Snakes from temperate climates such as Europe and North America, go through periods of reduced activity or hibernation depending on the severity of the winter and are not inclined to breed until temperatures are high enough during the spring. Breeding cycles of species confined to the tropics usually coincide with dry and wet seasons when there is a reduction in temperature. Other more opportunistic species may not have a well defined breeding cycle and will breed regardless of the seasons. However, breeding usually coincides with temperature, humidity and the abundance of prey for the young snakes to feed upon.
Mating
Female snakes ovulate at the beginning of the breeding season. At this time they go through various hormonal changes and in turn, secrete certain pheromones which enable male snakes to follow and seek them out. When two males meet on the trail of the same female, they will often compete in stylised combat or wrestling where they coil around one another and try to force the opponent to the ground. During these wrestling matches the male snakes never bite each other. Odour probably plays a significant role in the courtship and the male will crawl along the back of the female, making regular jerking movements to stimulate her. Members of the Boidae which have rudimentary spurs, will use these to tickle and scratch the female. Once the female is ready to mate she allows the male to coil his tail around hers and when their cloacae come into contact with one another, the male everts one of his hemiphines and copulation takes place. This may continue for several hours and there are usually multiple couplings over several days. Once joined, the snakes are not easily separated as the male has spines or bristles which cover the hemiphines, (refer to the image on the anatomy page). The hemiphines fit snugly into the female’s cavity and the shape of the organs prevents cross breading. Two different species have to be closely related to cross breed successfully. One example is the Puff adder, Bitis arietans and the Gaboon adder, Bitis gabonica.
Sperm production
Male snakes in temperate climates produce sperm during summer and autumn when they are most active. It is then stored in a bladder-like structure which is formed from part of the ureter until the following year when breeding commences in the spring. In tropical climates the males of certain species usually have viable sperm throughout the year, while others produce sperm immediately prior to the breeding season.
Ovulation
The first stage of ovulation is the formation of small egg follicles which are produced in the ovaries. This is a lengthy process, in which the eggs mature slowly and may take years in large, long lived species. Once the egg follicles are developed, vitellogenesis or maturation takes place and the egg follicles are surrounded with yolk. Maturation only takes place immediately before the breeding season. After maturation the eggs are released into the body cavity and then collected by the funnel shaped end of the oviduct or infundibulum.
During ovulation the infundibulum grows around the ovaries to reduce the chances of the eggs being lost in the body cavity, although some eggs may become lost and end up in the opposite oviduct. Therefore eggs are transferred from one oviduct to another. If sperm is present, the eggs are then fertilized and start developing. If any eggs are infertile they are reabsorbed by the body, however once the eggs have been fertilized they cannot be reabsorbed. If the eggs are fertilized they move into the lower end of the oviduct and providing the species is oviparous, they are coated with a parchment-like egg shell which is secreted by a special gland. In ovoviviparous species each egg is enclosed in a very thin, transparent membrane.
Developing embryos are nourished by the yolk surrounding them and there is no transfer of nutrients between the female and the eggs. The one exception may be the Australian, Pseudechis porphyriacus or Red-bellied black snake, which transfers nutrients in the form of amino acids and electrolytes to the developing embryos. The flower pot snake, Ramphotyphlops braminus, is a parthogenic species. All the specimens which have been collected are females and therefore this specie is easily introduced to new locations as a single specimen can reproduce to form a colony. Some primitive species such as Typhlops bibroni lay eggs with very thin shells and well developed embryos that hatch in a matter of days.
Embryonic developmentThe amount of eggs and the shape of the eggs depends on the size of the specie. Smaller species tend to lay fewer and larger eggs which are more elongated. While larger species tend to lay larger amounts of smaller eggs in comparison to their size and these are more rounded in shape. This is probably due to the minimum size of hatchlings or neonates that will be able to survive. Very small snakes will have difficulty in sustaining adequate body temperatures and finding food. Generally speaking, an egg is 3% of the body weight of the female snake. Clutch size varies between species, the largest is by the Puff adder, Bitis arietans, with 157 live young. Other species that produce large clutches are the Asian pythons P. molurus, P. reticulatus, African rock python, P. sebae, the Fer de Lance, Bothrops atrox, and four colubrids, which include the Mud snake, Farancia abacura, Green water snake, Nerodia cyclopion and lastly the Common garter snake, Thamnophis sirtalis. The primitive Leptotyphlopidae and Typhlopidae, including a number of small colubrid snakes lay only one or two eggs. The average clutch size, taking all snakes into account is around seven.
The external egg shell which develops in the lower ends of the female’s oviducts, encompasses the shell membrane, the chorion, the albumin and the yolk sac. The embryo which is surrounded by the amnion membrane, starts development as a flat disc which sits on the yolk. As it begins to form it lifts away from the yolk sack which continues to nourish it throughout its development. All of the three membranes which include the amnion, the chorion and the allantois, help to conserve water and only start developing after the egg is layed. The albumin consists of proteins and through osmosis, it attracts water into the egg. During its development the egg swells to about twice its weight, due to the influx of water passing through the permeable shell and then being used by the embryo. The small air bubble acts as a pressure regulator and controls the exchange of oxygen and carbon dioxide through the shell. Later on in the embryo’s development, it extracts calcium from the shell to form its skeleton, the shell becomes thinner and oxygen passes through more easily, although eventually the young snake will have to break through the shell to breath.
Hatching Once the young snakes are fully developed their need for oxygen becomes more apparent, they are restless and can be seen moving inside their egg shells. Before they emerge from their eggs, the neonates absorb the remaining yolk and then slit the shell with a special thorn-like growth located on the snout, called the egg tooth. Neonates rest before emerging, sometimes for several days and will poke their heads out of the shell to breath. In ovoviviparous species, which include viperids, the boas, sea snakes and some colubrid species, the embryos are surrounded by a thin membrane, complete with blood vessels and the neonates absorb the yolk prior to being born. In arboreal species this membrane stops them falling to the ground by sticking to branches or foliage. These young snakes do not require the egg tooth as in oviparous species, the membrane is delicate and they simply wriggle out. Although they also rest for some time before leaving their membranes. The hatchlings will disperse within the following few days, however in some species the neonates will stay in or nearby the nest until they have shed their skins, which usually happens within a week or two. The young snakes do not feed until the first sloughing of their skins. The egg yolk which was absorbed prior to hatching, sustains them during this period. After the first shed they will seek out their first meal, which will be digested over the next few days. In most species, hatchlings feed once every five to ten days and shed their skins at three to four weekly intervals. They grow rapidly within the first year and the intervals between feeding and skin sloughing gradually lengthens until they reach maturity at three to four years, with the male snakes maturing a little earlier than the females. Generally, mature snakes feed once every fortnight and slough their skins after several meals. Although very large species such as some pythons may only feed once or twice a year and digest a large meal over the following months.