Ostrich - land animal with the largest eyes

With a diameter of about 5cm, ostrich eyes are five times larger than human eyes and larger than their own brains.

Ostriches ( Struthioniformes ) are strange, but they also hold many records on Earth. Not only are they the largest group of birds on the planet, reaching up to 2.8 m tall, but they are also the fastest group of flightless birds. Another impressive record is that they have the largest eyes of any land animal. Even ostrich eyes are larger than their brains.

Ostriches have unusually large eyes. (Photo: Evgenia Bolyukh).

Ostrich eyes are about 5 cm in diameter, about 5 times larger than human eyes. In the ostrich eye, the lens and cornea contribute equally to the total optical power.

Analysis shows that ostrich eyes are almost large-scale versions of the eyes of other birds, such as the green starling ( Sturnus vulgaris ) and the golden owl ( Strix aluco ). This is quite impressive because the eye shapes of these groups of birds appear to be different. Experts hypothesize that the cause may stem from their different weights.

For example, the overall shape of an owl's eye may have resulted from the evolution of large eyes - both in terms of pupil and axial length (measured from front to back of the eye) - that needed to be fitted into a small skull. and light enough to fly effectively. However, because they do not fly, ostriches are not limited by the same problems related to weight. As a result, their giant eyes have evolved into their current characteristic shape. This can have many benefits, especially in terms of visual field size.

Ostriches have single ocular fields - vision in each eye - that extend to about 155 degrees, which helps them scan the horizon for potential dangers. However, they have significant blind spots on the top and back of their heads due to the way their eyes are positioned in the skull to avoid damage under the tropical sun. This natural shielding may be necessary to prevent retinal damage, but makes the ostrich look rather odd.

Ostriches also have binocular vision (two eyes looking in the same direction to receive a three-dimensional image of their surroundings) in front of their beak. This ability is very useful in finding food. This visual field extends vertically to 80 degrees and widthwise to 20 degrees.

Such a range is quite small but also helps ostriches find food on the ground. Because they mainly eat grass, they need to be able to focus on small objects such as leaves, seeds, and flowers. They also sometimes eat insects and small vertebrates, so this visual field helps them target moving prey.

Contrary to its giant eyes, the ostrich's brain is modest in size. Their brains are about 5.9 cm long and about 4.2 cm wide. However, ostriches have evolved in a way that does not require a large brain. If hiding doesn't work, they can often run away quickly.