Animal eyes vs Human eyes (Part-1)
Humans are living in a mysterious and fascinating environment surrounding with lots of animals, birds, and reptiles etc. Scientists are curious about their physical and visual characteristics and use it for modeling various scientific inventions. Indeed, scientists are able to unlock so many interesting facts. Compare to the human visual system,
animals visual system and visual behavior are quite enthralling and distinctive to render the visual demand for daily living. Animals are using their supersonic eyes to scan their visual field looking for prey and predators.
How the human eye works?
Human eye works as a window of the multilayered visual system, which process image in the brain. Human has binocular vision, extending a 170-degree horizontal field of view and 135-degree vertical field of view while facing forward at primary gaze.
- A light ray from objects all around reflected off and enters our eye.
- Light rays refracted by cornea then enter through pupil and bent by crystalline lens, focuses on the retina.
- The retina contains two photosensitive pigments (Cone: Responsible for day vision and color vision. Rod: responsible for night vision) converts the light into an electrical signal, which brain receives and process image in visual cortex. Cone cells contain three types of photopigments sensitive to Red, blue and green color.
- The optic nerve carries the signal from retina to brain.
- So, a human perceives objects, color, depth and movement of objects.
- A human eye can see visible spectrum ranges from 390(blue) to 700(red) nm.
How animal eyes are different from human eyes?
Animal eyes are designed in different shape, size, light sensitivity, and color sensitivity depending on their living environment and visual need.
- The shape of the eye and cornea differs in various animals. Eagle can change the shape of cornea and lens to focus clearly from far away.
- Complex eye: Some insects like bee have a compound eye which contains each individual eye named Omnitidia. Each omnitidia contains cornea, lens, and photoreceptors.Images from each omnitidia superimposed and give an overall image in pixel form.
- Human has binocular vision. Animals have a wider field of view as their eyes are situated infront or side of their head.
- Some birds, insects, bees, mantis shrimp having extra cones to see various types of color and shades than the human eye.
- Animal eyes can detect light spectrum ranges from UV light to Infra Red helps them to detect their target more efficiently. Bees use UV light to differentiate the nectar from a flower.
- Animals eye like a cat, cows, shark, crocodiles, zebra contains a reflective mirror-like membrane just behind the retina called Tapetum. It reflects the light back to retina helps to capture more light in dim illumination .some nocturnal animal we see yellow or green color in their eye at night.
- Pupil: Most attracting parts in animals eye is the various shape of the pupil and its various colors. Scientists are trying to find out the reason behind the different shape the of the pupil in animals and how it helps them in their vision. The scientist has found that there is a relationship between the orientation of pupil in animals and their ecological behavior, either they are predator or prey. The orientation of pupil varies like vertical, horizontal and diagonal. Cat, sheep and other animals can enlarge their pupil size by 350 folds from constricted position to dilatation at night.
Scientific observations revealed that animals (foxes, domestic cat ) with vertically oriented pupil are mostly nocturnal ambush predator which hide and attack the prey from closer distances. Vertically oriented pupil creates astigmatic depth of field which helps to see clearly the images of vertical contour at different distances, at the same time blurred the images of horizontal contour. This unique mechanism helps them to judge the distance of prey accurately without moving their head much.
Herbivorous grazing animals (domestic sheep, goats, horse, white-tailed deer) need to scan clearly all around for food and need to run away rapidly in case of attack from other threats, having the horizontally oriented pupil. Their eyes are positioned sides of their head enabling wider visual field. Horizontal pupil receives more light from horizontal space while reduces light from vertical space. This produces the clear image of an object the in a horizontal plane with a panoramic vision all around the visual field. Close observations revealed that animals eye(goat, horse, sheep) having laterally situated eye with horizontally oriented pupil showed cyclovergence (rotatory movement of the eye) to keep align the eye with earth.