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.)
All mothers need to do one thing for their offspring: provide nourishment. That’s...