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In what seems like an inefficient use of resources, these penguins always lay two eggs, but then ignore, discard, or just straight-up destroy the first one. What gives, penguins?

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[♪ INTRO] Imagine you were born into a different species.

You’re a penguin migrating to your breeding site, crossing the frigid ocean in hopes of finding The  One you’ll raise a little penguin family with. And you do.

And the two of you settle in. You lay your first egg… and then just kind   of forget about it without  giving it a chance to hatch. What the heck, penguins?

This behavior, honestly,  doesn’t make a lot of sense, but scientists think there  must be good reasons for it. Well, not good reasons, exactly, but reasons  that help us understand how these birds evolved. Erect-crested penguins always lay two  eggs, but they only keep the second one.

They’re a pretty understudied  species endemic to New Zealand, and they breed on the remote, sub-Antarctic  islands of Bounty and Antipodes. And much of what we know about them  comes from data collected in the 1990s. Yeah, the decade that brought  us the Backstreet Boys and the Macarena and many of my worst decisions.

I don’t actually know if someone found the  data collecting dust in a filing cabinet during COVID lockdowns, but the study  I’m talking about was published in 2022 in the journal PLoS One. It describes erect-crested penguins practicing  something called obligate brood reduction. Each breeding female lays two eggs:  a small one, followed by a big one.

Laying different sized eggs  isn’t unheard of in birds. But for most birds that do  this, the big egg comes first. These penguins are different because  they lay the small egg first.

This also has a name and it’s  called reverse size dimorphism. Now, other crested penguins, such as rockhoppers,  lay two eggs, but they incubate both of them, though the parents generally put more  effort into raising the second chick. Erect-crested penguins don’t attempt  to incubate the first, smaller egg.

Sometimes, they roll it out of the  nest in a deliberate act of egg-icide. More often, they seem to just lose track of it, since their nests aren’t so much  “nests” as spots of bare rock. As soon as number two showed up, though, the  parents rushed to give it all of the attention.

In fact, to make sure the penguins  weren’t just being weirdly consistent about accidentally knocking  that first egg out of the nest, researchers even surrounded some nests with stones to prevent the parents from losing track of it. The penguins in the modified nests still  refused to incubate the smaller egg and sometimes even broke it. Now, this behavior is not what  you’d expect to see in a bird.

It’s just not super efficient  to invest all those resources into making an egg you’re just going to yeet. Animals use up most of their energy  just on basic metabolic needs, and what's left over goes toward reproduction. So what gives, erect-crested penguins?

Well, there are a few theories. Scientists wondered if the second egg is necessary because fighting in the colony  is likely to smash the first. But that’s probably not it, because these  penguins don’t actually fight that much.

Another theory is that the  first egg acts as insurance in case the second one is  destroyed or isn’t viable And that’s not it either, because  the parents abandon the first egg by around the time the second one is laid. So, scientists also wondered if  this is not really an adaptation. You see, that first egg is formed  while the penguin is still migrating, so perhaps it’s smaller because the penguin  just has less energy to contribute to it.

But if that’s the case, why  don’t they just lay one? The researchers think it’s because erect-crested  penguins are descended from ancestors that always laid two eggs, so they are  evolutionarily obligated to do the same. But they've also evolved to be offshore foragers, so they can't really procure  enough food for two chicks.

So investing less energy in that  first egg is the best they can do. It may make sense for them to do this with  the first egg because it’s still forming while they’re at sea, when they  need a lot of energy for themselves. And smaller chicks have a  poorer shot at survival anyway.

So in the erect-crested penguin, we see  a pretty good example of why organisms have to work with what evolution hands them. Their ancestors probably had better  access to resources than they do, so they have adapted to compensate. So if erect-crested penguins are given two eggs, perhaps egg-iciding the first one  is just the sensible thing to do. [♪ OUTRO]