Why do some animals lay eggs, and others give live birth?

Today's question comes from 11-year-old Robin Yi-Sewell in Calgary, Alberta. He asks:  

Why do some animals lay eggs, and others give live birth?

Robert Montgomerie, an evolutionary biologist with Queens University explains that scientists still don't really know why this happens, but it all seems to come down to costs versus benefits. The reason that some animals lay eggs is because that is more beneficial to them than than it is to give birth to a live offspring.

Birds, for example, all lay eggs because they can't fly with the weight of their babies inside of them. However, in situations where it's dangerous to have your offspring out in the environment, susceptible to predators and bad weather, then animals have adapted to keep their babies inside of them as long as possible.

Millions of years ago, every animal laid eggs, and slowly, they evolved to give birth to live babies based on what would help them survive best. And there are some species, like the yellow-bellied three-toed skink, where some animals have evolved to give birth and others lay eggs depending on where they live.

The skinks that live high on the mountain give live birth because it's too cold to incubate eggs, whereas the skinks that live down on the coast lay eggs, because they can just bury them in the sand to keep them safe and warm.

Written and produced by Amanda Buckiewicz