V.

Reproduction and Life Cycle

In climates with cool winters, snakes typically begin mating in the spring; in tropical regions, they mate year-round. Females signal their readiness for mating by producing pheromones, chemicals with a smell that attracts adult males of the same species. Females leave behind pheromones on the ground as they move about, and males follow the pheromone trail to its source.

A male snake may track down a female only to discover that other males are already nearby. Among most species, males ignore rivals and continue with their courtship efforts. In some species, however, competing males engage in ritual combat dances, in which they intertwine their bodies and try to force down one another’s heads. The larger male usually wins the fight. Biting is generally not involved, although in a few species—typically ones that do not produce venom—males may engage in fierce biting battles.

Courtship follows a similar pattern among all snakes. The male rubs the underside of his head along the female's back, stimulating her and orienting his body against hers. To enable copulation to occur, the male may lift the female's tail with his own tail. Pythons use their spurs for this purpose. Male snakes have two penises or hemipenes, one on either side of the body, which are usually contained within small sacs at the base of the tail. During mating, males insert one of their hemipenes into the cloaca of the female. In many snake species, hemipenes are covered with small spines to hold them in place during copulation. Mating lasts from several minutes to several hours. Each female typically mates with several males during her reproductive season, and some females mate with the same male more than once.

After mating, sperm is stored in the female’s tubelike oviduct. One to two months later, the female’s ovary releases large eggs, each with an abundant yolk, which are fertilized by the sperm waiting in her oviduct. The fertilized eggs are funneled to the cloaca, from where they are released and deposited in a shallow hole in the ground or an area under a rock or log. Among a few types of snakes, including the king cobra and the diamond python, the female constructs a nest for her eggs out of vegetation. In these and some other snake species, the female remains with the eggs and guards them against predators until they hatch. Female pythons warm their developing eggs by twitching their muscles in a way that resembles human shivering.

In about 20 percent of all snake species, females give birth to live young. This way of reproducing is most common in cold areas, possibly because it helps females to control the temperature of the developing eggs. By basking in sunlight, a pregnant snake can keep her eggs warm and speed up the rate at which they develop so that the young will be born before the onset of cold winter weather. Live births also benefit some snakes that do not live in cold climates. For example, most sea snakes give birth to live young. This frees them from the need to lay eggs on land, where they are less agile and more vulnerable to predators.

One snake species—the flowerpot blind snake—reproduces without mating. This species consists entirely of females who reproduce by a process called parthenogenesis. In this type of reproduction, the chromosomes within an unfertilized egg replicate, and the embryo develops as if the egg had been fertilized. One advantage of parthenogenesis is that females can reproduce whenever environmental conditions are optimal, without waiting to make contact with a mate.

Most snakes reach maturity at two to four years of age, and those that survive to old age live for about 20 to 30 years. However, the odds are against snake survival. Most succumb to predation or disease long before they reach the end of their natural life span.