RATITES: EVOLUTION, CHARACTERISTICS, TAXONOMY - BIOLOGY - 2024

The ratites (Struthioniformes) are a group of flightless birds that do not fly. The name ratite comes from the Latin word ratis, which means raft without a keel; As they do not have a keel or fairing in their sternum, the wing muscles have nowhere to fix themselves and for this reason these vertebrates cannot fly.

Ratites are made up of five families: Struthionidae like the well-known ostrich; Casuariformes or Casuariidae as is the case with cassowaries; the Reiformes, which are the rheas; the Dromaids or emus. The fifth family, Apterygidae, includes kiwis, which stand out for being smaller, having shorter legs and for being the only birds of the group with nocturnal habits.

By Loïc Ventre from Croissy-sur-Seine, France (IMG_3249), via Wikimedia Commons

These flightless birds are the giants of the bird kingdom, and zoologists have argued for more than a century about their origins. Until now, many paleontologists believed that ostriches and their allies shared a flightless ancestor in the ancient supercontinent known as Gondwana, from the Cretaceous era, located south of the globe.

Subsequently, the various ratite lineages split apart when Gondwana fragmented into the southern continents that we know today.

This would explain why living ratites are uniquely located in Gondwana-derived land masses, as follows: ostriches in Africa, rhea in South America, and emus in Australia.

Examples of ratites are: ostrich, emu, rhea, cassowaries, kiwi, moa (recently extinct post-human) from New Zealand, and elephant birds from Madagascar.

Also included are 9 genera and 47 species of tinamou that are found in South America, they are foraging birds and are not large fliers.

Evolution

Most of these birds are large vertebrates with long legs, belonging to the class Paleognathae, with the exception of the kiwi. The time at which the diversification of modern birds (Neornithes) occurred remains in controversy.

The majority criterion to explain the loss of flight capacity of ratites is based on evolution after continental drift, because the organisms could not fly anywhere else.

By not using the upper appendages (wings), they atrophied and in their place the lower appendages (legs) of these birds were developed, for a better adaptation to the conditions.

Later studies propose that the absence of flight may have evolved much later, thus challenging the conventional biogeographic theory that associates the distribution of ratites in the southern hemisphere with plate tectonic theory, the origin of continental drift.

In ancient times there were ratites not only on the supercontinent Gondwana. Flightless paleognates were also present in Europe, during the Paleocene and Eocene eras.

In the Holocene era, ostriches were on the Asian continent, however studies suggest that they originated in Africa.

General characteristics

To better understand the characteristics of the birds in this group, it is useful to break down the specimens by family, as follows:

Rhea americana: Rhea. Rheidae family

It is considered the American ostrich, reaching an approximate height of 1.50 meters and a weight between 20 and 27 kilograms approximately, in adulthood.

The color of their feathers ranges from gray to brown, with darker areas between the neck and back, and white feathers on the abdomen and thighs.

Males are larger than females, predominantly gray in color and have patches on the neck and back. In juvenile stages they are gray with darker stripes.

This bird has very large wings, although it cannot fly, which play an important role in the balance of the rhea when it turns while running. In addition, it has long legs with three fingers that help it run at speeds of up to 60 km / h

Dromaius Novaehollandiae: Emus. Dromaius family

This is the only living species belonging to the genus Dromaius. After the ostrich, the emu is the second largest living bird on the planet, whose specimens can measure up to 2 meters and weigh up to 45 kilograms. As a characteristic detail, the females are larger than the males.

Juvenile specimens have a cream color with dark brown stripes. As they grow, the stripes disappear and the soft feathers of youth are replaced by dull brown plumage.

The color in both sexes is similar, but the females have black feathers on the head and neck during the mating season.

Struthio camelus: Avestrúz. Family Struthionidae

It is the largest living bird species, reaching a height of 1.8 to 2.75 meters and a weight of approximately 150 kilograms. It has long limbs, a large neck, large and widely separated eyes that give it a wide panoramic vision both forward and down.

Their feathers are symmetrical and fluffy. In the juvenile stages, both males and females have a similar coloration on their wings that is mottled between yellow, brown and orange and black feathers on the back.

In adults, the female is grayish-brown in color and the tail feathers and wings are gray to white. The males are predominantly black, their tail and wing feathers are white and the neck feathers are gray.

When studying the ostrich's bone structure, it suggests that it evolved from a flying ancestor: existence of air sacs, presence of a pygostyle (terminal part of the spinal column fused into a tail) and presence of pneumatic bones.

Taxonomy

Modern birds belong to the Neornithes group, which has branched, based on palatal characters (structure of the bony palate), into two superorders: Neognathae, representing 99% of existing birds, and Palaeognathae, where ratites are found. and you are.

This primary division is supported by genetic analyzes that allow us to conclude that ratites are paleognaths, that is, running birds that do not fly, have herbivorous or omnivorous food and are large.

Within the superorder Palaeognathae, ratites are assigned to the order Struthioniformes. However, lately it is estimated that only the ostrich belongs to that order.

The Palaeognathae group, although small (1% of modern birds) is considered of great importance for understanding the evolutionary process of birds. The morphology and behavior of the ratites suggest a common ancestry, although adaptation to a different way of life is also suggested.

Much more recently, it has been pointed out that the ratites are a paraphyletic group, that is, that the members of the group share a common ancestor, which is not shared by the offspring).

From the above it follows that the inability to fly is a characteristic that the descendants of the ratites developed independently, on several occasions.

Due to the uncertainty that revolves around the phylogenetic relationships (relationship or kinship between species) of these paleognathic birds, they have become one of the most interesting branches of study in the avian tree of life on the planet.

References

1. Bunney, S. (1987). Did ostriches run all they way from Europe? New Scientist, 34.
2. Glatz, P., Lunam, C., Malecki, I. (2011). The welfare of farmed Ratites. Berlin: Springer.
3. Harshman, L., Braun, EL, Braun, MJ, Huddleston, CJ, Bowie, RCK, Chojnowski, JL, Hackett, SJ, Han, K., Kimball, RT, Marks, BD, Miglia, KJ, Moore, WS, Reddy, S., Sheldon, FH, Steadman, DW, Steppan, S., Witt, C., Yuri, T. (2008). Phylogenomic evidence for multiple losses of flight in ratite birds. Proceedings of the National Academy of Sciences of the United States of America, 13462-13467.
4. Roots, C. (2006). Flightless birds. London: Greenwood Press.
5. Torti, MV and Scataglini, AD (2003). Guide for the management and rearing of the Rhea or suri Rhea americana linneo. Colombia: Andrés Bello Agreement.