Natural history ( reptile )
Tagged: animal, animals, cat, mammalReproduction and life cycle
Courtship
Courtship in some form is such a widespread prelude to mating among modern reptiles that it must have characterized many extinct groups as well. When courting, the male of some freshwater turtles, such as the red-eared turtle (Chrysemys scripta elegans) of the eastern United States, orients himself in the water so that he is directly in front of a female and facing her. With his forefeet close together, the male vibrates his claws against her head. If the female is receptive, she swims forward slowly while the male backs away. Finally the female sinks slowly down. The male then mounts her from behind, clutching her shell with all four feet. His tail is brought under hers and his penis introduced into her cloaca. The male of some terrestrial turtles of North America (e.g., the gopher tortoises, Gopherus) begins courtship by extending his neck and bobbing his head up and down. The courted female may bob her head in return. The male advances, nips at the female, and then circles her as she turns away from him. As soon as she shows signs of response, the male mounts her from the rear and begins a series of pumping movements that bump the rear of his shell against the ground. Finally the female extends her tail, and copulation begins. Males of the smaller box turtles (Terrapene) nip and butt at the female. Male crocodilians bellow during the mating season, and in many cases the females respond with an answering call. The male American alligator, which copulates in water, grasps the female’s neck with his jaws and slips the rear part of his body under hers to enable him to insert his penis into her cloaca. Lizards have rather elaborate courtship patterns usually involving display and posturing by the males, who often have distinctive patches of colour on their throats or low on their sides. The male bobs up and down, thus exposing the patches of colour, which may be blue, orange, red, or black depending on the species. Males of some species, such as the green anole (Anolis carolinensis) of the southeastern United States, have brightly coloured folds of skin at the throat (dewlaps) that are expanded during courtship. If the female seems receptive, the male straddles her back, often gripping her back or legs with his jaws. Just before copulation, his tail is bent under hers. The courtship patterns of snakes are simpler; they usually consist of the male’s crawling over the back of the female and adopting every curve her body takes, then vibrating against her body or nudging it with waves of his own. Male boas and pythons stroke or scratch the female’s body with their vestigial hind limbs. The male water snake rubs his chin against the female’s back. The male rattlesnake (Crotalus) frequently nudges the female with his head. The male bull snake (Pituophis) grasps the female’s neck with his jaws during copulation. When the male king cobra (Ophiophagus hannah) crawls onto the back of a receptive female, he flicks his tongue against her repeatedly. The female raises her head and spreads her hood. The male nudges her neck and head with his snout and lifts the rear of her body with his tail. Copulation between cobras may last more than two hours.
The embryo
The embryo of a reptile develops a thin sac, the amnion, that envelops the embryo and becomes filled with a watery fluid. The entire structure serves to protect the embryo from desiccation and mechanical injury. A parchment-like shell, which also contributes mechanical protection, is produced by the female parent and surrounds the amnion; between the shell and the amnion a second sac, the allantois, becomes inserted. The allantois, which is supplied with many fine blood vessels, serves as a respiratory organ, absorbing the oxygen and emitting the carbon dioxide that pass through the somewhat porous shell.
Egg laying
The typical mode of reptilian reproduction is oviparous (i.e., the female lays eggs in which the young develop). The eggs are laid shortly after fertilization, and development of the embryos takes place largely after the eggs have been laid. This pattern characterizes crocodilians, turtles, the tuatara, most lizards and snakes, and many extinct reptiles. The size of eggs laid by lizards and snakes varies according to the size of the females. The banded rock lizard (Petrosaurus mearnsi) of the western United States, which ranges from 7.5 to 10 centimetres (three to four inches) in length, lays eggs that are about one centimetre (0.4 inch) long; those of the 30- to 60-centimetre (12- to 24-inch) ringneck snake (Diadophis punctatus) from the eastern United States are 1.25 centimetres (0.5 inch) long. Eggs of the three-metre Komodo dragon lizard (Varanus komodoensis) and of the six-metre (20-foot) Indian python (Python molurus) are about 11.25 centimetres (4.5 inches) long. A minority of modern and extinct reptiles are (and were) live-bearing, or viviparous. Strictly speaking, most of this minority are not truly viviparous but ovoviviparous, because the embryos develop with their shells or shell membranes intact and are nourished wholly by the yolk. In a few modern reptiles the embryonic membranes and the tissues lining the oviducts of the females come into close contact and are modified in one of several ways to provide a temporary organ, through which food and respiratory gases are exchanged; i.e., a structure similar to the placenta of mammals. In the simplest reptilian “placenta,†the most superficial layer (the lining of the oviduct partially degenerates, thereby bringing the blood vessels of embryo and mother closer together. The approximation of the two bloodstreams facilitates the exchange of oxygen and carbon dioxide, this gas exchange being the only function of the organ at this stage of evolution. Several Australian snakes (Denisonia superba and D. suta) and a number of lizards e.g., the common East Indian brown-sided skink (Mabuya multifasciata) and the cylindrical skink (Chalcides ocellatus) of southern Europe and North Africa are known to have this type of organ, as presumably do many ovoviviparous reptiles. The best developed reptilian “placentas†consist of apposed, thickened, folded elliptical areas of the outer embryonic membrane and lining of the oviduct. The ridges of the oviductal areas are filled with blood vessels, and the epithelium between ridges is thickened and glandular. Usually, eggs developing with this type of “placenta†have less yolk; food and oxygen are transmitted from mother to embryo. Several species of Australian lizards, American water snakes, and the common European viper (Vipera berus) are known to provide this type of internal environment for their developing young. The line between oviparity (egg laying) and ovoviviparity (hatching of eggs in the mother’s body) is arbitrary. Females of some lizards and snakes retain the fertilized eggs in their bodies for a few days before laying them. Other species retain the eggs for most of the developmental period, hatching occurring shortly after laying. For the grass snake of England (Natrix natrix), the lapse of time between copulation and egg laying is usually two months; the young hatch six to ten weeks later. The interval between mating and egg laying is one month in the Texas horned lizard (Phrynosoma cornutum). A given species may be ovoviviparous in parts of its range and oviparous elsewhere.
The nest
The eggs of modern reptiles may be deposited in a nest prepared by the female or simply laid under some convenient cover, such as a rock or log. Crocodilians invariably prepare a nest, and the female invariably does the work. Most turtles dig their nests, scooping out a flask-shaped cavity in the ground with their hindfeet. When the hole has reached the proper size, the oval or spherical eggs, 1.5 to 3.75 centimetres (0.6 to 1.5 inches) in diameter depending on the species, are dropped into the nest from the female’s cloaca. The female scratches soil over the eggs, usually obliterating the nest site. Crocodilians either dig a nest along the bank of a river or lake or heap together a mass of dead vegetation in which the eggs are laid; their oval eggs are usually about five centimetres (two inches) long. Most oviparous lizards merely hide their eggs under some convenient cover such as under a rock or in a hole in a tree. Nest construction among lizards, though appearing in such diverse families as the iguanids, skinks, and true chameleons, is neither so elaborate nor so rigid in pattern as among turtles. The nest consists of a small hole made by either the snout or the limbs. Soil or leaves usually are pushed on top of the eggs to hide the nest, although the entrance to the nest cavity is kept open in a few species. Snakes, like lizards, usually lay their eggs under natural, pre-existing cover. The king cobra, one of the very few nest-building snakes, drags dead vegetation into a low heap by bending its body. The eggs are laid in a cavity at the centre. Other snakes deposit their eggs in holes they have scooped out of sand or soft earth with their snouts.
Number of offspring
The number of eggs in a clutch or offspring in a brood varies from one to 200 among living reptiles; presumably similar variation occurred among the extinct types. Crocodilians lay from 20 to 70 eggs, turtles from one to 200. In turtles, more so than in crocodiles, the number varies with the species and roughly with the size attained by the females. The big marine turtles have the largest clutches (usually more than 100); the smaller land and freshwater turtles have much smaller ones. The number of eggs or young is not so closely related to the size of mature females in species of lizards and snakes. With few exceptions, lizards of the family Gekkonidae lay two eggs at a time, regardless of the size of the female. Lizards of the family Scincidae have broods varying from two to about 30; one of the largest members of this family, the 30-centimetre-long (12-inch) stump-tailed skink (Tiliqua rugosa) of Australia, has only two young at a time, whereas the Great Plains skink (Eumeces obsoletus) of the United States usually lays between ten and 20 eggs in a clutch. Clutch size in the 1.5-metre (five-foot) bull snake (Pituophis catenifer) of western United States is usually ten or 12; in the grass snake of England and Europe, which measures 60 to 90 centimetres (two to three feet), it is 30 to 40; in the giant reticulated python of Southeast Asia and the East Indies, it may reach 100.
Parental care
Parental care of eggs and newborn young is neither well developed nor elaborated among reptiles. Female crocodilians generally remain in the vicinity of their nests and chase would-be predators from the site. In a few lizards the female returns to the nest between feeding excursions to coil around the eggs and turn them at intervals. The male and female king cobra remain in the vicinity of the nest, and one of the parents usually is coiled above the egg cavity. Female pythons coil around their eggs and pull them into a heap. Females of some species remain with the eggs for the entire two-month incubation period; others leave the eggs only to drink. In at least one species (Python molurus) the female provides heat by muscular contraction to keep the eggs at incubation temperature on cool nights. Turtles and the majority of egg-laying lizards and snakes abandon their eggs after they are laid.
Incubation period
The incubation (or gestation) period of reptilian eggs is affected by many factors and to such an extent that it is difficult to assign a figure characteristic of a given species. One source of complication is the combination of oviparous and ovoviviparous habits by certain species. The developmental period of the embryo, whether it occurs within the female’s body or outside it, is referred to as the gestation period. In general, the gestation period lasts from 60 to 105 days in most American and European reptiles. The eggs of the American alligator hatch about 63 days after they are laid, those of the small Eastern fence lizards (Sceloporus undulatus) in about the same time. The gestation period in the common European viper lasts from 60 to 90 days. Eggs of marine turtles hatch between 30 and 75 days after they are laid, depending on the site. The temperatures to which a brood is subjected shorten or lengthen gestation according to whether the temperatures are high or low.
Growth and longevity
Giant Galápagos tortoises kept under nearly ideal conditions have been known to increase their weight from 3.2 to six kilograms (seven to 13 pounds) to about 82 kilograms (180 pounds) in nine years. Smaller species also grow rapidly. The box turtle of the United States has a shell about 3.75 centimetres (1.5 inches) long at the end of its first year; at the end of five years, the length has doubled. Under favourable conditions a one year old American alligator is about 60 centimetres (24 inches) long and weighs about 1.8 kilograms (four pounds). At the end of six years, males average about 190 centimetres (about six feet) and about 36 kilograms (80 pounds). The red diamond rattlesnake is about 30 centimetres long at birth, grows to about 65 centimetres in its first year, reaches about 85 centimetres (33 inches) by the end of its second year, and grows more slowly after that. The pattern for lizards is much the same: rapid growth early in life and slow growth afterward. The significant difference between growth in reptiles and that in mammals is that a reptile has the potential of growing throughout its life, whereas a mammal reaches a terminal size and grows no more, even though it may subsequently live many years in ideal conditions. The length of time needed to attain sexual maturity varies greatly among reptiles and, although roughly related to the size usually attained by the species, is even more closely related to the climate in which the animal lives. The red diamond rattlesnakes in southern California, for example, bear their first young when three years old; on the other hand, the northern Pacific rattlesnake (Crotalus viridis oreganus) bears its first litter when four years old. The much smaller common garter snake is sexually mature shortly before the age of two years. The 12.5- to 15-centimetre (five to six inch) northwestern sagebrush lizard (Sceloporus graciosus gracilis), living in the Sierra Nevada range of California at an elevation (about 1,800 metres [6,000 feet]) where it has, at most, six months of activity each year, requires two years or more to reach sexual maturity. Another lizard, the green anole (Anolis carolinensis), similar in size to the sagebrush lizard but living in the lowlands of the southern United States, may reach maturity in four or five months in Florida. Turtles mature at a slower rate. Females of the red-eared turtle of the central United States lay their first eggs when they are from three to eight years old, depending upon how long it takes them to reach a shell length of 15 centimetres (six inches). Females of the musk turtle, or stinkpot (Sternotherus odoratus), in Michigan require nine to 11 years to mature, at which time their shells are 7.5 to 10 centimetres (three to four inches) long. Presumably, turtles living in the tropics mature more rapidly. The maximum age, meaning the potential longevity, of modern reptiles varies greatly and can be determined only from records of captive animals. Turtles as a group seem capable of living longer than the others, and about 30 species have been kept in captivity more than 20 years. Several species, said to have lived 150 years or more, may be cases of two individuals whose periods of captivity overlapped. There is no reliable evidence for believing that the giant land tortoises live much longer than some smaller species. Two crocodilians (Alligator mississippiensis and A. sinensis) have survived in zoos for more than 50 years. Several species of pythons and boas have lived longer than 20 years. Lizards seem to have an upper limit near that of snakes. A slowworm, Anguis fragilis, has been kept in captivity for more than 30 years.
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