Light, a form of electromagnetic radiation, encompasses a vast spectrum, only a small portion of which is visible to human eyes. This visible light typically ranges from approximately 400 nanometers (violet) to 700 nanometers (red) in wavelength. Beyond the violet end of this spectrum lies ultraviolet (UV) light, characterized by shorter wavelengths, generally between 10 and 400 nanometers. Many organisms can see UV light, a range invisible to humans. This unique visual capacity offers them a perception of their environment hidden from human observers.
How Animals Perceive UV Light
Animals perceive UV light through specialized visual adaptations. Many species have UV-sensitive cone cells. These cones contain opsin proteins tuned to absorb UV light, often peaking around 360 nanometers. Optical structures also play a significant role. Unlike humans, many animals with UV vision have lenses and ocular media that do not filter UV radiation, allowing it to reach the retina. This transparency allows UV light to be detected by specialized cones, contributing to their color perception.
Animals with UV Vision
UV vision is widespread across the animal kingdom, offering diverse species unique ways to interact with their surroundings. Insects are prominent examples; bees and butterflies rely heavily on UV perception. Bees navigate flowers using UV patterns as nectar guides, while butterflies use UV markings on wings for species recognition and mate selection. Spiders, particularly jumping spiders, also use UV vision for courtship displays and prey capture.
Many bird species have tetrachromatic vision, with one of their four cone types sensitive to UV light. This allows them to see UV patterns in other birds’ plumage, often invisible to humans. Similarly, various fish species, including goldfish, salmon, and anemonefish, use UV sensitivity for foraging and social signaling.
Reptiles like snakes and certain lizards also perceive UV light, aiding low-light vision and prey detection. Once thought rare in mammals, many species, including mice, rats, gerbils, reindeer, dogs, cats, pigs, and ferrets, can detect UV light. Reindeer, for example, can discern UV-absorbing mosses and lichens against UV-reflective snow.
The Purpose of UV Vision
UV vision provides animals with distinct advantages in various ecological contexts, influencing foraging, mating, and survival behaviors.
Foraging
Foraging animals often use UV light to locate food sources camouflaged or appearing different in visible light. For instance, flowers display UV patterns, known as nectar guides, directing pollinators like bees to nectar.
Similarly, birds of prey, such as kestrels, detect UV-absorbing urine trails of voles, helping them locate prey.
Communication and Mating
In communication and mating, UV patterns play a significant role in species recognition and mate choice. Many birds exhibit UV-reflective plumage used in courtship rituals, allowing individuals to assess potential mates based on these unseen signals.
Fish and certain reptiles also use UV patterns on scales or skin for social communication and identifying conspecifics.
Navigation and Predator/Prey Detection
UV vision also assists in navigation and predator/prey detection. Some insects use polarized UV light patterns in the sky for orientation. For predator avoidance, rodents can more easily spot bird silhouettes against the UV-rich sky.
Conversely, some predators use UV vision to identify camouflaged prey or detect biofluorescent signals, invisible in visible light.
Human Perception of UV Light
Humans cannot see UV light due to specific characteristics of the human eye. The human eye’s crystalline lens acts as a natural filter, absorbing most UV radiation between 300 and 400 nanometers before it reaches the retina. This filtering mechanism helps protect the retina’s delicate photoreceptor cells from potential damage by high-energy UV photons.
Additionally, human retinas primarily contain three types of cone cells sensitive to red, green, and blue wavelengths, lacking a dedicated UV-sensitive cone. While blue-sensitive cones show some sensitivity to near-UV light, the lens blocks these wavelengths. Individuals who have had their natural lens removed, such as during cataract surgery, and replaced with an intraocular lens that does not block UV, have reported perceiving UV light as a whitish-blue or desaturated blue-violet. However, modern intraocular lenses are designed to block UV light to prevent retinal damage.