AnimalsExplainer

This frog’s babies erupt out of its back—and other surprising ways animals give birth

Mammals aren't alone—some reptiles, amphibians, and insects give birth to live young.

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A Suriname toad, Pipa pipa, at the Saint Louis Zoo. Females of this species birth their young from holes in their backs.

Of the many ways to be born, live birth may be the most familiar to humans. We mammals deliver live, squirming babies, and we think of many other animals as laying eggs—but in reality, animals have found a variety of ways to bring their young into the world.

Live birth, also known as viviparity, is common throughout the animal world, and not just among mammals. It has emerged in fish, amphibians, insects, and arachnids, to name a few.

In fact, viviparity has evolved independently about 150 times in various animal species, including at least 115 times in living reptiles, a number three times higher than in all other vertebrates combined, says Henrique Braz, a herpetologist at the Butantan Institute in São Paulo, Brazil.

There are benefits—and drawbacks—to laying eggs and live-bearing, but these modes of reproduction aren’t an either/or proposition. Egg-laying and live-bearing are two points on a continuum, with many species straddling the middle. (Read about a lizard evolving from egg-laying to live birth.)

Halfway there

All mothers need to do one thing for their offspring: provide nourishment. That’s either as yolk in an egg or, for live-bearing animals, often directly from the mother’s body. (In the unique case of seahorses, it's the father's body that feeds the young.)

Some species manage to give birth to live young, yet the mother contributes little to no food in utero. They do this by retaining the babies in eggs inside the mothers’ bodies, letting the young grow and develop using the...

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