Seventy percent of the world’s snakes lay eggs (oviparous). The rest give birth to live young (viviparous). Oviparous snakes tend to live in warmer climates, where the substrate they lay their eggs in is warm enough to incubate the eggs. (Most egg-laying snakes deposit their eggs and then depart, relying on the substrate to incubate the eggs.) Viviparous snakes tend to live in cooler regions, where the ground is too cold to provide incubation.
There is a distinction between egg-laying snakes. The majority of snakes that lay eggs do so outside their body, in a protected area such as a rotting log. These snakes are known as oviparous. There are also egg-laying snakes that retain their eggs inside their bodies until they’re ready to hatch. These snakes are called ovoviviparous. Ovoviviparous snakes, such as the Common Gartersnake, appear to give birth to live young, but they actually don’t. Unlike viviparous species, there is no placental connection, or transfer of fluids, between mothers and babies, because the developing young snakes feed on the substances contained in their individual eggs. The snakes emerge from the mother when they hatch from their eggs, giving them the appearance of “live” births. The gestation period for oviparous snakes is generally longer than those of ovoviviparous snakes and vary from a few weeks to a few months in length. (Photo: very young Common Gartersnake, Thamnophis sirtalis sirtalis, consuming an earthworm)
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Common Gartersnakes begin mating in the spring as soon as they emerge from brumation (a reptilian state of dormancy similar to hibernation in mammals, but involving different metabolic processes). The males leave the den first and wait for the females to exit. Once the females leave the den the males surround them, forming what is called a mating ball (one female and many males). The males give off pheromones that attract the female. After the female has chosen her mate and mated, she leaves. while the males stay to re-mate with other available females. The females have the ability to store the male’s sperm until it is needed and thus a female may not mate if she does not find a proper partner.
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Common Gartersnakes mate soon after emerging from hibernation in the spring, in March or April, and four months later the females give birth to live young. The newborn snakes are 5 to 9 inches long at birth and from day one have to fend for themselves. Their diet at this early stage consists of earthworms, insects, slugs, tadpoles, small frogs and fish. If there is an abundant supply of food, the young snakes can grow as much as 1 ½ inches a month during their first year. Earthworms are their preferred diet and gartersnakes are known for their ability to find them, even underground. It turns out that earthworms produce a chemical substance in their skin that is easily detected by (and attractive to) Common Gartersnakes. (Thanks to Eli Holland, who located the worm-eating newborn Common Gartersnake in the photograph.)