How Animals Use UV Light
Recently, scientific researchers have made the discovery that a variety of animals possess UV vision. This ability is due to the presence of an extra cone cell in the eye that is sensitive to wavelengths of light in the near UV spectrum (300-400nm). Often times, these organisms with the UV sensing cone also have all three of the “visible” light sensing cones and therefore can perceive a whole new dimension of colors. Scientific experiments and photography show that these animals make use of their UV vision and superior color perception when hunting food and mating thus explaining the fact that evolution has preserved these traits over millions of years.
We rely heavily on all our senses in everyday life. Of particular importance is the sense of sight. Every day, we humans depend on our eyes to perceive the light around us in order to gather information about the presence, size, and color of objects. As the most highly evolved organisms on our plant, it is understandable that we consider our vision to be among the best of all these organisms. In our own superiority, we have even classified the light on the electromagnetic spectrum that we can see as “visible” light and implied
that all the other light is “invisible” However, many other animals including birds and
certain insects, have both the ability to detect near UV light and the ability to differentiate far more colors that we can. Many birds and insects have this ability due to the presence
of a fourth cone in their eyes, specifically sensitive to the UV wavelengths. They have acquired this evolutionary adaptation due to its importance in mating rituals and food gathering.
II. Origination of UV light
The sun is the earth’s primary source of light
Before the sense of sight can activate, there must be a light source present. In this case, the sun serves as Earth’s primary provider of light. The sun generates light through the process of incandescence, which is the emission of a broad spectrum of electromagnetic radiation through heat. This spectrum of radiation depends on the temperature of the source. As shown in the curve below, higher temperatures lead to a greater percentage of higher energy photons (such as those in the visible and UV spectrum) being emitted.
Plank’s Curve for Black Body radiation
Most objects are not hot enough to emit visible light and can only emit Infrared radiation (which is felt as heat). However, the sun’s temperatures are high enough to generate very
high energy photons (including harmful gamma and X-rays) as well as visible and UV light. Fortunately, the atmosphere shields us from the worst of the radiation, so the strongest forms of radiation to reach the earth in significant quantities are visible light and near UV light. These relatively strong forms of radiation are ultimately what animal eyes utilize for sight.
III. The Sense of Sight
A cross section of the retina showing rod cells, cone cells, and the intermediate molecules that help transmit signals to the brain and are responsible for our sight.
Despite the fact that our eyes may not be the best in the animal kingdom, our sense of sight is still a complex and important adaptation that requires intricate coordination between our eyes and our brain. The eye itself is the organ that traps energy from light and conveys the information it receives to our brain. Photons of light enter our eyes through the pupil and get focused by the lens onto the retina, which contains all the light sensing cells, in the back of the eye. These light sensing cells are broken up into two categories, cone and rod cells. Both cells are similar in structures, containing “photosensitive pigments [which] are composed of a form of Vitamin A, the
chromophore, and a membrane protein called opsin,” (“An Introduction to the Biology of
Vision”, McIlwain, 1996, 67). The following picture illustrates the photopigment
complex, and its constituent molecules, in more detail.
This complex plays a very crucial role in the transfer of energy from light into a signal that our mind receives and interprets as sight. “When the 11-cis isomer of retinaldehyde
(11-cis retinal) absorbs a single photon, it is isomerized to the all-trans form…[This
change] converts the opsin to an activated state,” (McIlwain, 1996, 67). Obviously, the all-trans form is at a higher energy level than the 11-cis form. However, this difference in energy is a specific amount. In order to change the molecular structure of 11-cis in such a way, a very specific quanta of energy is required. This quanta of energy is conveniently provided by a photon of light. The energy contained in the photon is
inversely proportional to its wavelength. As a result, the photon must have a specific wavelength of light and this wavelength happens to fall in the visible spectrum.
Once the initial activation of opsin occurs, the photoreceptor must then activate as well and convey its message to the brain. This process occurs when the activated photopigment “converts the protein transducin (T) to its active form (T*), [which then] activates phosphodoiesterase (PDE*), [which converts] cGMP to GMP.” The decrease of “cGMP concentrations closes the cation channels…hyperpolarizing the photoreceptor
and slowing transmitter release,” (McIlwain, 1996, 68). This complex process of one molecule activating other ultimately “leads to the activation of the retinal neurons, one set of which fires impulses in the optic nerve, conveying information to the brain,” (“What
Birds See”, Scientific American, July, 2006, Goldsmith, 71). Once the brain receives such information, it can begin to put together a picture of the surrounding world.
However, not all photoreceptors perform the same roles. Due to some structural and molecular differences, certain photoreceptors will absorb different wavelengths and intensities of light and convey different messages to the brain. The two major classes of photoreceptors are rod and cone cells, aptly names for the difference in structure. Rod cells focus on absorbing light at lower intensities and medium wavelengths. These cells are responsible for our night vision. On the other hand, cone cells absorb varying wavelengths of light across the visible spectrum. These cone cells are differentiated by their “opsin-Vitamin A photopigments [each of which has a] characteristic absorption profile,” (Nature, Neurobiology: Bright Blue Times, Foster). In other words, certain cones are sensitive to specific wavelengths of light (be it long, medium, or short wavelengths). These cones are responsible for our color vision.
Even though the cone cells are amazing, they have their limitations in conveying information and our mind must compensate for the lack of information in order to generate a more detailed picture of our surroundings. A photoreceptor has only two states; it can either be activated or inactive. The cell itself is unable to tell the brain what wavelength photon is absorbed. However, our eyes have three different types of cones, with peak absorptions at 424nm, 530nm, and 560nm (Goldsmith, July, 2006, 70). When hundreds of thousands of these cone cells activate, “our brain must compare the responses of two or more classes of cones containing different visual pigments…in order
to differentiate color,” (Goldsmith, July, 2006, 71). Therefore, it is the specialization of the cones that allows the brain to create a sense of color. Furthermore, “the presence of more than two types of cones in the retina allows an even greater capacity to see different colors,” (Goldsmith, July, 2006, 71). Compared with many mammals, which have only 2 cones, we have a much greater ability of differentiating color with our 3 cones. IV. Advantages of UV Vision
However, many animals are a whole level beyond us in terms of color
differentiation due to the presence of a UV sensitive cone in their eyes. In the case of birds, they have 4 cones with peak absorptions of 370nm, 445nm, 508nm, 565nm (Goldsmith, July, 2006, 71). Like humans, birds have cones focused towards high, medium, and low wavelength visible light. However, they also have an extra cone which picks up near UV light (at 370nm). This extra cone has major implications because these birds can detect light that is invisible to our eyes (UV light) and perceive a whole new dimension of colors which we cannot even imagine.
On top of these advantages, birds and reptiles also posses other substances in their eyes that allow them to differentiate color even better. “Each cone…contains a colored
oil droplet…which contain[s] high concentrations of molecules called carotenoids…The oil droplets function as filters, removing short wavelengths and narrowing the absorption spectra of visual pigments,” (Goldsmith, July, 2006, 72). As a result, each specific cone
absorbs a narrower range of photons and this fact ultimately allows for greater differentiation of color.
These 4 cone animals, tetrachromats, are blessed with such sensory superiority because their ancestors evolved very differently than our ancestors, the mammals. Initially most of the highly evolved animals had 4 cones in their eyes. However, most mammals now only have 2 cones. Such a deficiency can be explained through evolution. Back “in the Mesozoic (245 million to 65 million years ago), mammals were small,
secretive and nocturnal,” (Goldsmith, July, 2006, 72). This was in stark contrast to other animals, such as dinosaurs, which dominated the planet and needed to make full use of their color vision. Mammal eyes “evolved to take advantage of the night” thus making them “increasingly dependent on the high sensitivity of rods and less dependent on color vision,” (Goldsmith, July, 2006, 72). Without such selective pressures, mammals “lost two of the four cone pigments…that [still] persist in most reptiles and birds,” (Goldsmith, July, 2006, 72). Though humans and other primates were able to reclaim one cone through mutations and evolutionary adaptation, they still cannot match the 4 cones that other organisms possess. In conclusion, we have inferior color vision because our ancestors did not need to rely on such an ability to survive, and tetrachromats have
superior vision because they are evolved to take advantage of their fourth “UV sensing” cone.
V. UV Light in Mating Patterns
Spider eyes are able to detect UV Vision
Since evolution preserved the presence of 4 cones and/or UV sensitivity in many animals, these traits must therefore be critical for the survival of these animals. One of the most important qualities for the survival of a species is the ability of the animals within that specie group to be able to reproduce successfully. With many animals including the jumping spider “Cosmophasis umbratica”, UV coloration is critical in mating rituals. In a particular experiment, male and female jumping spiders were tested in environments with and without UV light to see if courtship rituals occurred. Under lights that included UV wavelengths, “males and females began courtship rituals,” which is to be expected since that is the control of the experiment (“UltraViolet Light Is Key to Spider Mating”, January 25, 2007, Schmid, The Associated Press). However, without the
UV lights, “females turned away and males…reduced their actions,” (January 25, 2007, Schmid). To make this more understandable in human terms, this experiment would be similar to putting humans in a room where no red light was emitted. Obviously, without the input of red light, the humans would be unable to perceive the “color” associated with objects that reflect red. Likewise, the spiders could not perceive the “UV colored” body parts of their mates. The important conclusion made in this experiment was that spiders can sense UV light and that sensitivity was critical in mating rituals. VI. UV Light for Food Gathering
Likewise, the ability of an animal to consume food is crucial for its survival, and therefore its ability to pass on its genetic material to the next generation. However, in order for an organism to consume its food, it must first be able to find the food source. This point is where the UV sensitivity and superior color perception of many animals comes into play. After numerous studies, bees were discovered to “have a visual system based upon three-colors,” (“How Bees See Their World”). In other words, bees are
trichromats just like us and perceive roughly the same amount of colors as we do. However, these bees can see UV light thought they lack the ability to see long wavelength red light. (“How Bees See Their World”) Therefore, bees see different colors
than we do and they make extensive use of the UV detecting abilities in hunting for pollen.