Chinchillas are exotic pets, and exotic pets can be surprisingly different to those we’re familiar with. So are chinchillas one of the small number of mammals that lay eggs?
Do chinchillas lay eggs? They don’t, as chinchillas are mammals, and almost all mammals give birth to live young. Chinchillas aren’t marsupials, either, so when they give birth they don’t keep their young in pouches. The earliest evidence of giving birth to live young is found 250 million years ago in ichthyosaurs; mammals first began birthing live young 150 million years ago. These mammals are the ancestors of all of today’s species, including chinchillas (and including us).
The guide below first answers the question directly, before looking in detail at why mammals give birth to live young, and don’t lay eggs. After all, it’s inconvenient to be pregnant!
Do Chinchillas Lay Eggs?
Chinchillas don’t lay eggs. Chinchillas are mammals, and there are only a very select few mammals that lay eggs: the platypus, and some species of the echidna, which looks like a porcupine crossed with an anteater.
While we don’t recommend that new owners allow their chinchillas to breed, you would see this in action if you did. Chinchillas mate just like any other mammal. Around 110 days after becoming pregnant, the female will give birth to live young. There are never any eggs involved, unless you count the eggs in the female’s ovaries. Once born, the young aren’t kept in a pouch, because chinchillas aren’t marsupials.
Which Animals Lay Eggs?
More species lay eggs than give birth to live young. Many reptile species, and almost all invertebrates (animals without spines) lay eggs in one way or another. Many do so in the way you’re likely imagining: laying eggs like the ones you find in the store. But many animals that live in water release clouds of eggs that males then fertilize external to her body.
Contrary to popular belief, there isn’t just one kind of animal that gives birth and one kind that lays eggs. Mammals aren’t the only animal that birth live young, for example, and not all reptiles lay eggs. Lizards and fish are the most common animals that lay eggs rather than give birth. Birds never evolved live birth, either, likely because holding the young in their bodies would make flying difficult or even impossible.
Why Don’t Mammals Lay Eggs?
There is strong evidence that laying eggs is ancestral to live births. That means that all animals that give birth to live young—people, chinchillas, other small mammals and so on—have ancestors that laid eggs. The fossil record shows that all life on earth evolved from life in the oceans, and these early animals all laid eggs. So what made them begin giving birth, instead?
This evolutionary leap would have been a difficult one to make. That’s because when the young is inside an egg, inside its mother, it’s protected; but the mother is also protected from it. You’ll be familiar with the idea of the body rejecting tissue from learning about medical procedures like heart transplants. The body recognizes the new tissue as a foreign substance and rejects it, and the immune system starts attacking it. Well, since a mother’s young only shares 50% of the mother’s DNA, the same should happen: the mother’s body should reject it, which would mean that the young dies before it’s born.
To achieve this, scientists think that early live-bearing animals repurposed sections of DNA intended for other things and used them in this context. The mother needed a way to dampen down her immune response, but only in her uterus (otherwise she could easily get sick and pass away, which wouldn’t be helpful). DNA related to progesterone, the female sex hormone, was repurposed and reused to achieve this. This made it safe for the mother to hold young inside her without the dividing wall of an egg between her and her offspring.
But why go to all this bother?
Live Birth Provides Protection to Young
The key advantage of live birth is that it helps, or allows, the mother to care more closely for her young.
Many egg laying animals lay their eggs and then leave them be. They produce large litter sizes to counteract the effects of the elements, predation and unfertilized eggs. So, even if three or four die, four or five will still hatch. But giving birth to live young means that the mother can keep the young inside herself until they’re ready to fend for themselves; avoiding predators and environmental harm becomes the mother’s responsibility.
The likelihood of the young surviving isn’t the only advantage, either. Because more are likely to survive, the mother is free to have smaller litters. This means she needs invest less energy into creating new life, which is, biologically, a very costly process. There are downsides too, of course: the mother is less agile and has to eat more than usual while pregnant. But the advantages seem to outweigh these drawbacks.
Some Mammals Still DO Lay Eggs (Just Not Chinchillas)
Biology is rarely clear-cut. While there are rules that generally apply, there are often exceptions to those rules, too. In this context, that means there are both mammals that lay eggs and many, many reptiles that give birth to live young.
Examples of animals halfway between these forms of giving birth are common. The platypus, for example, is a mammal that lays eggs. On the other side of the equation, there are snakes that appear to give birth to live young, but in fact, hold their eggs inside themselves until they hatch—only then letting their offspring into the world. One animal, the Australian three-toed skink, can even give birth to live young or lay eggs; miraculously, it can do so in the same litter!
As of today, scientists still haven’t found the earliest animals that gave birth to live young. New species are still being discovered that push the date of the evolution of this trait further and further back.
What’s interesting are our misconceptions about the birth of live young. We assume that because it’s something we do, it must be the best way to reproduce, but it isn’t necessarily. People and other animals die in childbirth frequently, and it’s only because of modern medicine that this is now rare in the western world. We also assume that it evolved once, when it certainly did not—it has evolved dozens of times in different families of animals, not just mammals.