Why Is the Kiwi’s Egg So Big?

The flightless kiwi is so unbirdlike that many biologists call it an “honorary mammal.” Flightless and nocturnal, the kiwi’s feathers evolved into softened, fur-like filaments and its nostrils migrated to the tip of its long beak, which it uses to snuffle in the dirt of its forested New Zealand habitat for the archipelago’s famously giant earthworms. It’s a member of the ratites, the avian order that includes ostriches, emus, and rheas, but the largest kiwis are only the size of a plump laying hen, while the smallest is the size of a guineafowl.

Strangest of all, it lays an egg that can weigh up to a quarter of its body mass. Proportionally, this is by far the largest of any bird in the world. Imagine a chicken laying a one-pound egg, or, more graphically, a human giving birth to a fully formed four-year-old. Yowza.

An adaptation so bizarre is like a magnet for evolutionary biologists, and a slew of ideas about how the petite bird ended up with such a ginormous egg have been published over the past century. But new DNA analysis has begun to radically rewrite the ratite family tree—which, for reasons I’ll address shortly, means that the prevailing theory about how the kiwi got its egg has got to be wrong.

The Tiny Giant Theory

Prior to this new research, the accepted theory about the kiwi egg—most famously and eloquently expounded upon by Stephen Jay Gould—was that it was simply a holdover from when the kiwis were much bigger birds.

The thinking went like this: Before humans got there and killed them all, the kiwi’s only ratite relative on the isolated New Zealand archipelago was the moa, a very big flightless bird that stood up to 12 feet tall and weighed more than 500 pounds. Since, as far as Gould knew, all the other ratites were similarly large (ostriches, emus, cassowaries, the extinct elephant bird, etc.) it made sense to posit that all ratite ancestors were similarly big birds that speciated after Gondwanaland (the landmass that eventually became South America, Africa, India, Australia, and New Zealand) split apart and drifted into the continents of today, a process of speciation also known as vicariance. Therefore, Gould reasoned that kiwis had shrunk down from moa-sized ancestors, kept the moa-sized egg for a while, found it to be not particularly harmful if not exactly helpful, and the big egg stuck.

In short, it wasn’t an adaptation that gave any competitive edge—it was just a relic that natural selection hadn’t bred out. This supposition makes even more sense considering that, until Polynesian settlers arrived with rats in the 13^th century, there were no major ground-dwelling, egg-eating predators to encourage a shrunken egg and discourage the loss of mobility that comes with carrying such a huge clutch (and in evolutionary terms, the 13^th century was incredibly recent).

When Moas Could Fly

In 2010, though, new research investigating the avian family tree started to make this theory seem a little more dubious. There’s an order of roughly partridge-sized ground birds called tinamous, which includes species that range from Mexico down to the southern tip of the Americas, some of which lay astonishingly brightly colored eggs, and most of which can fly, if badly. Despite obvious outward differences, tinamous share a primitive skull structure with the ratites, and the tinamous and ratites are the only birds in the taxon of paleognaths (the other 99 percent of the bird species in the world are all called neognaths). Researchers had long thought the tinamous and the ratites to be sister groups, but in a 2010 paper published in Systematic Biology, mitochondrial DNA analysis found that the giant, extinct moa’s closest relative wasn’t the kiwi, emu, or even the ostrich—it was the unassuming little tinamou, workings wings and all.

This finding changed the assumption that big birds from Gondwanaland had given rise to all of the ratites—maybe a smallish flying bird had landed on each continent at a different time, independently lost the power of flight, and evolved into the big birds of today, possibly filling the ecological niches left open by the then-recent mass extinction of the non-bird dinosaurs.

Even if this isn’t exactly how it all went down, the new research still meant that the kiwi was, in all likelihood, closer to the size of its ancestor than the moa was, which put the kibosh on the big-bird-big-egg theory.

Furthermore, in the past two years, two separate papers have put their own respective pins in that theory. First, a team of researchers, whose results were published in a 2013 paper, analyzed the fossil remains of an assumed direct ancestor of the modern kiwi, and found them to correspond to a much smaller bird—only a quarter of the size of the smallest extant kiwi species. And, if that wasn’t enough, a 2014 paper published in Science found that the kiwi’s closest genetic relative wasn’t even the emu, let alone the moa, but the extinct and unsurprisingly giant elephant bird of Madagascar. If the vicariance theory was correct, and big, flightless birds had split with Gondwana, that would mean the elephant bird should be closest, genetically, to its African neighbor the ostrich—the fact that the kiwi was its next of kin meant that, in all likelihood, they shared a flying ancestor that winged its way to Madagascar and New Zealand long after the islands became islands.

Big Egg, Better Babies

So, far from being the weighty baggage of an evolutionary journey from huge to humble, the kiwi’s egg, in all likelihood, must be an adaptation it picked up as it evolved from a smaller flying bird to the stumpy-legged, chickenish weirdo of today. But what possible advantage could a cripplingly huge egg confer?

Both recent papers include brief notes on the implication of their research for the longstanding kiwi egg debate, and both say the same thing: it’s probably about precocity. The giant egg means that kiwi chicks hatch pretty much ready to run, with a belly full of yolk that they can live off of for their first two and a half weeks of life. In a world with few ground-dwelling egg-eating predators but many chick-eating flying predators (i.e. New Zealand before the mammals arrived), having a chick that can better evade death from above might just be worth the pain of the outsized egg. Take it a step further, and you could even think of it as the kiwi out-mammalling the mammals and evolving a mechanism for an eggy approximation of a live birth, with a whole lot of extra yolk taking the place of mother’s milk.

Since the DNA evidence suggests it’s an evolved adaptation, it’s likely more genetic research will fully explain why the egg is so improbably outsized. Until then, we'll continue to stare at the above photo in wonder.