Animal vision

Colour vision depends on cone photoreceptors, and different animals carry different numbers and types of cones. People are usually trichromatic, with three cone types. Dogs are generally described as dichromatic, with two cone types sensitive to shorter (blue) and medium (yellow-green) wavelengths, so they are thought to distinguish fewer colours than people and to have difficulty separating reds from greens. Cat colour vision is also limited, though the exact details remain debated in the research literature.

Having fewer cone types does not mean an animal sees a poor or broken version of human vision. It means the visual system is tuned differently, often in ways suited to the animal's habits and habitat. Comparisons such as 'better' or 'worse' than human vision are usually the wrong framing unless a specific, measurable property is being described.

Many diurnal birds are tetrachromatic, carrying four cone types including one sensitive to violet or ultraviolet wavelengths, and their eye media transmit ultraviolet (UV) light that the human eye largely blocks. This means some birds can perceive differences in plumage and markings that are invisible to people. Studies link UV sensitivity in certain species to behaviours such as foraging and mate choice, though specifics differ from species to species.

Many bees are trichromatic with sensitivity shifted toward the ultraviolet, blue, and green range, and tend to respond weakly to red. Some flowers carry ultraviolet 'nectar guide' patterns that are invisible to people but that bees can perceive. Because humans cannot see UV directly, any image meant to show 'what a bee sees' is a reconstruction or interpretation, not a literal view through the animal's eyes.

Vision in dim conditions relies heavily on rod photoreceptors, which are more light-sensitive than cones but do not convey colour. Animals active at night or twilight often have rod-rich retinas. Cats, for example, have a high proportion of rods, which is associated with a lower light threshold for vision than people have.

Many nocturnal and crepuscular animals also have a reflective layer behind the retina called the tapetum lucidum. It reflects light that passed through the retina without being absorbed back across the photoreceptors, giving them a second chance to detect it; this is also why such animals' eyes can appear to glow when caught in light at night. These low-light adaptations typically come with trade-offs, such as reduced fine detail or colour discrimination in bright conditions.

Some snakes, including pit vipers and many boas and pythons, have specialised pit organs that detect infrared radiation, essentially the heat given off by warm objects. This ability appears to have arisen independently in different snake groups. It is important to be clear that these pit organs are a separate sensory system from the eyes: they respond to radiant heat rather than to visible light, and the information is processed differently in the brain.

Describing this as snakes 'seeing heat' is a loose analogy. The pit organ does not form a sharp image the way an eye does; research indicates it provides relatively coarse spatial information about thermal contrast. It is included here only to draw a clear line between true vision and heat sensing, which are sometimes confused.

Where the eyes sit on the head reflects a common trade-off. Forward-facing eyes, often associated with predators such as cats, produce a large region where the two eyes' views overlap (binocular vision), which supports depth perception. Side-facing eyes, common in many prey animals, give a wide, panoramic field of view that can approach a full circle in some species, helping them detect movement from many directions.

This pattern is a useful generalisation, not a strict rule, and there are exceptions across the animal kingdom. The broad point is that an eye arrangement good for judging distance to a target and one good for wide all-round surveillance pull in different directions, so each species' visual layout reflects a balance between competing demands.

Mantis shrimp are often cited because their eyes contain an unusually large number of photoreceptor types, reported to be around a dozen, spanning ultraviolet to far-red wavelengths. It is tempting to assume this means they experience the richest colour vision of any animal, but behavioural research suggests otherwise.

Experiments testing how well mantis shrimp distinguish between similar wavelengths found relatively coarse colour discrimination compared with what their many receptors might imply. Researchers have proposed that their visual system may identify colours quickly in a simpler way rather than finely comparing them as the human system does. The mantis shrimp is a clear reminder that more photoreceptors do not automatically mean 'better' or more detailed colour vision, and that animal senses resist simple rankings.