THE RATITES all share several traits: large size, flightlessness, strong legs for walking or running, and the absence of a keel on the sternum. The sternum is the breastbone, and the keel is where wing muscles normally attach. On a bird with no wing muscles to speak of, there is no keel. So for a bird known only from a handful of subfossils, the sternum can be an eloquent datum, revealing at a glance whether the species was capable of flight.

The living ratites, with their keelless sternums, are all big gawky wonders with useless little wings. Like the extinct ratites, they are mostly restricted to islands. The ostrich is an exception, a ratite that inhabits the African mainland. The rheas, native to South America, are not quite so exceptional, since South America itself was an island until the rise of the Panama land bridge. Other living ratites are the cassowaries of Australia and New Guinea, the emu of Australia, and the kiwis of New Zealand.

One of the intractable riddles about ratites is whether their flightlessness came as a consequence of gigantism or their gigantism evolved as a compensation for flightlessness. The smallish size of the kiwis provides a false clue toward solving that riddle, because their smallness developed as a late evolutionary modification of the earlier body plan. Another lingering uncertainty is whether the ratites are a genuine kinship group, reflecting common evolutionary origin, or just an unrelated cluster of species that incidentally share bigness and flightlessness. That question implies still another. Did early ratites arrive in New Zealand, Madagascar, and Australia by wing power or did they walk?

If they flew, then the moas and the kiwis and the elephant birds and the emus and the cassowaries all lost their disperal abilities independently. Under that scenario, the losses may have happened within just the past five or ten million years. If they walked, on the other hand, they must have done it in the more distant past, before the great southern continent now known as Gondwanaland had split into pieces. The walking-ratite scenario has certain merits over the flying-ratite scenario. For one thing, it permits the parsimonious assumption that the ratites inherited flightlessness as a common ancestral condition rather than acquiring it independently by a whole series of coincidences¹. But the walking-ratite scenario was befuddling for biogeographers of earlier generations, before continental drift theory had been developed and accepted. How could a giant bird walk from Africa to New Zealand?

Only a few naturalists of the last century came to grips with any of the ratite questions. One was our man [Alfred Russel] Wallace¹. He guessed presciently, back in 1876, in his book The Geographical Distribution of Animals, that the giant birds of New Zealand, Madagascar, Africa, and South America did derive from a common ancestor. He took them to be "a very ancient type of bird, developed at a time when the more specialized carnivorous mammalia had not come into existence, and preserved only in those areas which were long free from the incursions of such dangerous enemies." But he had no explanation for how such flightless giants could have colonized their remote islands. Continental drift was the crucial missing piece.

The idea of giant flightless birds walking from Africa to New Zealand, during the Mesozoic era, is nowadays a reasonable hypothesis. How could they have gotten there? By cutting across Antarctica, of course. It was just a matter of migrating from one province of Gondwanaland to another. The big continent began breaking apart about 130 million years ago, at which time the earliest birds had already evolved from their dinosaur ancestors. About 90 million years ago, when South America separated from Africa, the early ratites were probably dispersed throughout Gondwanaland, so South America could have taken some with it. Madagascar may have ripped away from eastern Africa at about the same time, and New Zealand from western Australia, each of those new islands carrying its share of ratites. Australia detached from Antarctica somewhat later, drifting northward toward its convergence with New Guinea. This is all speculative, but it represents the informed speculation of credible geologists.

The ratites are both instructive and misleading. They exemplify much about island ecology, but not so much about island evolution as a person might suppose. They tell less about the process of innovation than about the process of survival. The distinction I'm alluding to now is between endemism and relictualism.

An endemic species is one that has arisen evolutionarily in the same place where it's presently found--say, on an island. A relictual species is one that has survived in a given place while disappearing elsewhere. This dichotomy is not applicable only to species; larger taxonomic groups, such as a genus or a family, can be endemic or relictual as well. The ratites, though very old as a group and previously more widespread than now, have continued to generate new species. So the ratite lineage combines aspects of both endemism and relictualism--endemism at the level of species, relictualism at the level of the group. Aepyornis maximus was endemic to Madagascar, having evolved from its immediate ancestor on that island. But the ratite lineage in general, with its heritage of gigantism and flightlessness, is relictual on Australia, New Guinea, and New Zealand (and was until recently on Madagascar), having held out in those places while long ago disappearing from the mainlands.

The lineage has also persisted, barely, in Africa. The single species of living ostrich, Struthio camelus, has survived there against seemingly bad odds--on a continent populated by lions and jackals and hyenas and other predators, as well as by huge herds of herbivorous mammalian competitors. How has this giant flightless bird avoided extinction? With some difficulty. Even the ostrich family is relictual in its present distribution, having once also been represented in Europe and Asia. Struthio camelus survived in Africa by adapting to a fleet-footed way of life on the great grasslands. Its leg bones are drastically different from the leg bones of Aepyornis maximus, those of the ostrich being suited for much greater speed. Its swiftness, its kicking ability, and several other characteristics have made the ostrich capable of coping with life on a continent.

Close ancestors of Aepyornis maximus probably once lived on the African mainland too. But as the mammalian predators became formidable, as the mammalian herbivores became abundant, as the climate changed and the wet forests gave way to drier and more open grasslands, the ancestral elephant birds couldn't cope. Evolution, which sometimes (but not always) supplies new solutions to new problems, didn't move quickly enough to save them. So in continental Africa the elephant birds went extinct. On Madagascar, a haven from such pressures, they became relictual.

There and in their other havens, the ratites continued to evolve. New species arose, endemic to each place. If they had enemies, those enemies were endurable. So the lineage of huge flightless birds survived, at least until humans arrived, on most of those scattered fragments of Gondwanaland that we now know as the planet's great southern landmasses--Australia, New Zealand, New Guinea, South America, Africa, Madagascar. The only big Gondwanan fragment where ratites didn't last so long is Antarctica², and the reasons for that are presumably climatic³.

If the ostrich is an exceptional ratite, the ratites themselves stand as exceptions to virtually every rule except one: the rule that says islands show us fresh and extravagant definitions of what's biologically possible. My advice is to savor them for their oddity, then forget them. With the ratites set to one side, flightlessness as a characteristic of insular evolution can be more clearly and broadly understood. Most of the flightless birds and insects that exist on the world's islands did not get to those places by walking across Antarctica⁴.

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Notes on this text

I think the author's point is well taken: that a theory to explain what appears to be monophyletic descent among the ratites should not be held hostage to the current limitations of scientific knowledge. Wallace was a gifted naturalist who is unjustly overlooked for his contributions to Science (especially since "Evolution = Darwin" in the minds of many, despite the fact that some notion of Evolution predates Darwin's inquiries into the matter). Wallace could not help noticing the similarities among the giant flightless birds, so he postulated a common post-dinosaur origin to explain them. As Quammen notes, the eventual acceptance of Continental Drift supplied the missing piece of Wallace's puzzle.

Nevertheless, we are left with a different puzzle now: where are the intermediate ratite fossils? The record is spotty at best, and undermines the theory of a common ratite origin which allowed them to take their respective places throughout Gondwanaland before the supercontinent went to pieces. Advances in molecular biology & the discovery of further intermediate fossil forms may solve this one day: just as Science did not have all the answers in Wallace's day, neither does it have them now.

Particular points:

1. To use Occam's Razor to explain ratite flightlessness seems like a misapplication in this instance: surely it is far easier to LOSE the ability to fly, than it was to evolve it in the FIRST place. This is demonstrated by the many flightless rails of the Pacific, which unquestionably flew to the islands to which they are now restricted. Given a large enough island, an absence of serious predation, and enough time, is it not possible that the next "ratite" could evolve from a medium-size bird?



2. India was a larger piece of Gondwanaland than Madagascar: what is the ratite fossil record there?



3. Antarctica may be cold now, which hampers fossil hunts, but it wasn't always. If it once held ratites, it must still hold fossils of them somewhere.



4. To take Quammen's advice and forget the ratites may be ironic: if they did fly to their islands and devolve into flightlessness, then they are prime examples of insular biogeography, a field to which he has devoted this book.