Rhino Eyes: Does Size Matter?

A Sumatran rhino's eyes are small compared to the size of its head and body.

A Sumatran rhino’s eyes are small compared to the size of its head and body.

Rhinos are known for their keen sense of smell and hearing, but they apparently have poor vision.  The white and greater one-horned rhinos are the third and fourth largest land mammals behind the African and Asian elephant, and all four species have eyes that are much smaller than would be predicted based solely upon body size.

Like those of rhinos, the eyes of elephants are small given their body size

Like those of rhinos, the eyes of elephants are small given their body size

Much of what we believe about rhino eyesight is based on anecdotal reports by big game hunters. For example, one common statement about black rhinos is that they are remarkably nearsighted, and unable to tell the difference between a man and a tree even as close as 20 meters away. There doesn’t appear to be any empirical evidence to support this assertion, but it seems to explain the species’ sometimes aggressive behavior when approached by humans.  However, a recent anatomical study of the black rhino’s retina at an Australian university suggests that the human form should be detectable at nearly 200 meters – 10 times the distance previously suspected.

Black rhinos surprised in thick brush may have difficulty distinguishing the human form from the surrounding landscape.

Black rhinos surprised in thick brush may have difficulty distinguishing the human form from the surrounding landscape.

Overall, rhino don’t stack up very well against the rest of the Animal Kingdom when it comes to vision.  Frogs and toads have humongous eyes for their size, great stereoscopic vision, and are quite adept at snatching quick-flying insects from the air. Lizards and snakes also have large eyes for their size, good visual acuity, and see the world in color as well.  So do birds, which not only have the largest eyes relative to body size among vertebrates, but also seem able to detect ultraviolet and polarized light, which is important for migrating species. Pigeons, in fact, have been referred to as “two eyes with wings”.

Frogs and toads can have incredibly large eyes for their size.

Frogs and toads can have incredibly large eyes for their size.

Snakes and lizards have good visual acuity and see the world in color.

Snakes and lizards have good visual acuity and see the world in color.

The term "eagle eye" applies to this bird of prey's incredible vision.

The term “eagle eye” applies to this bird of prey’s incredible vision.

Among mammals, eyes tend to be large in small species and small in large species, but there are exceptions.  Small burrowing species such as moles and shrews have tiny eyes, and fast species such as cheetah and pronghorn have larger eyes than would be predicted for their body size.   With eyes located on opposite sides of the head, rhinos don’t have good binocular vision like humans and other primates do, and also lack color vision. However, recent anatomical studies of the black rhino’s retina indicate that this species’ vision may be as good as a rabbit’s, and is probably better than that of seals, dolphins, bats and rats.

Fleet predators such as the cheetah tend to have larger eyes than would be expected for their body size.

Fleet predators such as the cheetah tend to have larger eyes than would be expected for their body size.

One study suggests that rhino vision may be comparable to that of rabbits.

One study suggests that rhino vision may be comparable to that of rabbits.

The same study suggests that rhino vision is superior to that of marine mammals such as seals and dolphins.

The same study suggests that rhino vision is superior to that of marine mammals such as seals and dolphins.

So which animals have the largest eyes?  Based on body-size, it’s difficult to compete with the insects, many of which have enormous compound eyes.  The largest eye overall would be that of the giant squid, a 40-feet long, several hundred pound mollusk that can dive to depths approaching a half-mile and must withstand pressures almost 100 times greater than those experienced at the water’s surface. Its eye falls somewhere between the size of a soccer ball and a basketball.

The compound eyes of insects are off-the-chart on the eye-to-body size curve.

The compound eyes of insects are off-the-chart on the eye-to-body size curve.

The largest eye in the Animal Kingdom belongs to the giant squid, an almost mythical creature of the ocean depths.

The largest eye in the Animal Kingdom belongs to the giant squid, an almost mythical creature of the ocean depths.

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