Largest Eyes Among Land Animals
With a diameter of around 5 cm, an ostrich's eyes are five times larger than a human's and larger than its own brain.
Impressive Records
Ostriches (Struthioniformes) are not only the largest living birds on Earth, reaching heights of 2.8 meters, but also the fastest-running birds that are flightless. Another impressive record they hold is having the largest eyes among land animals, with each eye even being bigger than their brain.
Unique Eye Features
Ostrich eyes have a diameter of roughly 5 cm, approximately five times larger than human eyes. In an ostrich's eye, the lens and cornea contribute equally to the total optical power.
Analysis has revealed that ostrich eyes are essentially scaled-up versions of the eyes of other bird species, such as the common starling (Sturnus vulgaris) and the tawny owl (Strix aluco). This is remarkable given that the shapes of these birds' eyes appear quite different. Scientists theorize that this difference may stem from variations in weight.
For instance, the overall shape of an owl's eye may be a result of the evolutionary pressures of fitting a large eyeâ€"both in terms of pupil area and axial length (the measurement from the front to the back of the eye)â€"into a small and lightweight skull for efficient flight. However, as flightless animals, ostriches are not constrained by the same weight-related concerns. Therefore, their enormous eyes have evolved into the distinctive shape we see today. This may provide advantages, particularly in terms of visual field coverage.
Visual Acuity
Ostriches have a monocular field of viewâ€"the range of vision in each eyeâ€"of approximately 155 degrees, allowing them to scan the horizon for potential threats. However, they have substantial blind spots above and behind their heads due to the placement of their eyes in their skulls to avoid damage from the tropical sun. This natural shielding may be necessary to prevent retinal damage but gives ostriches their rather comical appearance.
Ostriches also have binocular vision (both eyes looking in the same direction to receive a three-dimensional image of their surroundings) in front of their beaks. This capability is useful for searching for food. This area of visual field extends to 80 degrees vertically and 20 degrees horizontally.
While this range may seem small, it aids ostriches in foraging on the ground. As grazers, they need to be able to focus on small objects, such as leaves, seeds, and flowers. They also occasionally eat insects and small vertebrates, so this binocular field of view helps them target moving prey.
Brain Size
In contrast to their enormous eyes, ostriches have rather small brains. Their brains measure approximately 5.9 cm in length and 4.2 cm in width. However, ostriches have evolved not to require a large brain. If camouflage fails, they can often simply outrun predators.
Summary
Ostriches hold the record for having the largest eyes among land animals, with each eye being about five times larger than a human's. Their unique eye structure allows for a wide field of view, aiding in detecting predators and foraging. Despite their impressive eyes, ostriches have relatively small brains, relying on their speed and agility to ensure survival.