known to have existed in North and South America, Australia, and Africa. They became extinct as humans appeared, and it is assumed humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands. They can grow to over 51 inches in length and weigh about 660 lbs.
The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 15 feet long.
The smallest living turtle is the speckled padloper tortoise of South Africa. It measures no more than 3.1 inches in length and weighs about 4.9 oz.
Neck retraction Turtles are divided into two groups, according to how they retract their necks into their shells, something the ancestral Proganochelys could not do. The mechanism of neck retraction differs phylogenetically: the suborder Pleurodira retracts laterally to the side, anterior to shoulder girdles, while the suborder Cryptodira retracts straight back, between shoulder girdles. These motions are largely due to the morphology and arrangement of cervical vertebrae. In recent turtles, the cervical column consists of nine joints and eight independent vertebrae. Since these vertebrae are not fused and are rounded, the neck is more flexible, bending in the backwards and sideways directions. The primary function and evolutionary implication of neck retraction is thought to be for feeding rather than protection. Neck retraction and reciprocal extension allow the turtle to reach out further to capture prey while swimming. Neck expansion creates suction when the head is thrust forward, and the oropharynx is expanded. This morphology suggests the retraction function is for feeding purposes, as the suction helps catch prey. The protection the shell provides the head when it is retracted is on this hypothesis not the main function of retraction, but a shift in the function during evolution. Both Pleurodirans and Cryptodirans use the quick extension of the neck as a method of predation, so the difference in retraction mechanism is not due to a difference in ecological niche.
Head Most turtles that spend most of their lives on land have eyes positioned to look down at objects in front of them. Some aquatic turtles, such as snapping and soft-shelled turtles, have eyes closer to the top of the head. These species can hide from predators in shallow water, where they lie entirely submerged except for eyes and nostrils. Near their eyes, sea turtles possess glands that produce salty tears that rid their body of excess salt absorbed from the water they drink. Turtles have rigid beaks and use their jaws to cut and chew food. Turtles appear to have lost their teeth about 150–200 million years ago. Their upper and lower jaws are instead covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. They use their tongues to swallow food, but unlike most reptiles, they cannot stick their tongues out to catch food.
Shell Its upper shell is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined on the turtle's sides by bony structures called bridges. The inner shell layer is made up of about 60 bones that include portions of the backbone and the ribs, meaning the turtle cannot crawl out of its shell. In most turtles, the shell's outer layer is covered by horny scales called scutes that are part of its outer skin or epidermis. Scutes are made up of the fibrous protein keratin. Keratin also forms the scales of other reptiles. These scutes overlap the seams between the shell bones and add strength. Some turtles do not have horny scutes; for example, the leatherback sea turtle and the soft-shelled turtles have shells covered with leathery skin instead.
The shell's shape gives clues about how a turtle lives. Most tortoises have a large, dome-shaped shell that makes it difficult for predators to crush the shell between their jaws. One of the few exceptions is the African pancake tortoise, which has a flat, flexible shell that allows it to hide in rock crevices. Most aquatic turtles have flat, streamlined shells, which aid in swimming and diving. American snapping turtles and musk turtles have small, cross-shaped plastrons that give them more efficient leg movement for walking along the bottom of ponds and streams, and the Belawan turtle from Cirebon, West Java has a sunken-back soft shell.
The shell's color may vary. Shells are commonly colored brown, black or olive green. In some species, shells may have red, orange, yellow or grey markings, often spots, lines or irregular blotches. One of the most colorful turtles is the eastern painted turtle, with a yellow plastron and a black or olive shell with red markings around the rim.
Tortoises have rather heavy shells. In contrast, aquatic and soft-shelled turtles have lighter shells that help them avoid sinking in water and swim faster and with more agility. These lighter shells have large spaces called fontanelles between the shell bones. The shells of leatherback sea turtles are extremely light because they lack scutes and contain many fontanelles.
Jackson in 2002 suggested that the turtle shell can function as a pH buffer. To endure through anoxic conditions — such as winter periods beneath ice or within anoxic mud at the bottom of ponds — turtles utilize two general physiological mechanisms: their shell releases carbonate buffers and uptakes lactic acid.
Senses At least some turtles species can see color and different species prefer different colors. Indeed, the males of some species such as the painted terrapin change color during breeding season which is thought to help individuals find a suitable mate. Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near ultraviolet to red. Some land turtles have very poor pursuit movement abilities, which are normally found only in predators that hunt quick-moving prey, but carnivorous turtles are able to move their heads quickly to snap.
Communication While typically thought of as mute, turtles make various sounds when communicating. Tortoises may be vocal when courting and mating. Various species of both freshwater and sea turtles emit numerous types of calls — often short and low frequency — from the time they are in the egg to when they are adults. These vocalizations may serve to create group cohesion when migrating. The big-headed turtle is said to growl when removed from the water in an effort to scare off predators. Turtles also have non-vocal ways of communicating that may be employed during courtship. For example, male slider turtles use their long foreclaws to stroke the female's face while courting.
Social behavior
Most turtle species do not display social behavior outside of mating season, but a few exceptions have been noted. It has been suggested that gopher tortoises — more than most other tortoise species — exhibit social behavior. Gopher tortoises live in well-defined colonies which are similar to those of highly social animals such as the prairie dog. The distribution and proximity of burrows might be the consequence of social relationships between tortoises. Some females have been observed visiting the burrows of a particular female repeatedly, even if there are other tortoises nearer to them. Some researchers have called this "a sort of friendship."
Hatchling Arrau turtles or South American river turtles are thought to communicate with each other and with females that wait for the hatchlings in order to migrate from the nesting site to the foraging sites.
Intelligence It has been reported that wood turtles are better than white rats at learning to navigate mazes. Case studies exist of turtles playing. They do, however, have a very low encephalization quotient — relative brain to body mass, and their hard shells enable them to live without fast reflexes or elaborate predator avoidance strategies. In the laboratory, turtles can learn novel operant tasks and have demonstrated a long-term memory of at least 7.5 months. Similarly, giant tortoises can learn and remember tasks, and master lessons much faster when trained in groups. Remarkably, tortoises that were tested 9 years after the initial training still retained the operant conditioning.
Reproduction Turtles have a wide variety of mating behaviors, but do not form pair-bonds or social groups. Once the eggs have hatched, neither parent provides care for the offspring. Females generally outnumber males in various turtle species — such as green turtles, and as a result, most males will engage in multiple copulation with multiple partners throughout their lifespan. Most terrestrial species are sexually dimorphic, with males larger than females, and fighting between males often determines a hierarchical order for access to mates. For most semi-aquatic species and bottom-walking aquatic species, combat occurs less often. Males belonging to semi-aquatic and bottom-walking species instead often use their larger size advantage to forcibly mate with a female. In fully aquatic species, males are often smaller than females, and rely on courtship displays rather than strength to gain mating access to females.
Male competition In some terrestrial species such as wood turtles, the males have a hierarchical ranking system based on dominance through fighting; the males with the highest rank and thus the most wins in fights have the most offspring. Galápagos tortoises are another example of a species which has a hierarchical rank that is determined by dominance displays, and access to food and mates is regulated by this dominance hierarchy. Two male saddle backs most often compete for access to cactus trees, which is their source of food. The winner is the individual who stretches their neck the highest, and that individual gets access to the cactus tree, which can attract potential mates.
Force mating The male scorpion mud turtle is an example of a bottom-walking aquatic species that relies on overpowering females with its larger size as a mating strategy. The male approaches the female from the rear and often resorts to aggressive methods such as biting the female's tail or hind limbs, followed by a mounting behavior in which the male clasps the edges of her carapace with his forelimbs and hind limbs to hold her in position. The male follows this action by laterally waving his head and sometimes biting the female's head in an attempt to get her to withdraw her head into her shell. This exposes her cloaca, and with it exposed, the male can attempt copulation by trying to insert his grasping tail.
Male radiated tortoises are also known to use the force mating strategy wherein they use surrounding vegetation to trap or prevent females from escaping, then pin them down for copulation.
Courtship displays Red-eared sliders are a fully aquatic species. The male courts the female by extending his forelegs with the palms facing out and fluttering his forelegs in the female's face. Female choice is important in this method, and the females of some species— such as green sea turtles — are not always receptive. As such, they have evolved certain behaviors to avoid the male's attempts at copulation, such as swimming away, confronting the male followed by biting or a refusal position in which the female assumes a vertical position with her limbs widely outspread and her plastron facing the male. If the water is too shallow to perform the refusal position, the females will resort to beaching themselves, which is a proven deterrent method, as the males will not follow them ashore.
Ecology and life history Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air and must surface at regular intervals to refill their lungs. Immersion periods vary between 60 seconds and one hour depending on the species. Some turtles spend much or all of their lives on dry land. Aquatic respiration in Australian freshwater turtles is currently being studied. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called papillae, have a rich blood supply and increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.
Like other reptiles, turtles lay eggs that are slightly soft and leathery. The eggs of the largest species are spherical while the eggs of the rest are elongated. Their albumen is white and contains a different protein from bird eggs, such that it will not coagulate when cooked. Turtle eggs prepared to eat consist mainly of yolk. In some species, temperature determines whether an egg develops into a male or a female: a higher temperature causes a female, a lower temperature causes a male. Large numbers of eggs are deposited in holes dug into mud or sand. They are then covered and left to incubate by themselves. Depending on the species, the eggs will typically take 70–120 days to hatch. When the turtles hatch, they squirm their way to the surface and head toward the water. There are no known species in which the mother cares for her young.
Sea turtles lay their eggs on dry, sandy beaches. Immature sea turtles are not cared for by the adults. Turtles can take many years to reach breeding age, and in many cases, breed every few years rather than annually. Researchers have recently discovered a turtle's organs do not gradually break down or become less efficient over time, unlike most other animals. It was found that the liver, lungs and kidneys of a centenarian turtle are virtually indistinguishable from those of its immature counterpart. This has inspired genetic researchers to begin examining the turtle genome for longevity genes.
Diet A turtle's diet varies greatly depending on the environment in which it lives. Adult turtles typically eat aquatic plants and invertebrates such as insects, snails and worms, and have been reported to occasionally eat dead marine animals. Several small freshwater species are carnivorous, eating small fish and a wide range of aquatic life. However, protein is essential to turtle growth and juvenile turtles are purely carnivorous. Sea turtles typically feed on jellyfish, sponges and other soft-bodied organisms. Some species with stronger jaws have been observed to eat shellfish, while others — such as the green sea turtle — do not eat meat at all and, instead, have a diet largely made up of algae.
Cultural depictions
Tortoise incarnation of Vishnu from a temple in Garhwa, India from 4th century CE
Sea turtle in Aboriginal rock art, 1600–1900
Jefferson's Embargo Act of 1807 depicted as a turtle
In Hindu mythology, the World Turtle Kacchapa, an avatar of Vishnu, supports the whole world on his back.
Lewis Carroll's 1865 “Alice’s Adventures in Wonderland” features a Mock Turtle, named for a soup meat to imitate the expensive soup made from real turtle meat.
Food, traditional medicine and cosmetics The flesh of turtles, clipash or calipee, has long been considered a delicacy in a number of cultures. Turtle soup has been a prized dish in Anglo-American cuisine. Gopher tortoise stew has been popular with some groups in Florida. Turtles remain a part of the traditional diet on the island of Grand Cayman, so much so that when wild stocks became depleted, a turtle farm was established specifically to raise sea turtles for their meat. The farm releases some turtles to the wild to help repopulate the Caribbean Sea. Fat from turtles is used in the Caribbean and in Mexico as a main ingredient in cosmetics, marketed under its Spanish name “crema de tortuga.” The supposed aphrodisiac or medicinal properties of turtle eggs created a large trade for them in Southeast Asia. Turtle plastrons are widely used in traditional Chinese medicine; Taiwan imports hundreds of tons of plastrons every year. A popular medicinal preparation based on herbs with or without powdered turtle plastron is guilinggao jelly. The shell of the hawksbill turtle has been used for centuries to make jewelry, tools and ornaments around the Western Pacific.
Comments