American Oceans

The Animal With the Biggest Eyes on the Planet

The animal kingdom is full of fascinating creatures that have evolved unique features to survive in their environments. One such feature is the size of their eyes. Eyes are essential for survival in the animal kingdom as they help animals detect prey, predators, and navigate their surroundings. While most animals have eyes that are proportional to their body size, some animals have evolved to have disproportionately large eyes.

a squid with massive eyeballs

The question of what animal has the biggest eyes in the world is a fascinating one. The answer to this question is not as straightforward as one might think. There are several ways to measure the size of an animal’s eyes, including the size of the eyeball, the size of the eye socket, and the size of the pupil. Additionally, some animals have eyes that are not spherical, making it difficult to compare their size to other animals. Despite these challenges, researchers have identified several animals that have some of the largest eyes in the animal kingdom, and their findings are truly remarkable.

In this article, we will explore some of the animals that have evolved to have the largest eyes in the world. We will discuss how these animals use their large eyes to survive in their environments and how their eyes have evolved to meet their unique needs. By the end of this article, readers will have a better understanding of the fascinating world of animal eyes and the incredible diversity that exists in the animal kingdom.

The Colossal Squid

a rendering of a colossal squid on a white background

The Colossal Squid (Mesonychoteuthis hamiltoni) is a deep-sea creature that resides in the Southern Ocean. It is the largest invertebrate on Earth, with a body that can grow up to 14 meters in length and weigh up to 750 kilograms. The Colossal Squid is known for its massive eyes, which can measure up to 27 centimeters in diameter, making them the largest eyes in the animal kingdom.

Eye Size in Relation to Body Size

Eye size is often related to body size, and the Colossal Squid is no exception. The size of its eyes is proportional to its body size, with larger individuals having larger eyes. However, the Colossal Squid’s eyes are still larger than what would be expected for an animal of its size. This is because the Colossal Squid lives in the deep sea, where light is scarce, and its large eyes help it to detect prey and predators in the dark waters.

The Colossal Squid’s eyes are positioned laterally on its head and are circular in outline. Each eye has a single, large lens that is capable of focusing on objects both near and far. The eyes are so large that they take up a significant portion of the Colossal Squid’s head, leaving little room for a brain.

The Swordfish

a close up of a billfish eye

Swordfish are known for their long, sharp bills, but they also have some of the largest eyes in the animal kingdom. Their eyes can be as big as a human head and are adapted to living in the deep ocean where there is very little light. The swordfish’s large eyes allow them to detect even the slightest amount of light, giving them an advantage when hunting for prey in the dark depths.


close up of an ostritch

The ostrich is the largest living bird in the world and also has the largest eyes of any land animal. Their eyes can be up to two inches in diameter, which is about the size of a billiard ball. The ostrich’s eyes are adapted to living in the harsh African savannah, where they need to be able to see predators from far away. Their large eyes also allow them to see more light, which is helpful in the early morning and late evening when they are most active.


a tarsier in a tree

Tarsiers are small primates that live in Southeast Asia. They have some of the largest eyes relative to their body size of any mammal. In fact, their eyes are so big that they cannot move them in their sockets. Instead, they have to move their entire head to see. Tarsiers are nocturnal animals, and their large eyes allow them to see in the dark and hunt for insects. Their eyes are also adapted to provide a high level of detail, making them effective at catching fast-moving prey.

Other animals with large eyes include owls, horses, slow lorises, and lemurs. These animals have evolved large eyes for different reasons, such as improved night vision or the ability to see in distorted environments. However, in all cases, their large eyes provide them with a significant advantage in their respective environments.

Eye Structure and Function in Animals

close up of a massive squid eye

The eyes of animals come in a variety of shapes and sizes, but they all share a common function: to gather light and convert it into neural signals that the brain can interpret as images. This section will explore the structure and function of animal eyes, including the unique compound eyes of insects.


The pupil is the opening in the center of the iris that controls the amount of light entering the eye. In bright light, the pupil constricts to limit the amount of light entering the eye, while in dim light, the pupil dilates to allow more light in. The size of the pupil is controlled by the muscles of the iris.

Cornea and Lens

The cornea and lens work together to focus light onto the retina at the back of the eye. The cornea is the clear, dome-shaped surface that covers the front of the eye and is responsible for most of the eye’s focusing power. The lens, located behind the iris, fine-tunes the focus of light onto the retina. In fish eyes, the cornea is spherical, which allows for a wider field of view.


The retina is the screen at the back of the eye where light is converted into neural signals. The retina contains two types of photoreceptor cells: rods and cones. Rods are responsible for vision in dim light, while cones are responsible for color vision and visual acuity. In crustaceans, the retina is located on the top of the eye, allowing for a panoramic view of the surroundings.

Unique Compound Eyes of Insects

Insects have unique compound eyes made up of many individual lenses, each with its own photoreceptor cell. This allows insects to see a wide field of view and detect motion very well. The resolution of compound eyes is lower than that of vertebrate eyes, but the panoramic view compensates for this. Insects also have ocular heating, which allows them to see in the infrared spectrum.

Predators and Eye Size

blue shark round snout and large eyes characteristics

In the animal kingdom, having large eyes can be an advantage for predators. It allows them to have better vision and detect prey from a distance. However, not all predators have large eyes. Some predators rely on other senses such as smell or hearing to locate their prey.

One animal that has the largest eyes relative to its body size is the tarsier. Each eyeball is approximately 16 mm in diameter and is as large as the tarsier’s brain. This unique feature allows the tarsier to have excellent night vision and detect small insects in the dark.

Another animal with large eyes is the giant squid. Its eyes can be as large as a basketball, measuring up to 10 inches in diameter. The giant squid lives in the deep sea, where there is little to no light. Its large eyes allow it to detect prey and predators in the dark depths.

Despite having large eyes, not all predators rely on vision to hunt. Some predators, such as snakes, rely on their sense of smell to locate prey. Others, such as bats, use echolocation to navigate and locate prey in the dark.

In battles between predators, having large eyes may not always be an advantage. Predators with large eyes may be more vulnerable to attacks and injuries. In contrast, predators with smaller eyes may be more agile and better able to evade attacks.

Eye Size in Relation to Head Size

a mola mola sunfish in the wild

Eye size in relation to head size is an important factor in determining which animal has the biggest eyes in the world. Generally, animals with larger heads tend to have larger eyes, but this is not always the case. In some animals, the eyes can be disproportionately large compared to the size of the head.

Humans have relatively small eyes compared to the size of their head. The average human eye diameter is about 24mm, while the average head circumference is about 56cm. This means that the eyes make up only a small fraction of the head size.

In contrast, some of the smallest primates in the world have relatively large eyes compared to their head size. For example, the pygmy marmoset has eyes that are about the same size as a human’s, but its head is only about the size of a human thumb. This means that the eyes make up a much larger proportion of the head size in this species.

When it comes to which animal has the biggest eyes in the world, the answer is not straightforward. Some animals have eyes that are large in proportion to their head size, while others have eyes that are simply massive. For example, the giant squid has the largest eyes in the animal kingdom, with eyes that can be up to 10 inches in diameter. However, the giant squid’s head is also massive, so its eyes are not disproportionately large compared to its head size.

The Effect of Light on Eye Size

squid for swordfish to eat prey

The size of an animal’s eye is largely influenced by the amount of light available in its environment. Animals that live in bright environments with high levels of light such as birds of prey, have larger eyes compared to those that live in dimly lit environments such as nocturnal animals. This is because larger eyes allow more light to enter and be processed by the retina, which in turn enhances visual acuity.

In addition to the amount of light available, the wavelength of light also plays a role in eye size. Animals that are active during the day, such as diurnal birds, have eyes that are optimized for detecting short-wavelength light, which is abundant during the day. Conversely, animals that are active at night, such as owls, have eyes that are optimized for detecting long-wavelength light, which is more prevalent at night.

Nocturnal animals have evolved several adaptations to maximize their vision in low light conditions. They have larger pupils that allow more light to enter the eye and a higher density of rod cells in their retina, which are more sensitive to low levels of light. They also have a reflective layer behind the retina called the tapetum lucidum, which reflects light back through the retina, increasing the amount of light available for processing.

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