Structural Differences in the Eyes of Nocturnal Animals

Submitted by laurenplozano on Tue, 12/12/2017 - 22:39

Amniotes are a class of animals that have four limbs and lay their eggs on land, such as reptiles, mammals, and birds. Some of these species are nocturnal, active at night, and some are diurnal, and are active during the day, like humans. Because of the difference in lighting, nocturnal and diurnal terrestrial amniotic species with camera eyes have evolved distinctly different eye morphology, or anatomy, in order to maximize their activity in their time of the diel (24 hours) cycle.


The two photoreceptors in the retina are responsible for beginning the process that allows animals to see. Rod photoreceptors are activated in response to differences in brightness and thus are most sensitive in dim-settings and contribute to visual sensitivity or the ability to distinguish an object from other objects or its surroundings. Cone photoreceptors on the other hand, are most sensitive in light conditions and contribute information about color and visual acuity, or clarity. The photopigment found in rods, called rhodopsin, absorbs particles of light and is replaced at a rate rapid enough for the rods to remain active and thus maintain vision in dim settings in nocturnal species. In diurnal animals however, rhodopsin becomes inactive after absorbing a photon, and is not replaced fast enough to maintain adequate vision. 


In order for nocturnal animals to maximize their visual abilities in the dark, their eyes have evolved to increase visual sensitivity by changing their morphology. In doing so however, there is a tradeoff and visual acuity is lost.


Tarsiers are nocturnal

Tarsiers are nocturnal


Compared to diurnal species, nocturnal species have larger eyes overall. The purpose of having a large pupil is to increase the brightness of the image on the retina as more light is able to enter the eye. Since more light is in the eye, this maximizes the input to the ganglion cells. The ganglion cells  convert the electrical signal produced by the retina’s photoreceptors when they absorb a particle of light into a signal understood by the brain’s cells. The brain is then able to make sense of an image and allows us to "see". Because more light is able to enter their eyes in the dark via their large pupils, more rods are activated and the animals increase their visual sensitivity. By increasing their visual sensitivity, they use object's edges and border rather than differences in color to distinguish objects from their background.


Elephants are diurnal.

Elephants are diurnal


Nocturnal animals also have slit pupils which allow less light in during the day time when bright light could damage their retinas. Their lenses are multifocal and have different areas in a concentric pattern in which each “ring” refracts, or bends, different light wavelengths best. Multifocal lenses and split pupils work hand in hand as they allow each area of the lens with different refractive power to be exposed to light and therefore prevent the animal from suffering from blurry vision, decreased acuity, which would occur if their lenses were circular and constricted during the day time, preventing light from hitting the needed “ring” of the lens further from the center.


slit pupil of a cat


Another morphological difference is that the diurnal species’ lenses are usually smaller and thinner as the ability of the lens to refract, or bend, light is related to visual acuity. Nocturnal species have larger and thicker lenses as well as a short focal length, which is the distance from the middle of the lens to the retina. The larger the focal length, the larger the area of photoreceptors that are activated by photons. In a cumulative fashion, nocturnal species large pupil diameter and short focal length allow greater light collection in exchange for less visual acuity but a brighter yet smaller image.  A downside of having large eyes is that they must remain fixed in the bony orbit, however evolution has allowed nocturnal animals with fixed eyeballs to develop the ability to turn their heads at much greater angles.


lens anatomy


The final differentiating feature of nocturnal animals’ eyes is the tapetum lucidium, which lies just behind the retina and acts as a mirror to bound unabsorbed photons back onto the retina so that they might get absorbed. You have probably seen the effects of this biological apparatus when you see the “glow” of your pet dog or cat’s eyes in the dark if you shine a light on them. This is due to the tapetum lucidium reflecting the unabsorbed light back out of the pupil.


tapetum lucidium


There’s much truth to the saying “The eyes are the window to the soul”, as much can be determined about the lifestyle of an animal based upon their optical morphology. If a species has big, glowing eyes, odds are its nocturnal. If its eyes and pupils are smaller in size then its probably a diurnal species.………………