Dr Jon Bielby, Zoological Society of London
Easter Eggs are traditionally distributed by the benevolent Easter bunny. But thereís one problem, where would a rabbit find an egg? They certainly donít lay them, but why not? Graihagh Jackson took a trip to London Zoo to search for an answerÖ
Graihagh - I have four beautiful rabbits Ė Walnut and Hazel, Smudge and Pushkin. I've been wondering for a long time whether they might surprise me one year and lay a chocolate egg for Easter because thatís what the Easter bunny does, right? Well, maybe not and at least itís not only my pet rabbits that aren't grateful enough to lay me an egg. Of course, there's a huge evolutionary diversity in the animal kingdom when it comes to reproduction. In team one, you have the live-birthers and in team two, the egg layers. One of the top players in team 2 are the birds. But why do some animals lay eggs and others give birth? What's better, and who does what? To get the egg rolling, I played a game with ZSL zoologist Jon Bielby in the birdhouse of London Zoo. The game is, birther or egg layer. Winner gets a gold star. Ready, set, goÖflamingo.
Jon - Egg.
Graihagh - Giraffe.
Jon - That will be a live young.
Graihagh - Panda.
Jon - Live young.
Graihagh - Penguin?
Jon - Eggs.
Graihagh - Duckbill platypus.
Jon - Thatís a tricky one. Itís a mammal, but it lays eggs.
Graihagh - So, there's sort of a 50/50 split there between live young and eggs. Is that the same across the entire animal kingdom or is it less evenly distributed than that?
Jon - Across the whole animal kingdom, it would be less evenly distributed than that. You'd have many, many more animals that lay eggs rather than giving birth to live young.
Graihagh - These two main varieties, what's the benefit? Why does some animals lay eggs and others give birth to live young?
Jon - The main reason is that there are lots and lots of different ways to make a living. So, weíre walking around London Zoo, you see tons of different ways that animals do things Ė sizes, diets and that includes how they breed, how they reproduce and what their young do. We think of all of vertebrates, all the things with backbones so fish, amphibians, reptiles, birds, mammals. The default setting is laying eggs and thatís because the ancestor of all of those different types of animals laid eggs. But at various points across that family tree giving to live young has evolved in different ways. So, both are equally successful. Both allow animals to pass on their genes and thatís what natural selection and evolution is all about really.
Graihagh - If we all started as egg layers Ė I'm laughing at the image of myself laying an egg Ė why would we suddenly evolve and across so many different animal kingdoms as well, how did that happen if you like?
Jon - The reason why it might happen is that a population or individuals within a population would have a mutation to their genetics. What it meant is that instead of laying eggs, they'd maintain their eggs for a little bit longer in their oviduct and then they'd lay eggs that would hatch ouy more quickly and so on and so forth. And if they survive better then their genes would be passed on to the next generation.
Graihagh - Is there any examples of it going the other way around say, from live birth back to laying eggs?
Jon - As far as I'm aware, itís not gone the other way as yet.
Graihagh - But what about the Easter bunny because he or she famously carries around chocolate eggs? Jon and I sauntered down to the rabbit enclosure to find out if this myth could possibly be true. Could my pet rabbits lay me an egg this Easter?
Jon - I donít think you're going to go home and find that your rabbits have laid eggs. Have rabbits ever laid eggs? Sorry, no, I donít think so.
Graihagh - Would they ever evolve to lay eggs in the future?
Jon - I think itís really, really unlikely. There are a couple of reasons for that. One is that if you think of how different laying an egg is, compared to giving birth to live young. So, if you look at your egg that you might have for breakfast, you crack it up and youíve got the yellow bit which is the yolk and then there's the white bit which is a protein. The shell itself is really specialised piece of equipment if you like for looking after a chick. In a rabbit, what would happen is that the young rabbits inside the mother would feed through the placenta. Think of a placenta compared to an egg. Itís really, really different. Now, the first reason I think is, itís really unlikely that you're going to go home and find that your rabbits have laid eggs is that having evolved all of this really elaborate way of looking after the young via a placenta, itís really unlikely that the body is physiologically going to be able to change and go back to lay an egg. I donít know whether thatís impossible or not but if you think about if that happened over thousands or even millions of years, it wouldnít really be a rabbit anymore. It would be something else. But the other reason is that rabbits are really successful, so that really need to do that. Rabbits breed like rabbits. There are millions of them. So, there's not really the pressure for them to go back to laying eggs.
Graihagh - Youíve just smashed on my hopes and dreams of the Easter bunny ever coming to visit.
Jon - I'm really sorry about that. I mean, you might still get some Easter eggs this week and I'm not sure. But yeah, probably not laid by a rabbit.
Graihagh - So, did the Easter bunny steal his eggs from the chickens then? Is that what happened?
Jon - Itís possible, but as far as I know, the eggs that the Easter bunny delivers are chocolate. As far as I know, chickens donít lay chocolate eggs. I'm not sure how that happened biological speaking.
Graihagh - There aren't animals that lay chocolate eggs I'm assuming.
Jon - It would clearly melt inside of the oviduct, so itís just not going to happen. I suppose maybe thatís why they got protective tinfoil insulation or something.