Seventy percent of the world’s snakes lay eggs and only about thirty percent give birth to live young. Eastern Garter Snakes (Thamnophis sirtalis sirtalis) are among the latter, giving birth in August to between two and thirty-one offspring (averaging 23). Carrying and incubating developing embryos within their body is more common for northern snakes and there is good reason for this. Whereas eggs are subject to whatever temperature fluctuations occur where they were laid, a snake that carries her young to term within her is able to move to warm areas that are ideal incubation temperatures. This causes less stress for the developing embryos and also results in a greater number of viable young.
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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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