Ratites are a group of large, flightless birds that include ostriches, emus, cassowaries, rheas, and kiwis. While ratites were once found worldwide, most species went extinct millions of years ago.
The Extinct Ratites
The earliest ratites evolved during the Cretaceous period, over 65 million years ago. These ancient ratites, part of the extinct lineages Aepyornithiformes and Dinornithiformes, were found across the continents until their extinctions.
The African Aepyornithiformes included the largest birds to have ever lived. One species, Vorombe titan, reached over 3 meters tall and weighed over 700 kg. This giant elephant bird lived in Madagascar until it went extinct around 1000 CE, likely due to human activity.
In South America, the Dinornithiformes included the giant moas of New Zealand. The largest moa species, Dinornis robustus, reached over 3.6 meters tall and 250 kg. Moas were the dominant herbivores in New Zealand’s forest ecosystems before going extinct around 1500 CE after human colonization.
Surviving Ratite Lineages
While many ancient ratite lineages went extinct, several modern groups still survive today.
Ostriches are the largest living birds, native to Africa. Emus are found in Australia. Cassowaries inhabit New Guinea and northeastern Australia. Rheas live in South America. And kiwis survive only on the islands of New Zealand.
All five of these ratite groups trace their evolutionary origins back to the ancient supercontinent Gondwana over 80 million years ago. As Gondwana broke apart into the southern landmasses, it left ostrich ancestors in Africa, emu and cassowary ancestors in Australia, rhea ancestors in South America, and kiwi ancestors in New Zealand.
Ostriches
Ostriches (Struthio camelus) are native to Africa and are the only living members of the genus Struthio. They likely diverged from other ratites around 80 million years ago.
There are four living subspecies of ostrich:
- North African ostrich (S. c. camelus)
- Somali ostrich (S. c. molybdophanes)
- Southern ostrich (S. c. australis)
- Masai ostrich (S. c. massaicus)
Ostriches can stand over 2 meters tall and weigh over 150 kg. Though they cannot fly, their large legs allow them to sprint at over 70 km/h, the fastest land speed of any bird.
Emus
Emus (Dromaius novaehollandiae) are found across most of mainland Australia. They diverged from their closest living relatives, the cassowaries, around 40 million years ago.
There are three subspecies of living emu:
- Southern emu (D. n. novaehollandiae)
- Northern emu (D. n. woodwardi)
- Kangaroo Island emu (D. n. baudinianus)
Emus can reach up to 2 meters tall and weigh around 60 kg. Though not as fast as ostriches, emus can run at speeds over 50 km/h using their strong legs.
Cassowaries
Cassowaries inhabit the tropical rainforests of New Guinea, northeastern Australia, and some surrounding islands. There are three living species:
- Southern cassowary (Casuarius casuarius)
- Dwarf cassowary (Casuarius bennetti)
- Northern cassowary (Casuarius unappendiculatus)
Cassowaries reach up to 1.5 meters tall and weigh up to 60 kg. On their feet they have dagger-like claws up to 12 cm long which they can use defensively if threatened.
Rheas
Rheas inhabit open grasslands across much of South America. They comprise two living species:
- Greater rhea (Rhea americana)
- Darwin’s rhea (Rhea pennata)
Rheas grow over 1.5 meters tall but only weigh 20-40 kg. Their wings are vestigial, with small residual feathers under 3 cm long. Rheas can sprint up to 60 km/h.
Kiwis
Kiwis (genus Apteryx) are the smallest living ratites, native and endemic to New Zealand. There are five surviving species:
- Great spotted kiwi (Apteryx haastii)
- Little spotted kiwi (Apteryx owenii)
- Brown kiwi (Apteryx mantelli)
- Tokoeka (Apteryx australis)
- Okarito kiwi (Apteryx rowi)
Kiwis grow up to 50 cm tall and weigh 1-3 kg. They are the only birds with nostrils at the tip of their beaks. Kiwis are unusual among ratites for being nocturnal. Their feather lack barbules, giving them a hairy, flexible appearance.
Causes of Extinction
Most ancient ratite lineages went extinct millions of years ago due to natural causes like climate change and ecological shifts. However, human activity likely drove the extinction of the largest, most recently extinct ratites like elephant birds and moas.
Possible reasons for these human-caused extinctions include:
- Hunting and harvesting of ratites for food
- Habitat destruction and introduction of invasive species
- Persecution as pests
- Collection of eggs for food
- Natural disasters on isolated islands
For example, moas disappeared rapidly from New Zealand within 200 years of human colonization. Humans extensively hunted moas and destroyed much of their forest habitat through burning and land clearing for agriculture.
Comparatively, the surviving ratites like ostriches, emus, and rheas inhabit larger continental landmasses and did not suffer such extreme overexploitation by humans.
Timeline of Ratite Extinctions
Here is an approximate timeline of major ratite extinctions:
| Time | Group | Notable Extinctions |
|---|---|---|
| Cretaceous-Paleogene extinction ~66 million years ago | Multiple ancient ratite lineages across continents | Mass extinction of non-avian dinosaurs and many other species |
| Pleistocene extinctions 11,700-140,000 years ago | Elephant birds, moas, giant storks | Climate changes and human arrivals lead to megafauna extinctions worldwide |
| 1000 CE | Elephant birds (Vorombe titan) | Hunting and habitat destruction by humans in Madagascar |
| 1500 CE | Moas | Overhunting and habitat loss after Polynesian settlement of New Zealand |
Conclusion
Most ratite lineages went extinct millions of years ago from natural causes, but human activity likely drove the final extinctions of the largest species like elephant birds and moas. Climate changes and overexploitation due to hunting and habitat destruction led to their rapid disappearance within a few hundred years after human contact.
Today just five ratite groups survive worldwide, though even these face continued threats from habitat loss and invasive species. Conservation efforts aim to protect these unusual flightless birds into the future.
