Do All Snakes Lay Eggs? Egg-Laying vs Live-Bearing Snakes

Egg-laying and live-bearing: what is the difference?

Biologists use two main terms for how snakes reproduce:

• Oviparous snakes lay eggs. The embryo develops outside the mother’s body, inside a leathery, flexible shell, and hatches after several weeks.
• Viviparous snakes give birth to live young. The eggs stay inside the mother’s body until the babies are fully formed. Older books often call this “ovoviviparous,” because in many species each baby develops inside a thin membrane and is nourished mainly by yolk.

In practice, there is a continuum. Some egg-laying snakes keep their eggs inside the body for a long time and lay them when the embryos are well advanced. Some live-bearing snakes have a simple placenta that passes water, oxygen and some nutrients from mother to embryo. A newborn live-bearing snake usually emerges wrapped in a clear membrane, which it breaks within minutes.

Snakes that lay eggs

Egg-laying is the original method for snakes and is used by most species, especially in warm climates.

Most snakes simply lay their eggs in a warm, damp, hidden place, such as rotting logs, leaf litter, burrows or compost heaps, and leave. Clutch sizes range from just one or two eggs in small species to dozens in large ones. Incubation usually takes about six to ten weeks, depending on species and temperature.

A few snakes look after their eggs. Female pythons coil around the clutch for the whole incubation, and the Burmese python can raise the temperature of her eggs by shivering. The king cobra is famous for being the only snake that builds a true nest; see do king cobras build nests?. For egg-layers across the animal kingdom, from birds to the platypus, see animals that lay eggs.

Snakes that give birth to live young

Live-bearing has evolved many times in snakes, so it appears in several unrelated groups:

• Boas and anacondas. Most boas give birth. A female green anaconda typically has about 20 to 40 young, each around 70 cm (28 in) long. Boa constrictors usually have litters of 10 to 60 or so.
• Most vipers. Rattlesnakes, copperheads, cottonmouths and the European adder all give birth to live young.
• Garter snakes and water snakes. The common garter snake has litters that commonly number 10 to 40, and large females can have more. North American water snakes are also live-bearers.
• Most sea snakes. True sea snakes give birth at sea and never come ashore, while their relatives the sea kraits return to land to lay eggs.
• The smooth snake. This rare European snake, found in a few heathlands in southern England, also gives birth.

Why cold climates favor live birth

The leading explanation is often called the cold-climate hypothesis, developed largely by Australian herpetologist Richard Shine. Eggs left in the ground are at the mercy of the weather. In places with short, cool summers, soil may never be warm enough for the embryos to develop in time.

A pregnant female, by contrast, can move into the sun to bask and retreat when it gets too hot. In effect, she becomes a mobile incubator, keeping her developing young warmer than the soil would. Carrying the young also protects them from predators that dig up nests and from fungi that attack eggs.

The pattern is clear on a map. The northernmost snake in the world, the common European adder, lives beyond the Arctic Circle in Scandinavia and gives birth to live young, as do the garter snakes that live farther north than any other snake in North America. The same trend shows up in mountains, where high-altitude snakes are more often live-bearers than their lowland relatives.

Live birth has costs, though. A pregnant female is heavy and slow, may stop eating for weeks and is more exposed to predators. That is probably why many snakes in warm regions still lay eggs.

How baby snakes start life

Whether they hatch or are born, baby snakes are independent at once. Hatchlings use an egg tooth, a tiny sharp tooth on the tip of the snout, to slit the shell, and the tooth falls off soon after hatching. Newborn live-bearers only have to wriggle free of a thin membrane, which usually takes a few minutes.

Parental care after birth is rare. Some newborn rattlesnakes stay with their mother for a week or two until their first shed, and a few pythons stay near their hatchlings briefly, but most young snakes disperse right away and must find food on their own.

Unusual cases: virgin births

Females of several snake species, including Burmese pythons, boa constrictors, copperheads and cottonmouths, have occasionally produced offspring without mating. This is called facultative parthenogenesis. It has been confirmed by genetic tests, mostly in captive snakes but also in a few wild ones. It is uncommon, and it does not mean these species normally reproduce without males.

Myths and facts

Myth: snakes that give birth are mammals in disguise. Live birth is a reptile adaptation that has evolved independently many times. The young of live-bearing snakes still develop from eggs inside the mother.

Myth: you can tell whether a snake is venomous by whether it lays eggs. Both venomous and harmless snakes include egg-layers and live-bearers. Cobras lay eggs, while vipers mostly give birth, and harmless garter snakes give birth too.