Many animals use ultraviolet and infrared light to orient themselves or detect predators and prey

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Indeed, sometimes life is ultraviolet in color. Or infrared. And, other times, what we see is not what it seems. But let's move on. The colors that we humans see make up what is known as the visible spectrum—the portion of electromagnetic waves that we can see. Now, what colors are there beyond and how do other animals perceive them? We can start with ultraviolet light, for example, which occupies the spectral range of wavelengths slightly shorter than those visible to humans.


Have you ever stopped to think that the beauty and colors of flowers are not made to please your eyes? Its purpose is, quite simply, to attract pollinators (bees, butterflies, wasps, flies, etc.). Therefore, flowers satisfy the sense of sight of these insects. Many flowers accumulate ultraviolet pigments in their petals, which form patterns invisible to our eyes, but which most pollinators can see. Pigments that reflect ultraviolet rays are found mainly at the tips of the petals, and those that absorb them, at the base. This arrangement creates a "bull's-eye" pattern that serves as a runway to direct pollinators to the center of the flower, where the nectar and pollen are.


Multiplicity of functions


The functions of ultraviolet light are multiple, from guiding navigation and orientation behavior to detecting food and potential predators and facilitating such important tasks as mate selection and communication between individuals of the same species. The eyes of many birds, fish and reptiles have cells sensitive to ultraviolet rays. There are even mammals, such as some rodents and bats, that can see ultraviolet light quite well. Snakes that hunt during the day have lenses that block wavelengths of ultraviolet rays, but snakes that hunt at night have lenses that transmit them. Ultraviolet pigments are based on so-called opsins, the same family of proteins used to detect radiation in conventional light receptors. This means that the origin of ultraviolet vision is as old as that of visible light.


On the other side of the visible spectrum is infrared light. It is a type of radiant energy invisible to the human eye, but which we can feel as heat. What kinds of animals can see heat? Especially predators or parasites that need quick detection of their prey or hosts, such as mosquitoes, bats, snakes, bedbugs or beetles.


Our current understanding of vision outside the visible spectrum is almost entirely restricted to two groups of animals: arthropods such as arachnids, insects or crustaceans, and vertebrates such as mammals, birds or reptiles. What surprises does the vision of the large number of animal groups that are still unknown offer us? Surely more than one.


Eyes that are not eyes


In the wings of butterflies, in the shell of some molluscs, in the skin of some reptiles, fish and cats and in the plumage of certain birds there are concentric circles that look like eyes but are not. Why are eye-like patterns seen in such diverse groups of animals? The adaptive function of this common and constant pattern in nature has captured the interest of evolutionary biologists for centuries. One of the hypotheses to explain this holds that they serve to intimidate predators, who confuse them with the eyes of another animal, possibly their own predator, and therefore hesitate to attack. They have also been suggested to have a diversionary function: some animals have them in areas away from vital organs (e.g.


These patterns, called eyespots , can also function as detection signals. The false eyes would indicate to the predator that it has been seen, which it would interpret as attacking would require too much energy and let the hunt run. In some cases, eye spots also act as sex signals. Perhaps the most common example is that of the peacock: its success in mating is based on the ability to display its tail covered with eye-like circles.


And let's not forget that we are animals too. Although there is no direct evidence that eyespots affect humans, it has been suggested that the feeling of being watched also influences human behavior: for example, by reducing theft in areas with high incidences of crime. It's fascinating how a seemingly simple pattern like eyespots can create such an amazing variety of behaviors in various animal species.


Rosa Fernández is a researcher at the Institute of Evolutionary Biology (IBE), a joint center of the CSIC and the UPF.