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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.
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