Birds Aren't the Only Ones

Animals That Lay Eggs

April 17, 2023

by Aubry Hall, Membership and Volunteer Manager, Imagination Station

It’s that time of year where eggs are on everyone’s mind. We all know that only birds and reptiles hatch out of eggs, right? Wrong! But all reptiles hatch out of eggs, right? Wrong again.

Let’s take a look at some members of the animal kingdom that don’t follow these typical patterns.

First, the monotremes. Monotremes are an order of mammals that lay eggs. That’s right – mammals laying eggs like a chicken! (An Order is a group of animals that share common characteristics – think marsupials or carnivores.) The only living monotremes left on the planet are all found in Australia and New Guinea, and golly, are they cool! The platypus and four species of echidnas are all that make up this unique order. A platypus is a semiaquatic mammal that looks kind of like a small beaver with a duck bill, and echidnas look like very small porcupines with a long snout. Montremes hatch from their eggs about 10 days after being laid – much shorter than the typical incubation time for most birds. When baby monotremes (called puggles) hatch, they are small and helpless – much like a baby kangaroo.

What about the other end of spectrum – animals that are in a group of egg layers but instead give birth to live young? The word scientists use for animals that lay eggs is oviparous. (The Latin word for “egg” is ovo.) This includes most fish and reptiles, all birds, dinosaurs, and, of course, monotremes. The word for animals that give birth to live young, like the rest of us mammals, is viviparous – from the Latin vivus meaning “living”. While this may seem to be a trait only found in mammals, a few lizards and snakes, sharks, and some fish are born this way.

Let’s take our egg-ucation one step further. There is a third way that animals are born that combines the ovo and vivus parts of these words into one giant super word – ovoviviparous! In this case, eggs are created inside the mother, but rather than laying the eggs, she keeps them inside her body. When the young are developed, they hatch before exciting the mother’s body. This is a trait that can keep eggs from being eaten by predators, however, it can be more taxing on the mother’s body. Some sharks, rays, fish, and snakes are born this way.

Now that we are sufficiently confused, I think I will stick to birds. They have collectively agreed that egg laying is the way to go.