Testudines (Turtles and Tortoises)

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Testudines

(Turtles and tortoises)

Class Reptilia

Order Testudines

Number of families 14

Number of genera, species About 99 genera; at least 293 species

Evolution and systematics

Turtles first appeared in the fossil record during the Triassic period, about 220 million years ago. They were originally believed to have evolved from early anapsid reptiles (lanthanosuchids, millerettids, nytiphruretians, pareiasaurs, and the procolophonoids), but recent studies (mostly molecular) argue for a diapsid origin (the group that includes the squamate reptiles, the crocodilians, and the birds). Two mechanisms for retraction of the neck evolved in ancestral turtles. The members of the suborder Pleurodira (or side-necked turtles) retract their necks laterally between the carapace and the plastron, while those in the suborder Cryptodira (or hidden-necked turtles) retract their neck vertically. The pelvic girdle is primitive in shape, and fixed to the plastron in side-necked turtles.

Physical characteristics

These reptiles are easily recognized by the presence of a dorsal bony carapace and a ventral bony plastron, with the limb girdles located inside the ribs. All living forms lack teeth, have internal fertilization, and lay shelled amniotic eggs.

A turtle's shell first and foremost provides protection from predators and serves to buffer harsh environmental conditions. Most species can retract the head and limbs completely within the shell when distressed. The upper shell, known as the carapace, is typically joined to the lower shell or plastron by a bony bridge.

Bony plates, which develop from the dermal (lower) layer of the skin, widen and fuse to one another and with the vertebrae and ribs to form the carapace. Neural bones form along the midline, pleurals from the ribs, as well as the peripherals, which are outermost. The plastron is composed of nine bony elements; the paired hyo- and hypoplastra, epiplastra, xiphiplastra, and a single entoplastron are derived from the pectoral girdle, the sternum, and abdominal ribs (gastralia). The modified shoulder girdle remains inside the ribs, a remarkable arrangement found in no other vertebrate. The outer surface of the shell is generally covered by horny scutes derived from keratin in the epidermal (upper) layer of skin. The scutes overlap the sutures of the bone, which increases the strength of the shell and protects the growing portions.

Many variations on the basic shell structure have evolved over time. Softshell turtles, which lack horny scutes, have reduced pleurals, and most have completely lost the peripheral bones. Although softshell turtles lack a bony bridge, the carapace is firmly attached to the plastron by connective tissue.

The plastron is greatly reduced, and the bones are loosely connected to one another by cartilage. When fully formed, the plastral bones of an adult softshell turtle may be covered by two to nine leathery callosities, thick callous-like layers of epidermis that cover the plastral bones of softshell turtles. Callosities are generally absent in hatchlings, slowly developing with growth and attaining full size when the seaturtle reaches maturity.

An elastic cartilaginous hinge has arisen independently in many lineages. A box turtle can withdraw its head and limbs completely within its shell and draw the anterior and posterior lobes of the plastron tightly against the carapace. Some African tortoises have a carapacial hinge that allows the rear portion to close upon the plastron, thereby protecting the hind limbs. Female semiaquatic and terrestrial species may have a hinge in the posterior lobe of the plastron, providing the flexibility necessary to lay extremely large eggs.

The size and shape of the turtle shell may also be adapted to the environment. The broad, flattened carapace of some aquatic turtles functions like a solar panel. A basking turtle will change its position on a log or rock so that the greatest surface area is exposed to the Sun. In some northern species, the scutes of the carapace are darkened, allowing maximal absorption of solar radiation. Aquatic turtles have lower, more streamlined shells that offer less resistance while swimming. Extreme flattening is found among the softshells, which hide in shallow water beneath a thin layer of sand or mud. The flattened shell of the pancake tortoise (Malacochersus tornieri) in East Africa allows it to squeeze into narrow crevices within its rocky habitat. With the force generated by its legs and the natural elasticity of the shell bones, this tortoise is extremely difficult to extract once it has wedged itself between rocks.

Aquatic species that share their habitat with large crocodilians have traded the advantages of streamlining in favor of high-vaulted, strongly buttressed shells for protection from being crushed. Among Asian river turtles (e.g., river terrapins [Batagur baska], crowned river turtles [Hardella thurjii], and painted roofed turtles), these buttresses form bony chambers that enclose the lungs and prevent compression during deep dives. In desert-dwelling tortoises, the domed shell reduces surface area relative to volume, while a thickened keratin layer retards evaporative water loss. Some tortoises have even been observed gathering drinking water by angling the carapace

forward during a rainfall to catch water dripping from the shell.

Large openings, or fontanelles, between the pleural bones in the carapace have evolved in several genera. The leatherback seaturtle (Dermochelys coriacea) shows the greatest divergence from the characteristic turtle shell; tiny platelets embedded in the leathery carapace are all that remain of the bony shell. A reduced carapace decreases the physiological costs associated with building and maintaining a heavy shell (such as sequestering minerals to a substrate where they are not readily accessible for some physiological processes), as well as the energy cost for locomotion in terrestrial species, and provides greater buoyancy in the aquatic forms.

The turtle skull is also unique among living vertebrates for the absence of temporal fenestra. Formerly believed to be a true anapsid (i.e., without such openings), many researchers now believe that the turtle evolved from diapsid ancestors. In this scenario, the two pairs of temporal openings were secondarily lost, giving an anapsid-like appearance. Vestiges of the temporal openings can be seen in the slightly arched posterior margin of some turtle skulls. These large openings in the back of the skull allow the muscles of the jaw to expand beyond the confines of the adductor chamber.

Dietary preferences range from completely herbivorous to totally carnivorous; however, many turtles consume a mixture of plant and animal matter. In some species there is a dietary shift from the carnivorous diet of hatchlings to the mostly herbivorous adults. Although modern turtles lack teeth, there are many modifications of the maxillary, premaxillary, and dentary bones for feeding. A pronounced beak made of keratin may be used to hold and tear food. The palate of herbivorous turtles contains a series of ridges that assist in the maceration of plant matter. Macrocephaly, characterized by an enlarged head (as in many female map turtles), often develops in mollusk-feeding species. The broad crushing surfaces and powerful musculature allow them to exploit an abundant food item that may be unavailable to turtles that cannot extract this meal from the mollusk's protective shell.

The limbs of most aquatic turtles terminate in five independent digits; however, most terrestrial turtles and tortoises have reduced phalanges. The limbs of freshwater turtles are flattened laterally, and the digits are generally webbed. The sturdy limbs of tortoises are round in cross section. In some highly aquatic species, such as seaturtles and pig-nose turtles, the limbs are paddlelike, and digits are reduced to just a few claws.

The texture of turtle skin ranges from virtually scaleless and smooth in highly aquatic softshell species, to the coarse scaly texture of terrestrial tortoises. Keratin scales of various shapes and sizes are found on the head and limbs of most species. The large, thickened scales of tortoises are adapted to their dry environments. Epidermal appendages such as chin barbels, warts, and the fringe of matamatas provide cryptic camouflage that may assist in prey acquisition and/or provide protection.

The major organs of the turtle circulatory system are similar to those of other reptiles. Although heart rate is largely dependent upon temperature, the three-chambered heart beats slowly, especially in tortoises. The paired lungs are dorsal to the visceral organs and cannot be expanded by the action of

the rib muscles. Ventilation of the lungs is controlled by contraction of lung muscles; however, in the relaxed state the lungs are maximally filled with air. By manipulating airflow from one chamber of the lung to another, aquatic turtles can adjust their position in the water much like a fish uses a swim bladder. This ability is impaired in turtles with respiratory ailments and results in a diagnostic lopsided appearance.

Aquatic species may also respire through their skin, the lining of the throat, and through thin-walled sacks, or bursae, in the cloaca. The Fitzroy River turtle (Rheodytes leukops), an Australian sideneck living in well-oxygenated streams, maintains a widely gaping cloacal orifice and rarely surfaces. The turtle pumps water through the cloaca, which gapes in sequence to the pumping. Although common to most aquatic species, cloacal bursae are absent in softshell turtles. In these aquatic turtles, 70% of the submerged oxygen intake is through the skin and 30% is through the lining of the throat. In northern climates, turtles that spend most of the winter trapped below the ice must rely upon submerged oxygen uptake or tolerate long periods without oxygen. The mineralized shell of the painted turtle buffers the accumulation of

lactic acid formed under anaerobic conditions to maintain a stable blood pH through the winter.

The largest extant species is the leatherback seaturtle, which attains a shell length of 96 in (244 cm) and may weigh up to 1,191 lb (867 kg). Of the freshwater species, the alligator snapping turtle (31 in/80 cm; 249 lb/113 kg), the Asian narrow-headed softshell turtle (Chitra indica) (47 in/120 cm; 330 lb/150 kg), and the South American river turtle (42 in/107 cm; 198 lb/90 kg) attain impressive sizes. The Aldabra tortoise (55 in/140 cm; 562 lb/255 kg) is the largest living terrestrial species. With maximum shell lengths of less than 4.7 in (12 cm), the speckled cape tortoise, flattened musk turtle, and bog turtle are among the world's smallest turtles.

Distribution

Turtles and tortoises exist on all continents except Antarctica. The diversity of these species allows them to inhabit both temperate and tropical regions, as well as all bodies of water.

Reproductive behavior

Most turtle species exhibit sexual size dimorphism. Among aquatic species, males are generally smaller than females and have elaborate courtship behavior. However, in semiaquatic, bottom-walking species and tortoises, in which males are equal to or larger than females, courtship displays are generally

minimal, and combat for territories and/or mates is common. In temperate climates, courtship and mating may occur in the fall or the spring, but nesting usually occurs in the spring to early summer.

Although individual females may not reproduce every year, nesting in most species is annual and seasonal. Females of many species can store sperm in their oviducts for years and produce fertile eggs without mating annually. In addition, DNA analysis has shown that eggs within the same clutch are sometimes fertilized by more than one male.

The majority of turtles select nest sites from the available upland habitats found in the vicinity of their foraging areas. However, some sea and river turtles make extensive migrations to nesting beaches. Seaturtles, which nest every two to three years, may migrate over 2,796 mi (4,500 km) to nest in a specific location. During the arribada (a massive, coordinated arrival of seaturtles, and some freshwater species, at a nesting beach) of the olive ridley seaturtle, as many as 200,000 females nest on the same small beach over a period of one or two days. The large freshwater river turtles of South America and Asia similarly nest en masse. The predators on the nesting beach are overwhelmed by the reproductive output and many nests escape detection.

Turtle eggs are usually deposited in flask-shaped chambers excavated into the ground. However, some turtles may oviposit, or deposit their eggs, in decaying vegetation and litter, in nests of other animals, or even in a nest constructed while the female is completely underwater or underground. Some species quickly cover the eggs and leave the area, while others spend considerable time concealing the nest. Despite their vulnerability on land, leatherback seaturtles obscure the site completely before returning to the sea. Some species may construct a false nest some distance from the first or divide the clutch between two or three nests to confound predators. Although parental care is rare in turtles, the Asian giant tortoise, which nests in mounded vegetation, will defend her eggs from potential predators for several days following oviposition.

Reproductive output is related to body size, both within and across species. Smaller species lay one to four eggs per clutch, whereas large seaturtles regularly lay over 100 eggs at a time. The majority of species lay two or more clutches each nesting season. At higher latitudes there is also a general trend, both within and across species, toward the production of one large clutch of smaller eggs.

Turtle eggs are of two shapes: elongate or spherical. Although egg shape is usually consistent within a genus, members of diverse families such as the tortoises and side-necked turtles may lay eggs of either shape. The spherical shape has the lowest possible surface-to-volume ratio, and therefore is less vulnerable to dehydration. Turtles that produce large clutches (50 eggs or more) have spherical eggs to make efficient use of the limited space available.

The eggs of most turtles have flexible, leathery shells, but the shells of other turtles are more inflexible and often brittle. Eggs with brittle shells tend to be more independent of the environment, losing and absorbing less water than eggs with flexible shells. However, those with flexible shells often develop faster. Species that do not dig sophisticated nests, or those that nest in particularly dry or very moist soils, tend to lay eggs with brittle shells. Conversely, turtles nesting on beaches prone to flooding, or in areas with limited growing seasons, situations where rapid egg development is important, are more likely to lay eggs with flexible shells.

In most turtles, the temperature during incubation also determines the sex of the hatchling. In species with "temperature-dependent sex determination" (TSD), the temperature during the middle third of incubation affects the biochemical pathway that determines the sex of the hatchling. Two patterns of TSD have been described for turtles. Type I species have a narrow pivotal temperature range (usually between 80.6–89.6°F/27–32°C) above which only females are produced and below which only males result. Type II species have two pivotal temperature ranges, with males predominating at intermediate temperatures, and females predominating at both extremes. Sex determination appears to be genetically determined (GSD) in the Austro-American side-necked turtles, all softshells, and a few musk and pond turtles. Among species with GSD, only the wood turtle, two species of giant musk turtles, the black marsh turtle, and the brown roofed turtle have dimorphic sex chromosomes; all others have identical chromosome sizes in males and females. The evolutionary advantage conferred by these modes of sex determination remains unknown.

When fully developed, hatchling turtles use their caruncle, a small tubercle on the upper beak, to slice through the embryonic membranes and eggshell. Soon after hatching, most neonates emerge from the nest and head directly for cover of water or vegetation. Vibrational cues, such as movement by hatchlings within the nest, may help neonate seaturtles to coordinate the intense effort necessary for emergence from their sandy nest chamber. Hatchlings of a few temperate species (ornate box turtle, yellow mud turtle) delay emergence from the nest. After hatching, they immediately dig downward a few feet or more below the nest, presumably a behavioral adaptation to avoid the impending lethal winter temperatures in shallow water or soil. Hatchlings of a few other temperate species, such as the painted turtle, remain in the nest over the winter where they may be exposed to temperatures of 10.4°F (−12°C) or lower. Although these turtles tolerate freezing at high subzero temperatures (e.g., to 24.8°F/−4°C), they must remain supercooled (i.e., without the tissues freezing) in order to survive colder temperatures. Still other turtles, particularly those in highly seasonal tropical environments, must remain in their nests until rain softens the soil, allowing them to dig out. In dry years, the neonates may remain in the nest chamber for more than a year after hatching.

Growth may vary considerably even within the same clutch. Habitat, temperature, rainfall, sunshine, food type and availability, and sex have each been associated with growth rate in turtles. Growth can be conveniently studied in turtles because many species retain evidence of seasonal growth on their scutes. The rate of growth is also reflected in the ring-like layers of bone deposited on the femur and humerus. In most species, the turtle grows rapidly to sexual maturity; then the growth rate slows markedly. In later years, small species may stop growing completely.

Conservation status

Commonly known as turtles, tortoises, and terrapins, members of the order Testudines are distinguished from all other vertebrates by their bony shell. The protection conveyed by this morphological curiosity has contributed to the persistence of this group through more than 200 million years of evolution. Turtles and their shells have survived the conditions that resulted in the fall of the dinosaurs, the shifting of continents, and the ebb and flow of glaciers with little structural modification. They appear in the folklore, art, and creation myths of many human cultures, but humans are a major reason for the precipitous decline in turtle populations worldwide.

The characteristics of turtle life history (e.g., late maturity, extreme longevity, and low adult mortality) make them especially vulnerable to the habitat destruction and deterioration associated with the expansion of human activities. Indeed, nearly 50% of living species are listed as Endangered or Vulnerable.