BioGalleries Eye To Eye
How do animals see in the dark?
Article by Molly Kirk and David Denning
Photographs by David Denning
Barn Owl
Barn Owl

After the emergence of vertebrates from the water some 300 million years ago, the evolving landscape became more and more crowded with large herbivores and predators. However, a rich ecological niche remained available to competitors with the right adaptations — the abilities to find food and avoid predators at night. Although mammals first evolved over 200 million years ago, the majority of them remained primarily nocturnal until the demise of their major predators, the dinosaurs about 65 million years ago. After that, new ecological niches became available in the light of day. Then the modern complement of both diurnal and nocturnal mammals evolved.

In order to survive at night, these animals had to find food in the dark. Some developed a highly-advanced senses of smell or specialized hearing abilities such as echolocation. Others acquired eye adaptations for improved night vision

Big Eyes

The most notable feature of nocturnal animals is the size of their eyes. Large eyes, with a wider pupil, larger lens and increased retinal surface can collect more ambient light. For example, an owl's eyes fill over one half its skull. Likewise, some species have evolved tubular eyes as a means of increasing their size. By expanding the eyes in this way, they are very tightly fit in the socket. As such, many nocturnal animals cannot move their eyes within the orbit. Instead, they have evolved extraordinary rotational ability in the neck. Owls, for example, can rotate their neck through 270°.

Image

Some animals of the night have acquired a spherical lens and widened cornea to compensate for reduced eye movement. A spherical lens projects an equally clear image regardless of the light's incoming direction. This combined with a wide cornea effectively increases the animals field of view allowing the head and eyes to remain motionless.

Packed With Rods

The retina of nocturnal animals is almost entirely composed of rods. The other type of vision cells, cones, is absent or almost absent, leaving nocturnal animals with virtually no color vision. The photosensitive pigment inside the rods, rhodopsin, is particularly sensitive to low levels of light. During the day, in a daylight adapted eye, the rhodopsin breaks down so rapidly, it is ineffective for visual perception. At night-time, in the rod-rich eyes of dark-adapted animals, rhodopsin is created faster than it breaks down. Therefore, the threshold of light needed to stimulate the eye is reduced. It is just a minute fraction of the light needed to activate a cone cell for vision during the day.

However, despite being more sensitive to light, the low number of cones means nocturnal animals have sacrificed visual acuity. They must get by with somewhat fuzzy, unfocused images. Only by greatly exaggerating the size of their eyes (and therefore the retinal image), can dark-adapted animals develop reasonable resolution to their images.

Mirrors Add Intensity

On a dark night, flash a bright light at your dog or cat's eyes. What do you see? As if by some mysterious magic, their eyes glow in the dark. But, it is not magic at all. It is the tapetum lucidum (meaning "bright carpet"), an adaptation for night vision. The tapetum is a thick reflective membrane, 15 cells wide, directly beneath the retina. It collects and re-emits light back to the retina a second time, giving the rods a second chance to absorb the image information, thus maximizing the little light available to them. As this light is reflected off the tapetum, the animal's eyes appear to glow.

So, although nocturnal animals see mostly crude shapes, outlines and no color, by maximizing their sensitivity to low light levels with the above adaptations, it is enough for them to hunt, feed and survive in the dark of night.

Back In The Daylight

But what happens to these night-time specialists during the day? Most nocturnal animals are largely inactive during the day to avoid over-stimulating their highly sensitive eyes. Nocturnal animals have specialized pupils to shut out damaging bright light.

Pupils are usually circular; it's the most effective shape for allowing light into the eye. Indeed, nocturnal animals dilate their pupils to their circular maximum at night.

But by day, the circular pupil is inefficient at blocking light. Instead, a variety of pupil shapes have evolved that limit incoming light, the most advanced being the vertical slit. The slit pupil can shut out all light except a narrow band. Its vertical orientation is of significance too, as it works well with eyelids. As an animal squints, partially closing its lids at right angles to the vertical slit pupil, it further reduces the amount of light entering its eye. Perhaps the most noteworthy of all pupils is the stenopeic pupil of the gecko, a vertical slit lined with notches on each margin. When the pupil is entirely closed, tiny pinholes allow light to pass through to the retina creating sharp overlapping images.

 
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