What is the life cycle of snakes? Information about the courtship, mating, egg laying, birth and skin shedding of snakes.
Snakes, like other reptiles, are hatched or born as small replicas of their parents. They do not have a larval or tadpole stage as frogs and other amphibians do. Boas, most vipers, most sea snakes, and many water snakes give birth to live young, but pythons, cobras, rat snakes, racers, and most other snakes lay eggs that must be kept warm and moist for a period of time before they hatch.
Like all animals that live under natural conditions, snakes produce more young each year than can possibly survive. Ordinarily, of all the young that a snake produces during her lifetime, only two are likely to survive. Secretive species may produce only two or three eggs in a season, but those species that are exposed to the many hazards of living in view of other animals produce large numbers. For example, a python may lay more than 100 eggs in a single clutch, and a garter snake may give birth to more than 80 young in a litter. Most snakes, of course, have fewer young. The “average” number of young produced by a female snake in a season is probably between 8 and 15.
Generally, all the young snakes are on their own. They are not guarded by the parents, and they must find their own food and protect themselves as best they can from being eaten by other animals. Ordinarily it takes a baby snake two to six years to grow to maturity. Like other reptiles, however, it continues to grow after reaching maturity, but at a much slower rate. After the first spurt of growth, the life of the snake becomes a cycle of courtship and mating in the spring; maintaining body temperatures, capturing food, and avoiding enemies in the summer; and hibernation in the winter—perhaps with an interruption in the regular schedule to lay eggs or bear a litter of young. Once they attain sexual maturity, females grow at a slower rate than do males. In many snakes the production of eggs or young is biennial rather than annual.
COURTSHIP AND MATING
In the temperate regions of the world, spring is the time for courtship and mating. In tropical snakes, too, mating usually occurs at some change of season. Snakes just emerging from hibernation sites are already clustered in groups, and this aids the males in finding females.
As the weather becomes warm, snakes move out from hibernating quarters and become more active. Soon the males begin searching for the trails of the females that contain ripening eggs. Apparently such females have a distinctive scent. Males can follow the female’s trail by using their Jacobson’s organ sense as well as by the more usual sense of smell. Whichever is used, the trail of a female is as obvious to a male as is the fresh scent of a rabbit to a hound. Like a hound, sometimes the male follows the trail in the wrong direction for a while, but ultimately he discovers his error and retraces his path toward the female.
Courtship in snakes does not have the diverse patterns that are displayed by birds and lizards. Usually it seems merely to be an attempt by the male to attract the female’s attention. Upon discovering the female, the male brings his body parallel with hers. Meanwhile, she may continue to crawl along in an unconcerned manner. The male may then press his chin against the female and rub it along her back toward the head. At the same time, he may nudge her body with his coils, small ripples passing down his body and tail.
Finally the female appears to recognize his presence and slows her crawl. The male’s nudging actions increase. Boas and pythons have clawlike vestigial hind limbs that they use to scratch the back and sides of the female to help gain her attention. If the male’s attentions are acceptable, mating occurs. A female can store sperm and produce young several times after a single mating.
Most snakes lay eggs, and egg laying is characteristic of such common snakes as green snakes, rat snakes, racers, bull snakes, ring-neck snakes, hognose snakes, pythons, tree snakes, cobras, and many other species. In most cases, the female finds a warm and damp, but well-drained, hillside site for her eggs. She then pushes out a hole in the ground, perhaps under a stone or log, and lays her clutch of eggs in this protected spot. Very few snakes build nests. One that does— the female king cobra—constructs a nest of grass and leaves that may be 18 inches (45 cm) high on the forest floor. She deposits the eggs in a chamber near the top and then occupies a separate chamber above, apparently guarding them from predators.
Such parental care is most unusual, however, and in most cases the mother never visits the eggs after laying and has no means of recognizing her young after they hatch. Sunshine and rain and dew seem to provide the conditions necessary for hatching. A few exceptions exist, however. In North America the only snake that is known to remain with its eggs until they hatch is the mud snake (Farancia abacura). However, all pythons seem to have this habit. They remain coiled tightly around their egg mass for the entire incubation period. One species— the Indian rock python (Python molurus)— is known to raise its body temperature by a kind of shivering to keep her eggs at the proper incubation temperature.
Live births are characteristic of vipers, water snakes and their allies, and sea snakes. Yet all of these groups have one or more kinds that have retained egg-laying habits, including the bush-master (Lachesis mutus) of South America and the mountain viper (Trimeresurus monticola) of southern Asia. Rattlesnakes, copperheads, and cottonmouths all bear living young, usually 8 to 12 at a time. The European adder (Viper hems) is also a live-bearer, generally giving birth to four to six young at a time. Similarly, most of the lance-head vipers of tropical America and tropical Asia are live-bearing. The barba amarilla (Bothrops atrox) of America may bear 60 or more in a litter, but this is unusual, and most live-bearing snakes have much smaller litters.
In most live-bearers, the female seems simply to retain the eggs in her body until they are ready to hatch. The embryo snakes grow from the food stored in the yolk of their individual eggs just as they would if the eggs had been laid. However, in one group—the North American water snakes (Natrix)—a more complicated mechanism has evolved. The thin egg membrane becomes greatly folded in one area, and the female snake’s oviduct (which contains the eggs) becomes similarly folded to meet the egg membrane foldings. This junction of the egg membrane and the oviduct wall forms a kind of placenta, where the blood of the developing embryo can get into close proximity with that of the mother.
Although the separate bloodstreams do not mix, the close relationship does permit the exchange of gas molecules from mother to embryo and vice versa. Oxygen passes from the mother to the embryo and carbon dioxide from the embryo to the mother. It is not known whether food molecules can pass between the embryo and mother. If such molecules can pass, it would allow a reduction in the yolk needed in the egg. Oddly, the Old World water snakes do not have this adaptation of placenta formation, and one species —the common grass snake (Natrix natrix) of Europe—is an egg layer.
Young snakes appear to be fully prepared to take care of themselves immediately after hatching or after birth. Often snakes that hatch have a sharp “egg tooth” attached to their snouts, allowing them to slit the leathery egg shell. Venomous snakes appear with their fangs and venom glands ready to operate and with the instinctive ability to operate them properly. Cobras have been observed to spread their hoods and strike out at a threatening animal even before they were completely out of the eggshell.
Young snakes often have color patterns and some behavioral habits that differ from those of the adults. The length of baby snakes is generally about 20% that of the adult. Yet there are exceptions to this rule, especially among smaller snakes whose young tend to be fewer but larger than would be expected. Thus the common brown snake of North America, which averages about 1 foot (30 cm) in length, generally has 8 to 10 young that are about 4 inches (10 cm) long, or approximately one third the parent’s length. On the other hand, the Indian rock python, which is about 13 feet (4 meters) long, may lay more than 50 eggs, each of which produces a baby snake that is about 2 feet (60 cm) long, or approximately one sixth the parent’s length.
In temperate regions, the young snakes often go into hibernation shortly after hatching or birth. They may show no appreciable growth when they reappear from the hibernating site in the spring. In their first growing season, however, they commonly double their length and may even surpass this size. A few of the smaller »species of snakes become mature during the second growing season and may produce young themselves during the third season. Larger snakes, such as the pythons and boas, commonly take five or six years before they reach maturity, and the youthful surge of growth diminishes.
Soon after the young snake is born or hatches from its egg, its skin becomes dull and dry-looking, its eyes become foggy, and the young snake signals the beginning of its first molt. After two or three days, its skin brightens and its eyes clear. Usually on the following day, the snake begins to rub its snout against the ground or other rough surfaces. After a while the thin transparent outermost skin (the corneal layer) has been pushed back from the snout and jaws. Then the snake finds an overhanging rock or some other object that will hold the loosened edges and slowly and deliberately crawls out of its old skin. A similar layer of skin, just a little bit larger, has formed underneath. The shed skin, now inside out but showing each scale and a faint suggestion of the color pattern, is left behind. The process of skin molting is repeated at intervals throughout the life of the snake.
HIBERNATION AND TEMPERATURE CONTROL
To a temperate-zone snake, hibernation is a period when it almost stops living. Every autumn the snake must seek some protected spot where the temperature drops to near the freezing point, but not two or three degrees below freezing, which would mean the snake’s death. In the warmer regions of the temperate zone, the hibernating place may be a root hole near a stream, a sawdust pile, or just a large stump whose center has been rotted out and filled with the cuttings of squirrels and woodborers. Thus for snakes of ‘this region, hibernation is an individual act where one or a few snakes may repair to the same protected area for the brief cold season.
For snakes in the colder parts of the temperate zone, hibernation becomes a much more social type of aggregation. As the autumn days get cooler, snakes from the surrounding regions, some from more than a mile away from the hibernation site, begin to congregate. Deeply cleft rocks on southward-facing slopes are the preferred hibernating sites. As the days begin to cool, the snakes retreat into the rock crevices at night. Gradually as the warmth of the sun decreases with the winter season, they spend less and less time outside. By the time of the first killing frost, the snakes are safely tucked away in the deep crevices beyond the reach of the frost.
Frequently dozens of snakes of several species are found hibernating together. In eastern North America there may be timber rattlesnakes, copperheads, black racers, and common rat snakes. In the Plains region, bull snakes, whip snakes, and others are found along with the aggregations of prairie rattlesnakes (Crotalus viridis) or western diamondbacks (C. atrox).
During hibernation, the snake lies virtually oblivious to the happenings of the outside world. It is cold and motionless, its heartbeat slowed almost to a standstill and its breathing imperceptible. The body temperature of the hibernating snake lies between 32° F and 40° F (0° C-4.5° C). If the temperature drops much below 32° F (0° C), the fluid in the cells of the snake’s body will freeze, and the snake will die. If, on
the other hand, the snake’s temperature goes much over 40° F (4.5° C), the metabolism of the snake will speed up, and the snake may use up the energy reservoir of fat and starve before spring. This narrow range of acceptable temperatures is typical of the snake’s whole life. Like most reptiles, snakes probably spend more time trying to adjust their body temperatures than they do in searching for food. Most of their body heat must come from the outside.
After emerging from hibernation in the spring, the snake must bask in the sun for several days in order to raise its temperature and metabolism to an active level. A snake such as the black racer must raise its body temperature to about 86° F (30° C) to achieve its normal speed and agility.
For the remainder of the summer the snake will bask each morning to attain this “activity temperature.” It will maintain this temperature during the day by basking or by lying on sun-warmed rocks or soil, or by seeking the shade if the temperature goes too high. Then at night the snake’s temperature will decrease toward that of its surroundings. Thus most temperate zone snakes have three normal temperatures: active, resting, and hibernating. For a black racer, these would be about 86° F (30° C), 70° F (21° C), and 40° F (4.5° C), respectively.
There is virtually no information available on the life-span of snakes living in their natural habitats. Most of the actual records are of species that are often kept in captivity, such as pythons, cobras, rat snakes, and king snakes. However, the records that are available suggest that most snakes live between 10 and 20 years, with a few of the larger species living to “about 30 years. The maximum recorded age for boas and pythons is that for an Indian rock python that lived at the San Diego Zoo for 28 years and 3 months. Several other pythons and boas approach this age. Though most cobras do not live nearly so long, a black-lipped cobra (Naja mel-anoleuca) at the San Diego Zoo appears to hold the longevity record for all snakes—29 years, 1 month. Few snakes in the wild would have the opportunity to live so long. The average lifetime of wild snakes is probably less than half that of the captive snakes, but a few wild individuals may live much longer.