(Patricia Ferrer © 2002, University of Michigan Museum of Zoology) An 8-foot (2.4 m) ostrich (left) and 6-foot (1.5 m) emu. Photo used with permission.
What is the Difference Between an Ostrich and an Emu?
Delicious Reddit

AUGUST 01, 2008
April Holladay, HappyNews Columnist

Q: What is the difference between an ostrich and an emu?
Jason, Ottawa, Ontario, Canada
A: Differences? Yes, the birds are different but even more remarkable are their similarities. Even though they live continents apart — the ostrich in Africa and the emu in Australia — the look-alike long-necked, long-legged birds are both flightless runners. They are champions.
Differences: The emu is about 25% shorter than the ostrich and has three toes. The ostrich has only two (the only bird with two toes). They live in similar terrain — open country — but widely separated.
Both belong to the same order of birds but not the same family. That’s analogous to us being in the same order (primates) with Old World monkeys but not in the same family (ours being "great apes and humans").
Back to how similar the birds are. Why are they so similar since they are in even different animal families? Primarily because they are both superb runners.
"In fact, they are more specialized for running than almost any other animal," emails biologist A.T. White of Palaeos.
"...these two birds have relatively the largest total hind limb extensor muscles yet recorded for any tetrapod [four legged animal]," states locomotion expert John Hutchinson of the Royal Veterinary College, UK.
The ostrich and emu run better (not faster but a better combination of speed, power, and endurance) than the others. "This is particularly true," says White, "of the emu, which devotes almost one-third of its entire body mass, bones and all, to leg extensor muscles."
Both birds have compact bodies whose center of gravity hovers directly above their speeding feet — giving them maneuverability and good balance. Both have short almost horizontal thighbones and long shins and ankles — maximizing acceleration and mechanical advantage. Both have long necks and good vision that capitalize on speed in an open country.
Moreover, ostriches and emus have the same body plan. The birds apparently share a common ancestor who lived long ago on the gigantic landmass, Gondwana. When Gondwana broke up about 75 to 85 million years ago, Africa split off, and then Australia/Antarctica sailed slowly away (plate tectonics). See figure. With a widening ocean between them, the ostriches and emus were forced to evolve separately thereafter. At least, this is the best current theory.
The super continent, Gondwana, as it appeared in the Late Cretaceous (about 80 million years ago). Tectonic plates drift and wrench the land apart. Africa and South America have split off from Gondwana. Australia and New Guinea are splitting off from Africa. [Courtesy of A.T. White]
"...Africa was really quite isolated, even for flying birds, for a substantial period of time. Thus even if ratites were still flying birds in the Late Cretaceous [about 80 million years ago], they were stuck on the African ship once it left the dock," says White.
You might wonder why ostriches and emus still seem so similar since they have evolved on separate continents for about 80 million years.
Because they are superb runners. "Like specialization for flying, extreme specialization for running, or anything else, tends to make animals look the same if they started with the same body plan," says White.
. . .
I am indebted to A.T. White for determining (read fascinating details here) the best current theory on the origins of ostriches and emus.
. . .
Further Reading:
Palaeos.com: Flightless birds (Ratites) by A.T. White
University of Michigan Museum of Zoology: Order Struthioniformes (cassowaries, emus, kiwis, ostriches, and rheas)
Wikipedia: Ostrich (very good)
EggCrazy: Emu information
Australian Museum Online: Emu fact sheets
Wonder Oil: Emu oil
EggScape: Emu, Ostrich and other eggs
(Answered Nov. 15, 2005, updated June 11, 2008)