How do Animals See in the Dark?
Mother Nature has intrigued us in various ways by bestowing every creature with the necessary adaptations for their survival. Some animals have been gifted with the ability to see in the dark. This unique ability comes in handy when they have to hunt in the dark or successfully hide from those who want to hunt them.
What animals can see in the dark?
Contrary to the popular belief that all nocturnal animals can see well in the dark, many of them actually have poor eyesight. Animals can see in the dark to a certain degree depending on two aspects: the spectral range of light they can see and the structure of their eyes. The thing to remember here is that there is always some light available to use unless there is pitch darkness like that in a cave.
1. Spectral Range
The electromagnetic spectrum is a map of all the types of light that we can see and identify. The electromagnetic spectrum separates all the types of light by their wavelength that depends on how energetic a particular wave is. Waves that are more energetic have shorter wavelengths while waves that are less energetic have longer wavelengths. Human vision is restricted to a small portion of the electromagnetic spectrum; there are many kinds of electromagnetic waves that we cannot see. Nocturnal animals, on the other hand, have access to the wider sections of the spectrum and can see the infrared or ultraviolet spectrum as well.
2. Structure of the Eyes
The eyes of nocturnal animals are designed in a way that gives them the ability to sense very small quantities of light. Some of the special features include large eyeball, large lens, wider and much sensitive pupil, more rods in the retina, presence of additional tissue layers, etc. Many nocturnal animals do not have the ability to move their eyes but they have extraordinary rotational ability of the neck. For instance, owls can rotate their neck to almost 270° that augments their night vision. Those who cannot move their eyes or neck much have a spherical lens and large cornea to make up for reduced eye movement. So they can see better in night even without moving the head.
Rods and Cones in Nocturnal Animals
The eyes of nocturnal animals are made up of basically two types of photosensitive cells known as rods and cones. Rods are elongated cells mostly present in the peripheral region of the retina and act as the light receptors, i.e. they receive the available light and send it to the brain where it gets processed. A nocturnal animal’s eyes have far greater number of rods than a human eye, and can therefore receive more of the light available which aides the animals to see better in low light situations. Rods are extremely sensitive to light—their sensitivity being around 500 times greater than that of cones. Merely one photon of light is enough for a rod to be stimulated to send a signal to the brain. Rods have a photosensitive pigment called rhodopsin. Rhodopsin is extremely sensitive to light and plays a vital role in the night vision.
Cones are pointed cells present in the central part of the retina that help the animals to differentiate between colours. Many nocturnal animals have relatively few cones, and are generally colour-blind. This is a sacrifice they have to make for the boon of excellent night vision.
Why animal eyes glow in the dark?
Nocturnal animals have an additional tissue layer at the back of their eyes known as ‘tapetum lucidum’ which reflects light back through the retina. This increases the amount of light entering into the retina and since they have more rods and cones, they can use the available light better than anyone else to see. So, what might be too dark for humans could be just dim light setting for these animals. There is no tapetum lucidum in the human eye. Have you ever observed how the eyes of cats and dogs shine when your car’s headlight falls on them? It makes them look really scary! Well, it is the tapetum lucidum that makes the eyes of dogs and cats shine in vehicle head light. When there isn’t a lot of light in a room, tapetum lucidum works like a mirror and reflects any light available back towards the front of the nocturnal animal’s eyes. It’s almost like these animals get to use the light twice!
Fast Facts about Nocturnal Animals
- During the day time, a snake’s vision depends on the movement of prey. At night, snakes sense infrared rays using pit organs. Pit organs can detect infrared heat signals from the warm objects in their surroundings.
- Some fish can also see infrared wavelengths. They have a large number of rods in the retina which help them to detect the dim bioluminescence of their prey in the ocean depths.
So, perhaps these animals do not see the world as humans do with all the colours and their hues, but they certainly beat us hands down when it comes to finding way in the dark!
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