Mid-week Reptilian #12: Struthio camelus

Wednesday, March 3, 2010 at 10:36 AM Bookmark and Share
As the heaviest and tallest living bird, the Ostrich (Struthio camelus) is probably the most familiar of all flightless birds second only to penguins. While flightlessness has evolved numerous times in a variety of different bird groups, it's the norm among Ostriches and their closest relatives - the ratites.

Most people are familiar with these big, showy, eye catching animals. They are common in zoos, frequently seen images in popular media, and recently have even gained popularity as livestock (drumstick, anyone?).  Ostriches commonly appear in documentaries, such as this recent appearance (as cat food) in the latest BBC series, Life (see below).  

 
(Americans apparently can't handle Attenborough, so unfortunately we get Oprah instead. )

As you may have already noticed, Ostriches aren't just some stretched out version of your typical bird. So what makes them so distinct? To fully appreciate what makes these towering descendants of dinosaurs so darn interesting, it helps to look into their evolutionary history -- one that is shared with their closest living relatives, the other ratites.

Ratites are a group of flightless birds (order Struthioniformes, though some elevate the families below to order status) spread out over South America, Africa and Australasia.  They're a great example of allopatric speciation, with the different species having diverged from one another as the continents spread apart during the past hundred million years or so.


Figure 1: Pangea breaking up starting at ~225 million years ago.

As a group, the taxonomic relationships within the ratites falls cleanly over their geographic distributions.  Excluding extinct species like the freakishly huge Moas, and other groups that have recently been associated with the ratites (i.e., the Tinamous), their distributions are as follows: 

Australasian Ratites

1. Family Cassuariidae
  • Emu (Dromaius novaehollandiae)
  • Cassowary (3 species; Southern Casuarius casuarius, Dwarf C. bennetti, Northern C. unappendiculatus.)
2. Family Apterygidae
  • Kiwis (5+ species, all in the genus Apteryx)

African Ratites

3. Familiy Struthionidae
  • Ostrich (Struthio camelus

South American Ratites

4. Family Rheidae
  • Rheas (2 species; Greater Rhea americana, and Lesser R. pennata)
Along with having lost the ability to fly, ratites possess unkeeled breast bones reduced or absent furcula (wishbones), a stronger more dense bone structure, and legs that are well suited to walking and running.  Besides such adaptations for life as a terrestrial biped, Ostriches and other ratites also have what is perhaps the coolest and most under-appreciated feature - vestiges of their dinosaurian origins: they have vestigial claws on their wings.

If that's news to you, lets just say it out loud one more time:  Ostriches have claws!!

Figure 2: Details of the wingtip of the Ostrich, drawn circa 1898.
[Source: The structure and classification of birds (full text).]

Fortunately for zoo visitors and Ostrich farmers, they didn't retain TOO many of their ancestors' other dinosaurian features. Imagine getting a bite from one of these!

 
Figure 3:  The ostrich farmer's worst 
nightmare: avian atavism!  [Source]

2 comments:

Posted by: slybird | 3/05/2010 6:17 PM

I think Kiwis are a previously unrecognized example of convergent evolution: mammals that now vaguely resemble flightless birds.

Posted by: Paul | 3/05/2010 6:22 PM

Good call - although it is pretty obvious! I mean, those egg-laying echidnas are clearly just quadruped versions of the kiwi. ;)

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