Mwalimu-G
Elder Lister
Why Is the Kiwi’s Egg So Big?
This small(ish) bird lays one of the largest eggs in the bird world. New research might hold the keys to solving this long-standing mystery.
By Sam Dean
February 25, 2015
An X-ray of a female kiwi before she lays her egg. Photo: Otorohanga Kiwi House
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 13th 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 13th century was incredibly recent).
www.audubon.org
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This small(ish) bird lays one of the largest eggs in the bird world. New research might hold the keys to solving this long-standing mystery.
By Sam Dean
February 25, 2015
An X-ray of a female kiwi before she lays her egg. Photo: Otorohanga Kiwi House
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 13th 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 13th century was incredibly recent).
Why Is the Kiwi’s Egg So Big?
This small(ish) bird lays one of the largest eggs in the bird world. New research might hold the keys to solving this long-standing mystery.
www.audubon.org
