Ostriches are large, flightless birds that are natives of Africa. They are the largest living birds in the world and can grow up to 9 feet tall and weigh over 300 pounds. Ostriches are known for their long legs, long neck, and their ability to run at speeds up to 43 miles per hour. Despite their large size, ostriches cannot fly. Their wings are used primarily for balance during running and for courtship displays. Ostriches have several adaptations that allow them to thrive in the hot, arid environments of Africa. They can go for long periods without water and have specialized feathers that help insulate them from the heat.
When it comes to ostriches’ closest living relative, the answer is: the kiwi. Kiwis are small, flightless birds that are found only in New Zealand. Genetic studies have shown that ostriches and kiwis share a common ancestor and are each other’s closest living relatives. Kiwis may not look much like ostriches at first glance, but they do share some key traits that point to their evolutionary relationship.
Evidence That Ostriches and Kiwis Are Closely Related
There are several pieces of evidence from genetics, anatomy, and behavior that confirm ostriches and kiwis are more closely related to each other than to any other type of bird:
- Genetic studies show ostriches and kiwis share more DNA sequences in common with each other than with other birds.
- Ostriches and kiwis both cannot fly, despite being birds. This is an example of a shared derived trait since the ancestor of all birds could fly.
- They have similar egg structures – their eggs are thick-shelled and relatively large compared to body size.
- Both species haveHair-like structures on their bodies rather than feathers.
- Their chick’s have an egg tooth they use to break out of the egg shell.
- Ostriches and kiwis both build ground nests rather than nesting in trees.
- They have well-developed senses of smell, which is uncommon in other birds.
All these shared traits indicate ostriches and kiwis share a more recent common ancestor with each other than with other birds. This close evolutionary relationship means they belong to the same taxonomic clade called the Palaeognathae. All other living birds belong to the taxonomic clade Neognathae.
Differences Between Ostriches and Kiwis
Despite their close evolutionary relationship, ostriches and kiwis have many obvious differences in their size, appearance, habitat, and behavior:
Size
Ostriches are the largest living birds, growing 7-9 feet tall and weighing 220-320 lbs. Kiwis are much smaller, typically weighing 3-8 lbs as adults.
Appearance
Ostriches have long necks and legs with two toes on each foot. They have black and white plumage on their bodies and tails. Kiwis are more round-shaped birds with brown, hair-like feathers covering their bodies. They have very short necks and long, narrow beaks. Kiwis have four toes on each foot.
Habitat
Ostriches live in wide open grasslands and deserts of Africa. Kiwis live in forests and scrublands of New Zealand.
Behavior
Ostriches cannot fly but can run at speeds over 40 miles per hour. They live in nomadic herds. Kiwis move slowly on the ground and build burrows or hide during the day. They are mostly solitary, coming together only to mate.
So while ostriches and kiwis share a close evolutionary relationship and some unique traits rarely seen in other birds, they have adapted to very different lifestyles and environments.
Evolutionary History of Ostriches and Kiwis
Ostriches and kiwis belong to an ancient lineage of birds that diverged from other birds over 80 million years ago. Here is an overview of their shared evolutionary history:
- The earliest common ancestor of ostriches, kiwis, and all other living birds is thought to have lived 100-125 million years ago during the Cretaceous period.
- Around 80-90 million years ago, this ancestral population split into two distinct groups – the Palaeognathae (ostriches, kiwis, emus, etc) and the Neognathae (all other birds).
- Palaeognathae birds retained more primitive features like flightlessness, while Neognathae evolved traits like improved flight capabilities.
- Within the Palaeognathae group, the lineages leading to ostriches and kiwis diverged around 70-80 million years ago likely due to continental drift separating populations.
- Ostriches evolved in Africa while kiwis became endemic to New Zealand. They adapted to their local environments over millions of years.
Understanding the evolutionary split between Palaeognathae and Neognathae has helped explain many of the unique traits ostriches and kiwis share. Their common ancestor likely had some flightlessness and other primitive features that ostriches and kiwis retained while other birds continued to evolve enhanced flying abilities and adapt to ecological niches around the world.
Continental Drift Led to Separation of Ostrich and Kiwi Populations
Continental drift played an important role in the divergence of ostriches and kiwis. Their common ancestor lived during the late Cretaceous period when the continents were still joined together in a supercontinent called Gondwana. This included what is now Africa, South America, Antarctica, and Australasia.
As Gondwana broke apart around 80 million years ago, populations of ancestral ostriches and kiwis became separated. Early ostrich predecessors remained in Africa while early kiwis became isolated in the land mass that would become New Zealand. Separated for millions of years, these populations slowly evolved into the distinct ostrich and kiwi species we see today.
Other Close Relatives
In addition to kiwis, ostriches have a few other close living relatives within the Palaeognathae group of birds:
Emus
Emus belong to the same order (Struthioniformes) as ostriches but are native to Australia. They are the second largest living birds behind ostriches.
Rheas
Rheas include two species found in South America – the greater rhea and Darwin’s rhea. They are also large, flightless birds but not as tall as ostriches.
Cassowaries
Cassowaries are ostrich-like flightless birds that live in the forests of New Guinea and northeastern Australia. They have black plumage, colorful heads, and can grow over 5 feet tall.
Tinamous
Tinamous are found throughout Central and South America. Unlike other ratites, tinamous can still fly, although weakly. There are around 50 species, ranging from 9 to 16 inches long.
All these birds are part of the Palaeognathae clade along with ostriches and kiwis. However, genetic evidence indicates kiwis remain the closest living relative to ostriches even within this group of birds.
Characteristics of the Palaeognathae
The Palaeognathae are defined by several key anatomical and physiological traits that set them apart from all other living birds (Neognathae). These characteristics were likely present in some form in their common ancestor over 80 million years ago:
- Flightlessness – Most Palaeognathae species, including ostriches and kiwis, cannot fly at all, or can only fly weakly and for short distances.
- Large size – Many Palaeognathae like ostriches grow much larger than most Neognathae birds.
- Primitive wing structure – The wings have a more primitive skeletal structure compared to flying Neognathae birds.
- Large eggs – Relative to body size, Palaeognathae eggs are much larger than Neognathae eggs. The yolk is also larger compared to albumen.
- Simple vocal organs – Unlike songbirds, Palaeognathae lack a syrinx vocal organ and have simpler vocalizations.
- Slower growth rates – Palaeognathae grow and develop slower than comparably sized birds in the Neognathae group.
- Different head artery – A difference in the position of a key head artery separates the two groups.
- Thick-shelled eggs – The eggs have thicker, more heavily calcified shells compared to most other birds.
These ancestral traits connect ostriches, kiwis, and other ratites together as the Palaeognathae group separate from all other living birds. Even with many anatomical and behavioral differences, ostriches and kiwis still share core traits reflecting their close evolutionary relationship.
Conclusion
In summary, extensive genetic, anatomical, and paleontological evidence show that ostriches and kiwis are each other’s closest living relatives. These large, flightless birds diverged over 70 million years ago as ancient supercontinents broke apart, separating ancestral populations. While adapted for life in vastly different environments, ostriches and kiwis retain signature traits like flightlessness, strong legs, and thick-shelled eggs that distinguish them from all other living birds. Their shared evolutionary history provides a fascinating look at an ancient lineage of birds that split off from all other avian species over 80 million years ago.