Batrachemys zuliae m
Bones and scutes of the plastron and carapace of Cryptodira and Pleurodira. Carapace bones: nu=nuchal, pe=peripheral, ne=neural, pl=pleural, sp=suprapygal, py=pygal. Carapace scutes: nu=nuchal, m=marginal, v=vertebral, c=costal. Plastron bones: ep=epiplastron, en=entoplastron, hyo=hyoplastron, hyp=hypoplastron, x=xiphiplastron. Plastron scutes: i=intergular, g=gular, h=humeral, p=pectoral, ab=abdominal, f=femural, an=anal, ax=axial, in=inguinal. Some pleurodires have mesoplastrons between the hyoplastron and hypoplastron. (Illustration by Gillian Harris)
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 bur-sae, 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
Lateral view carapace-
Ventral view phalanges-metacarpals carpals -radius ulna humerus femur tibia fibula -tarsals horny layer, bony layer-
pelvic girdle metatarsals phalanges plastron vertebrae humerus
— fused to carapace
Skeletal structure of a turtle. (Illustration by Jacqueline Mahannah)
cervical vertebrae acromion scapula anterior coracoid vertebrae-
— fused to carapace pelvic girdle caudal vertebrae
Skeletal structure of a turtle. (Illustration by Jacqueline Mahannah)
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.
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.
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 gener-
ally 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 sea-turtles 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 ringlike 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.
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 world-
wide. 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 deterio-
ration associated with the expansion of human activities. Indeed, nearly 50% of living species are listed as Endangered or Vulnerable.
Ernst, C. H., and R. W. Barbour. Turtles of the World. Washington, DC: Smithsonian Institution Press, 1989.
Ernst, C. H., J. E. Lovich, and R. W. Barbour. Turtles of the United States and Canada. Washington, DC: Smithsonian Institution Press, 1994.
Pough, F. H., R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.
Zug, G. R., L. J. Vitt, and J. P. Caldwell. Herpetology: An Introductory Biology of Amphibians and Reptiles. San Diego, CA: Academic Press, 2001.
Patrick J. Baker, MS
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