Native to Southeast Asia, tarsiers are tiny primates in the family Tarsiidae. While their bodies are only about 9 to 16 centimeters (3.5 to 6 inches) long—excluding their tail—their eyes are about 1.5 centimeters (0.6 inches) in diameter, which is similar to the size of their brains.
They have the largest eye-to-body ratio of any mammal—human eyes would have to be the size of apples to have the same proportions. Tarsiers’ brains have extremely large visual cortexes, and their heads can rotate 180 degrees. They’re also the only completely carnivorous primates. Tarsiers have highly developed night vision, which allows them to hunt insects and other prey in the dark.
Colossal squids
As the largest invertebrates on Earth, colossal squids (Mesonychoteuthis hamiltoni) have the largest eyes in the animal kingdom—even larger than those of whales. Their massive eyes measure about 27 centimeters (almost 11 inches) across, making them about the size of soccer balls. These may be the largest eyes that have ever existed on Earth.
Vision is crucial for these squids, who live in the Southern Ocean at depths of 1,000 meters (3,300 feet) or more. Their habitat is extremely dark, but they need to spot predators and catch prey. To help them see in the dark, their eyes contain light organs, known as photophores, which produce light through a chemical reaction using bacteria. The ability of an animal to produce light is known as bioluminescence.
Ostriches
Ostriches (Struthio camelus) have the largest eyes of any bird—in fact, their eyes are the largest of any type of land animal. Ostrich eyes measure about 5 centimeters (2 inches) across, about the size of a billiard ball. Each eye is larger than the ostrich’s brain. They can see as far as 3.5 kilometers (2.2 miles) away.
These African birds have long eyelashes to prevent sand and dust from entering their eyes, much like we do, though their eyes are five times the size of a human’s. Their eyes are also protected by a nictitating membrane, which moves like a windshield wiper, preventing dryness.
Vampire squids
Found at depths of 600 to 1,200 meters (about 2,000 to 4,000 feet), the vampire squid (Vampyroteuthis infernalis) looks like a pink, cloaked figure—or like an umbrella—flying through the sky as it swims through the deep ocean. With eyes 2.5 centimeters (1 inch) long, these peculiar-looking creatures have the largest eyes in proportion to body size of any living animal.
Despite its name, the vampire squid is not a squid and does not feed on other animals’ blood. It’s a mollusk in its own order, Vampyromorphida, and it does not chase prey, opting instead to float around and scavenge for zooplankton and marine snow—which is not crystallized water but is rather the mucus, excrement, and particles of dead animals. This makes the vampire squid a detritivore.
Their eyes might appear blue in photos and videos, but they’re actually clear. The vampire squid’s keen eyesight allows it to find large pieces of marine snow in the darkness of the deep ocean.
Tree frogs
Tree frogs, amphibians of the family Hylidae, have some of the largest eyes in comparison to their bodies of any animal. The red-eyed tree frog (Agalychnis callidryas) is one prominent example, known around the world for its bright green, blue, and yellow skin, orange feet, and striking red eyes with large vertical slits.
The eye size of frogs is theorized to be linked to their habitats—frogs living in caves tend to have small eyes, while the eyes of frogs living in forests tend to be larger. Tree frogs have the largest eyes of all frogs because they need to make quick visual judgments while jumping from tree to tree.
Like ostriches, red-eyed tree frogs have nictitating membranes. They have these in place of true eyelids, which means that even when their eyes are ‘closed’ as they sleep, light can still enter them, so they can still ‘see’ predators and awaken when they come too close.
Bigeye thresher sharks
Chameleons have some of the best vision among lizards. Their eyes are some of the largest in proportion to body size of all vertebrates. One example is the veiled chameleon (Chamaeleo calyptratus), whose eyes are about 18 millimeters (0.7 inches) in diameter, compared to its 150-millimeter (5.9-inch) long body.
Unlike other animals, chameleons can move their eyes independently of each other, in many different directions, without moving their heads. Because their eyes protrude out of their heads rather than being more inset in their skulls, they gain an even larger field of view. Their vision is vital for hunting and staying safe from predators.
Chameleons also have a type of third eye, located on the top of their heads. It’s covered by scales, so it’s not easily spotted, and it can only detect violet and blue light.
Slow lorises
Slow lorises are small, nocturnal primates found in Southeast Asia, and thanks to the cute appearance of their large, round eyes, they have unfortunately been targeted for the exotic pet trade. Though they are protected by law, they are taken from their homes in the wild and trafficked to places where they are sold as pets. As wild animals, they don’t make fitting pets—especially considering they possess a venom strong enough to seriously harm humans.
The pygmy slow loris (Xanthonycticebus pygmaeus, formerly known as Nycticebus pygmaeus) is known to have one of the largest eye sizes in relation to its body size of all primates. Its eyes have a diameter of 14.9 millimeters compared to its cranial width of 35.8 millimeters. The Sunda slow loris (Nycticebus coucang) also has an impressive eye diameter of 15.8 millimeters compared to its cranial width of 45.5 millimeters.
Behind their retinas, slow lorises have a tapetum lucidum, a layer of tissue that reflects light and makes their eyes appear to glow. They have vision even when their surroundings are almost completely dark, thanks to the tapetum lucidum and the fact that their pupils can enlarge extremely wide.
