from CHAPTER 4: "Birds, Ancient and Modern"

Order   Aepyornithiformes

Family   Aepyornithidae · Elephant Birds · At least 11 species¹ · All extinct · The thick shelled eggs held up to two gallons and the birds may have weighed up to 500 kg.
Madagascar and southern Africa, where exterminated by man about 1000 years ago².

The ratites probably evolved from a common flying ancestor³. The Ostrich and rheas still have prominent wings, the cassowaries and emus have wing remnants but the kiwis are probably the nearest relatives of the extinct moas⁴.

   In some respects the Ostrich is the most distinct anatomically. It has a two-toed (3rd and 4th) foot while the other living ratites have three prominent toes (2nd, 3rd and 4th).

   Most ratites are long-necked birds that can spot and often outrun predators in the open country where they live. The cassowaries are somewhat different; a shorter neck and stockier build are suited to pushing through dense tropical forest. Finally, the squat kiwis mostly live in dense undergrowth.

   Although flightless, ratites can well defend themselves with their bills, feet and powerful legs. Cassowaries have the most dangerous weapon in the form of one toenail, the innermost, which may reach over a foot in length. It becomes pointed and sharp, and kicking cassowaries kill more New Guineans than any other animals. A cassowary kick literally slices open the victim.

Diet.   With a broad, slightly flattened bill, most ratites are basically vegetarians, although all will eat almost anything they can secure. The digestive tract is duly long, especially in the Ostrich and rheas. In addition, the rheas have two well developed caecae in which bacterial digestion of cellulose occurs. Chicks are insectivorous when small, and slowly become more vegetarian with age.

   Kiwis are specialized exceptions to this. They mostly feed at night, using a long bill with nostrils at its tip to probe for earthworms, their main food.

Reproduction.   The social structure of ratites is quite similar. In all except the Ostrich, where incubation is shared by male and female, the males do all incubation and rearing of downy chicks⁵. Several females lay eggs for each male. The system has been most thoroughly studied in rheas where each dominant male may display to and attract up to a dozen females, who will proceed to lay eggs for him in a nest scrape he makes. The female lays an egg every second day for up to 10-14 days before shifting to the next dominant male. Over the course of two months each female may lay up to 30 eggs in the nests of four to eight different males.

   Males incubate the eggs alone and then care for the chicks. Synchronized hatching appears to be advantageous to the rhea chicks. Since the eggs may be laid several days apart, the presence of other eggs in the nest seems to slow down the process of incubation by up to 3 to 5 days for the first egg and speed up the development of the last eggs laid by as much as 3 or 4 days. To do this, communication between eggs is required and experiments have demonstrated that communication by voice between developing chicks inside the eggs does occur.

   Clutch sizes and incubation periods are as follows: rheas, 15-30 yellow or green eggs, 35-40 days; emus, 10-20 dark green eggs, about 56 days; cassowaries, 4-8 light green eggs, about 50 days.

   Kiwis are again exceptional. One or two very large white eggs (about 25% of the female's weight; 60% of the egg is yolk, the highest proportion in any bird) are incubated by the male alone for 65-85 days. Chicks are active and fully feathered at hatching. It is possible parents feed the chicks in the first weeks after hatching; thereafter chicks feed themselves.

DB⁶

DID RATITES EVOLVE FROM FLYING BIRDS?

Ratites are an ancient group of birds, at least 80 million years old⁷. They probably evolved from a flying bird resembling modern tinamous, a group to which the ratites are certainly related.

   In becoming flightless, ratites lost the keeled sternum to which the flight muscles attach in flying birds. However, they retain other skeletal features that are associated with the ability to fly: fused wing bones; presence of an alula (bastard wing) on the wing; and pygostyle for anchoring tail feathers. In addition the structure of the ratite cerebellum, the part of the brain responsible for in-flight coordination, suggests a flying past.

   Ostriches and rheas have retained prominent wings which are now used in courtship display. Cassowaries and emus have lost all but remnants of their wings. In cassowaries these consist of several long feather shafts which appear like long rounded fingernails. The reasonably well developed wings of Ostriches and rheas, along with behaviours like the 'broken wing display' which draws predators away from the nest, further indicate that these birds have probably evolved from flying ancestors.

   The large numbers of flightless island birds, especially rails, is extra evidence that the loss of flight can occur quite rapidly in evolutionary terms⁸.

DB⁶

DID RATITES EVOLVE ONLY ONCE, OR ARE THEY SURVIVORS OF SEVERAL LINEAGES?

The current and historic geographic distribution of ratites in the southern hemisphere has resulted in much debate on their origins.

   One theory assumes that all ratites derived from some ancient bipedal dinosaur or tinamou-like ancestor while the southern continents were still attached. The suggestion is that after the break-up of Gondwanaland into Africa, Australia, South America and Antarctica occurred, between 130 and 80 million years ago, the common ratite ancestor that was present on all continents then evolved independently on each continental mass. In South America, Australia, New Zealand, Madagascar, and New Guinea, there were probably no large mammalian predators during this period. The ratites evolved and increased in size as a defense against the smaller, slower mammals that had developed in those areas. Without large cats or canids, ratites were able to compete in South America and the Australasian region. In North America and Eurasia none was able to cope with the large mammalian predators and the more rigorous climate.

   This theory does not fully explain the Ostrich in Africa. Perhaps the Ostrich (and the Elephant Birds in Madagascar) evolved before the large mammalian predators arrived in Africa⁹. Or perhaps the Ostrich evolved alongside those predators, depending for its survival on its large size and speed.

   A second theory suggests that all the ratite groups evolved from separate stocks on each continental mass to fill similar habitats. The evidence of similarities in anatomy and behaviour seems to refute this idea¹⁰.

DB⁶

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Notes on this text

1. Eleven species is higher than most sources put it; perhaps they're right, but this number may include species theorised on the basis of eggshell fragments alone (odder things have happened in the name of Science, you know).



2. There is evidence that Vorompatra became extinct as recently as the 17th century--1,000 years ago is a conservative estimate quite possibly based on early radiocarbon dating of egg shells.



3. As this selection points out later, the belief that all ratites are descended from a common tinamou-like ancestor is the traditional one, but may not be correct.



4. This sentence bothers me for stylistic reasons: the non-parallel structure seems like something which would cost you SAT points.



5. It seems reasonable to suppose that the Vorompatra is most closely related to the Ostrich (given the geographic proximity). So, did Vorompatra males share nesting duties with their mates, like the Ostrich, or did they follow the more usual ratite pattern?.



6. I presume "DB" is the contributor who wrote this part of the book; my fault here for not having noted who that was. I'll rectify this when I get the chance so that the author gets his credit.



7. If ratites are truly 80 million years old, can that mean AS ratites: that is, as non-flying birds? If so, it gets them distributed across Gondwanaland before that continent split. We're left with the question of why no Madagascan elephant bird fossils older than two million years have been found, though--were they just incredibly tidy & swept up after themselves?



8. Indeed, it seems to have taken very little time (in evolutionary terms) for many island birds to have folded their wings for good when their new homes allowed them to thrive without the investment that flight requires. This could readily explain how a flying bird made it to Madagascar, THEN gave up flight as unnecessary under the new circumstances in which it found itself. Giving up flight would allow such a bird to grow to elephantine proportions, since walking is a lot cheaper than flying.



9. I'm not sure quite when those large mammalian predators arrived, but isn't it possible that a long-legged bird evolved to escape on foot as a supplement to the trump card of flying away? Running away is cheaper, energy-wise, and as "proto-ostriches" got better at it, perhaps they could afford NOT to fly anymore. Getting taller (i.e., evolving longer legs) would allow for greater cursorial speed, even as it made flying less economic. Getting more massive would allow the Aepyornis to disregard the small predators of Madagascar, so it would eventually not need to run away at all! But the Ostrich still has lions to watch out for, so it continues to evolve into a more efficient runner: the two toes may dwindle to a single toe in time, just as the five-toed Hyracotherium graduated to the modern single-toed Horse.



10. Sure, there are similarities in anatomy between the various ratites, but the question really must be, are these similarities all derived, or could there be another source? Neoteny has been proposed as one mechanism whereby birds lose the flight option by arrested development. Baby birds learn to walk before they can fly, so their legs start out stronger than their wings. Perhaps the similarity among ratites is that they were not naturally-selected out of the gene pool despite their "refusal" to grow up. It is pretty well agreed that ratites are descended from flying ancestors, but if there is no compelling reason to take to the air, Neoteny is free to create new species that cut corners by not maturing when such an investment confers no "edge" to the organism. Presuming a common origin for ALL birds, there should be a lot of common Ontogeny (i.e., embryonic development) from which to draw; other features ratites have in common which cannot be thereby explained, might derive from Convergence, common evolutionary responses to like environmental pressures.