The Reluctant Mr. Darwin by David Quammen Page A

disagreed about just how many species to recognize, under just what scientific names, but currently the consensus seems settled on four: the North Island brown kiwi ( Apteryx mantelli ); the tokoeka ( Apteryx australis ); the great spotted kiwi ( Apteryx haastii ); and the little spotted kiwi ( Apteryx owenii ). The last was named for Richard Owen, who in 1838 presented a multi-part paper titled “On the Anatomy of the Apteryx” to the Zoological Society in London. Darwin, having heard at least some of Owen’s paper, referred to it in his notebook “D.” The most remarkable thing about the apteryx, Darwin thought, was its small respiratory system, suggesting that in the wild this must be a shy, patient, creepy little bird, with little inclination to exert itself much and therefore little need to breathe heavily. Owen had only a single specimen to examine, a male, and he was an anatomist, not a physiologist or a field naturalist; so he missed some kiwi traits just as peculiar as the reduced lung capacity. An extraordinarily acute sense of smell. A low body temperature, unusually cool for a bird. An odd mix of furtive and aggressive behavior. Darwin missed them, too, along with the single most notable fact about kiwi biology: These little birds lay humongous eggs.
    A female brown kiwi weighs less than five pounds. Her egg weighs almost a pound—constituting, that is, about 20 percent of her total weight. Among some kiwis, the egg-to-body weight ratio reportedly reaches 25 percent. A female ostrich, by contrast, lays an egg weighing less than 2 percent as much as herself. Certain other avian species—hummingbirds, for instance—lay more ambitious single-egg packages than ostriches, but few if any match kiwis. Relative to her body size, on a standard with other birds, the brown kiwi’s egg is about six times as big as it should be. It contains also a disproportionate allotment of yolk, on which the chick will survive just after hatching. This egg takes twenty-four days to develop and, once it has, fills the female like a darning egg fills a sock. Having gorged herself for three weeks to support the growth of such a large embryo, during the last two days she stops eating. There’s no room in her abdomen for another cricket. “Sometimes the egg-bearing female will soak her belly in puddles of cold water,” according to one source, “to relieve the inflammation and to rest the weight.” She is painfully replete with motherhood.
    An X-ray photo of a gravid female kiwi, taken fifteen hours before laying, shows this: a skull, with its long beak; a graceful S-shaped neck; an arched backbone; a pair of hunched-up femurs; and at the center of it all, a huge smooth ovoid—her egg—like the moon during a full solar eclipse. She herself is now just a corona. It seems impossible. How can she carry this thing? How can she deliver? Will it reward her efforts and discomforts, or rip her apart?
    The size of the kiwi’s egg raises interesting evolutionary questions. For starters: Why is it so big? What are the adaptive advantages for kiwi females (and for males, who do much of the incubating) of such heavy investment in a single chick? How has the kiwi lineage changed over evolutionary time? Did the egg evolve toward largeness? Or did the bird itself evolve toward smallness—a shrinking ratite, descended from moa-sized ancestors—while the egg stayed as it was? If the bird shrank and the egg didn’t, why not? Those questions could take us into a discussion of allometry (the study of growth rates and size disparities within organisms) and kiwi evolution, which might be amusing. But allometry isn’t the point here.
    The point is simply metaphor. Every time I see that X ray of the mama kiwi, I think: There’s Darwin during the years of gestation.
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    By spring of 1842 he was a famous author, thanks to the surprising success of his Journal from the Beagle voyage