The human is a diurnal species of animal – the human-animal. “Diurnal” means active during the daytime. I am referring to the domestic cat which does not comfortably sit within the diurnal or nocturnal category of animals. They can be active at night and during the day but are more likely to be most active at dawn and dusk and are therefore better described as ‘crepuscular’. Cat lovers with be very familiar with this.
The 10 reasons are listed below the pictures.



- And they are most active at these times because they expect to maximise their chance of catching prey animals. The cat’s anatomy is adapted to be a top predator. And because they are crepuscular, they need to see well in the dark. And therefore, they’ve relied upon evolution to provide many adaptations.
- The domestic cat’s eye is much larger in proportion to body size than that of the human. They have a large cornea and lens. The pupils have the ability to open very widely (see photo below). This anatomy allows for a large proportion of the retina to be illuminated.
- When the domestic cat’s pupil is fully open in lowlight conditions, it illuminates the retina of the eye to the same extent that occurs in bats and badgers. Both these are nocturnal animals.
- The human’s ability to illuminate the retina at a maximum is five times less effective than that of the domestic cat.
- There is a shorter distance between the pupil and the retina of the cat compared to the human which allows for the efficient illumination of the receptor cells in the retina and the minimal scattering of light.
- And behind the retina is a complex light-reflecting layer called the tapetum lucidum. This is Latin to mean “bright tapestry, coverlet”. Dr. Desmond Morris says that the literal meaning is “bright carpet”. The cells of this special layer reflect light back toward the visual receptors in the retina. This allows the retina to have a second opportunity for light absorption. It is this reflective layer which produces the green “eyes shine” that we see in domestic cats when they are peering out of the dark towards a light which has reflected on this layer and out of the eye towards the source of light (see photo above).
- The cat’s retina contains a high proportion of rods. These are the most sensitive visual receptors. In contrast, the cat’s retina contains far fewer cones in relation to rods. Cones are less sensitive to dim light but responsive to bright light and colours.
- As a result, the cat can see in very dim conditions but with reduced ability to see in fine detail and/or see colours (see my assessment as to how they see above. This is open to discussion).
- It is believed that when taken together these retinal adaptations enable domestic cats to utilise 50% more (at least) available light than humans.
- Cats have effective vision in one-sixth of the illumination under which humans can see.
Postscript: because a cat’s eyes are so sensitive, they have vertical slits for pupils in order to reduce the amount of light to a maximum that impinges on the retina during daylight hours. This helps to protect the retina. It partly explains why domestic cats have slits for pupils rather than a round aperture. A second reason, by the way, has been suggested: it allows cats to better measure distance. Tigers have round aperture pupils.

