The nighttime world is never closed for nocturnal animals. Unlike humans, many animals possess adaptations that allow them to see even when the night sky is very dark. Animals have developed amazing adaptations to their environments. Most nocturnal animals have large eyes relative to their body size. This adaptation, along with the ability to dilate their pupils far wider than humans can, maximizes the amount of light entering into the eye.
Night vision is the ability to see in a dark environment. Night vision is mainly due to two adaptive feautures.1. Sufficient spectral range2. Sufficient intensity range.
Human vision is confined to a small portion of the electromagnetic spectrum called visible spectrum. It extends between 450 and 750 nanometers wavelengths. Below 450 nm is the Ultraviolet spectrum and above 750 nm is the Infrared spectrum. Some animals can see using the infrared or ultraviolet spectrum than humans.
Sufficient intensity range is the ability to sense very small quantities of light. Although humans can detect single photons under ideal conditions, the neurological noise filters limit sensitivity to a few tens of photons. The night vision of animals is due to one or more differences in the morphology and anatomy of their eyes. These adaptations include large eyeball, large lens, large optical aperture, more rods in the retina, presence of a tapetum lucidum, etc.
Large eyes in Nocturnal Animals
Nocturnal animals have large eyes, wider pupil, large lens and increased retinal surface to collect more light. Some animal species have evolved tubular eyes as part of their evolution to collect more light. Many nocturnal animals cannot move their eyes but they have extraordinary rotational ability of the neck. For example. Owls can rotate their neck through 270°. This helps to increase the night vision.
Some animals have a spherical lens and widened cornea to compensate for reduced eye movement. This along with a large cornea increases the animal’s field of vision. So they can see better in night even without moving the head.
Physiology of Night vision
The vertebrate eyes have Photosensitive cells called Rods and Cones. Rods are elongated cells mainly confined in the periphery of the retina. These are meant for Dim vision in low light and for peripheral visions. Rods, are extremely light sensitive and their sensitivity is about 500 times greater than the sensitivity of cones. Only one pho is required to stimulate a rod into sending a signal to the brain. Nocturnal mammals have rods with unique properties that make enhanced night vision possible. The nuclear pattern of their rods changes shortly after birth to become inverted. Inverted rods have heterochromatin in the center of their nuclei and euchromatin and other transcription factors along the border. The outer nuclear layer in nocturnal mammals is thick due to the presence of millions of rods present to process the lower light intensities of a few photons. Light is passed to each nucleus individually.
Cones on the other had are pointed cells confined in the central part of the retina. These are meant for Central vision, Bright vision and Colour vision. Rods have photosensitive pigment called Rhodopsin and cones have Iodopsin.
Role of Rhodopsin
The molecules of Rhodopsin in the rods undergo a change in shape as light is absorbed by them. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. When exposed to light, it immediately bleaches, and it takes about 30 minutes to regenerate fully. Most of the adaptation occurs within the first five or ten minutes in the dark. Rhodopsin is less sensitive to the longer red wavelengths of light. So many people use red light to preserve night vision.
Role of Tapetum
Many animals have a tissue layer called the tapetum lucidum in the back of the eye that reflects light back through the retina. This increases the amount of light entering into the retina. This is found in many nocturnal animals and some deep sea animals. This causes the phenomenon of eye shine in these animals. Tapetum lucidum is absent in human eye.
Different types of visual sense exist in animals to adapt them with the environment. Here are some examples of how some animals see the outside world.
Dog and Cats - They have night vision and can see moving objects rather than the stationary ones. Their eyes are much more sensitive to movement. Dogs and cats are color blind and see only very pale shades of color but they have better peripheral and night vision .Tapetum lucidum is also present in them to reflect light into the retina.
Hawk - Hawk's vision is equivalent to 20/5. Normal vision for people is 20/20. Therefore Hawk can see an object from 20 feet that most people can see from 5 feet.
Snakes - They use their normal eyes during the daytime to see things. During the day time , a snake's vision dependents on the movement of prey. They ignore any prey that is completely motionless. At night snakes will use Pit Organs to sense infrared rays. Pit organs can pick up infrared heat signals from warm objects in their surroundings.
Falcon - Distant vision is high in Falcon. They can see a 10 cm. object from a distance of 1.5 km.
Bees - Bees can see light between wavelengths 300 nm & 650 nm and they can see polarized light.
Ants - Ants can see polarized light.
Octopus - Octopus is the largest mollusc and its retina contains 20 million photoreceptors. Their eye has a flicker fusion frequency of 70/sec in bright light.The pupil of the eye is rectangular in shape.
Fish - Some fish can see the infrared wavelengths. They have only rods in the retina. About 25 million rods/sq. mmof the retina.This high density of photoreceptors helps them to detect the dim biolumninescence in the ocean depths.
Fly - The eye of flies has a flicker fusion rate of 300/sec. Flicker fusion rate is the frequency with which the "flicker" of an image cannot be distinguished as an individual events of the electromagnetic spectrum.
Shark - Shark has no retinal cones, and therefore cannot detect colors. Shark's eyes are designed to pick up as much light as possible, in order to see in water . But their vision is not as acute.
Bats - Bats can detect warmth of an animal from about 16 cm away using its "nose-leaf".
Penguin - Penguin is well adapted to see under water.They have flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range
Not all animals see the world as humans do. For many animals, the world is seen in fuzzy shades of gray, or very "washed out" and pale colors. But some animals can see in total darkness, or even see colors beyond the visual spectrum, that humans have never seen. Still others can use binocular vision to spot prey from thousands of feet away.