Ratites are a family of flightless birds that includes ostriches, rheas, and emus, among others. They have a unique bone structure among birds that makes them unable to fly, as they lack the structure to which wing muscles attach in other birds, called a keel. There's debate about when and how they evolved, but some form of ratite has been around for at least 50 million years. Today, some species are cultivated both as a meat source and for their feathers and hides, but others are endangered.
The ratite family includes ostriches, emus, cassowaries, rheas, and kiwis. There are also several extinct ratites, including moas and elephant birds. Ostriches are the largest living species, and generally grow to be between 6 and 9 feet (2 and 3 m) tall, weigh about 360 pounds, (160 kg), and can reach speeds of up to 46 mph (74 km/h). They are native to Africa, but are raised commercially throughout the world. Kiwis are at the other end of the spectrum, and rarely grow larger than chickens. Several species of kiwi are endangered, and they are highly prone to predators like cats and dogs. They are known for laying the largest eggs in relation to their body size of any bird, with one egg reaching up to 25% of their body weight.
Rheas look somewhat like smaller ostriches, and live in South America. There are two species of rhea, each of which tends to form flocks with deer, similarly to the way that ostriches do with other animals like zebras. The larger of the two species, the Greater Rhea, stands about 3 to 5 feet (0.9 to 1.5 m) tall, and weighs about 50 pounds (20 kg), while the smaller one, called Darwin's Rhea, stands around 3 ft (about 1 m) tall, and weighs between 33 to 55 pounds (15 to 25 kg). A similar looking bird, the emu, lives in Australia, and grows to about 6 feet (2 m) tall. They are able to swim, and their legs are so strong that they can kick through metal fences.
Cassowaries are likewise extremely strong, and can disembowel an adult human with their kicks. They don't like to make contact with humans, but become aggressive when cornered. Adults are about 5 to 6 feet (1.5 to 1.8 m) tall and weigh about 130 pounds (60 kg). They are able to run at speeds of up to about 30 mph (about 50 km/h), and swim in both rivers and the ocean. After ostriches and emus, cassowaries are the third tallest birds in the world.
An extinct ratite, the elephant bird of Madagascar, was the world's largest bird at the time when it lived, though it's thought to have gone extinct around the 17th century. On average, they were about 10 feet tall (over 3 m), and weighed over 800 pounds (400 kg) Their eggs were around 160 times the volume of a chicken egg, with diameters of about 3 feet (1 m). They are thought to have been at least partially responsible for the legend of the Roc, a giant bird that features in the stories of Sinbad the Sailor. Another extinct type of ratite was the moa, which lived in New Zealand until they went extinct around 1300 AD. There were 11 species of moa, which ranged in height between 3 and 13 feet (about 1 to 4 m), and weighed between 30 to 520 pounds (14 to 236 kg), depending on the species.
Ostriches, emus, and rheas are cultivated commercially for their meat, skin, feathers, eggs, and other body parts. Emus in particular are used to make emu oil, which is a nutritional supplement made from their fat. Rheas and ostriches are more commonly cultivated for their meat, as well as their skin and feathers, which are used to make leather goods and decoration for garments. In most areas, including the US, EU, and Australia, there are regulations on ratite farming, including the type and height of fencing required to keep them and the cleanliness of the slaughter sites. The US Department of Agriculture (USDA) recommends cooking ostrich meat similarly to beef, making sure that it reaches an internal temperature of 145° F (about 63°C), or 160°F (about 71°) if it's ground.
The closest relatives to ratites are tinamous, which live in Central and South America. They're particularly closely related to rheas, both in terms of their body structure and in terms of DNA. Most species are fairly small, with the largest tinamou weighing about 5 pounds (2.3 kg), and the smallest weighing only 1.5 oz (43 g). Though they can fly a little, they walk or run unless they are greatly threatened. Unlike ratites, their muscles are adapted for flying, but it appears that their circulatory systems can't work properly for long flights.