Saturday, March 21, 2009
Ultraviolet Vision of Birds
On pages 110-112, Carroll talks about how "many birds have tuned an opsin to detect ultraviolet light, and have evolved body markings that reflect light in the ultraviolet part of the spectrum." Carroll also states that "ultraviolet vision has evolved at least four separate times in birds." Why do you think ultraviolet vision and body markings that reflect ultraviolet light are so important to birds? Why has ultraviolet vision evolved in some bird species but not in others? Is it because these birds have a differet habitat or lifestyle than other birds? Also, why has ultraviolet vision evolved more in birds than in other vertebrate?
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ReplyDeleteBirds need very good eyesight because they are flying through all types of weather, and birds have many adaptations that give them superior vision to that of other vertebrate. Birds have ultraviolet vision while many vertebrate do not because birds are flying through the air and must detect predators and food from a much farther distance away then many other animals. Therefore, it wouldn't be a necessity for all vertebrates to have such strong vision because they aren't flying miles about the ground, so other vertebrate have evolved in many other ways that have a stronger natural selection to their habitat. Birds, fish, and amphibians have four types of color receptors in their eyes, while mammals have two types of receptors and primates have three. The four receptors in birds' eyes allow them to detect ultraviolet light and polarized light. They are able to detect these types of lights because they have more light receptors in their retina and more nerve connections between their photoreceptors and their brains. In addition, some birds have more modifications to their eyes that are linked to how they live and where they live. For instance, birds who are large prey have a high density of receptors that increase their visual abilities. Also, their eyes are placed to the side of their head so they can have accurate judgement of distances to their prey and so they can detect their predators. In addition, there are nocturnal birds that have tubular eyes with a low number of color detectors, but many rod cell that function better in poor light. Gulls and albatrosses are types of seabirds that have red or yellow oil drops in their receptors that improve their distance vision for hazy conditions that may occur. Therefore, ultraviolet vision has evolved in some bird species but not in others because some birds don't need such acute eyesight because their niche doesn't require it in order for that bird to survive.
ReplyDeleteUltraviolet light is involved in courtship of birds. Birds have ultraviolet reflective patches on their feathers that help in mating, but these patches are invisible to the human eye. For example, a male Blue Tit has an ultraviolet reflective crown patch that is shown during courtship by the bird raising up their feathers. Also, the Blackbird has an ultraviolet beak that lures in female birds because they respond well to their orange coloring. In addition, many fruits reflect UV light so the bird can easily detect them. Therefore, the ultraviolet vision and body markings that reflect UV light are so important to birds because they are a necessity in mating practices and for detecting food.
http://www.bio.bris.ac.uk/research/vision/4d.htm
http://www.ncbi.nlm.nih.gov/pubmed/8023459
http://en.wikipedia.org/wiki/Bird_vision
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1617148
To begin, the mutations that occur in birds for them to see in the ultraviolet range include a specific site at position 90 in the bird SWS opsin. If the amino acid at position 90 is serine, birds see in the violet range; however if the amino acid is cysteine, the birds see in the ultraviolet range, "just this single amino acid difference shifts the absorption maximum by 35 to 38 nm, a very dramatic shift," (Carroll 111). Carroll has found compelling evidence that the evolution of ultraviolet vision has been affected by sexual selection where mates preferences are determined by colors and patterns visible in UV light. Furthermore, in a study conducted by Dr. Andrew Bennett and Innes Cuthill, which was published in the Proceedings of the National Academy of Sciences, confirmed the accuracy of this theory (source: http://www.pnas.org/content/94/16/8618.full?ck=nck). If you would like, I’d strongly recommend reading this article because it offers a very interesting experiment about the ultraviolet vision present in avian species.
ReplyDeleteLike Jackie has wonderfully put, birds use their keen vision and their ability to see ultraviolet light to hunt for prey and location food. However, I think the sexual advantages are just as important as the selective advantage of being able to easily find food. In an article by Deane Morrison of the University of Minnesota, she points out an interesting explanation for the possible characteristics of the male (bird) as a measure of fitness that relate to ultraviolet light; “for many animals, it’s not siz, but color” (source: http://www1.umn.edu/umnnews/Columns_SciFri/SciFri_2.11.05_Color_me_essential.html). Birds may signal their fitness by their color. A male bird that is well fed and strong can accumulate lots of carotenoid pigments from plants to give himself a colorful appearance. When a peacock displays to a pea hen, he is showing her how healthy he is-so healthy, he can afford to squander energy accumulating and arranging pigments in his magnificent tail. But melanin, a dark pigment that many animals can synthesize, also may signal fitness or dominance. For example, says Muir Eaton of the University of Kansas, a former postdoc fellow of Bell Museum director, “the size of the male house sparrow's black throat patch seems to signal his dominance in a flock."
The healthier the male is, the more pigments it has received and thus he can show his mate, the female, that through reproduction, their genes can be carried as well as their offspring will survive and reproduce.
The courtship behavior that we studied in the autumn describes the powerful preferences of females that shape secondary sexual characteristics and courtship behaviors of males, "for example, the showy displays of peacocks and other male birds during mating season have little to do with direct male-male competition and much to do with advertising robust health to choosy females," (Campbell 1141). Obviously the healthier birds would be the most fit since they would be receiving the right amounts of pigments to color their fanciful bodies. Through ultraviolet light, males may have different pigments showing that we cannot see which are a more accurate predictor of health and female birds would be able to choose the "fittest" males through their preference of color in the ultraviolet realm. Sexual selection pertains to the themes of evolution and heritable information in that sexual selection determined by a female's preferences increases the rate of evolution by selecting for the "fittest" males to reproduce with since "healthy mates provide the best opportunity for producing healthy offspring," (Campbell 1141) and health offspring are those that are most likely to survive and carry the genes/genetic code of their parents. These genes will be able to proliferate (heritable information) and eventually cause those that are not fit enough to reproduce to die off - call it "survival of the fittest" or put it as Carroll states, "making of the fittest." In essence, female (sexual) selection is a pivotal driving force in shaping the evolutionary time frame.
Birds have adaptations that give them very good vision. Since birds fly they must be able to find prey and predators from far distances. Most other vertebrates do not need ultraviolet vision like birds because they do not need to see as far as birds. Mammals have 2 color receptors in their eyes, while birds have four. The four receptors allow birds to see polarized and ultraviolet light, which two receptors can’t see. The birds with 4 receptors in their retinas and have more nerve connections between their photoreceptors and their brains. Birds that have many predators have a greater density of receptors. This is due to natural selection that allowed the birds to develop more receptors and a higher density of receptors. Another adaptation that birds have is the physical structure of their eyes. They are placed on the sides of their heads so they can measure distance more accurately. Birds that may not have developed the four receptors and ultraviolet vision may be related to their lifestyles. A bird that is nocturnal will not need so many adaptations for color vision. Birds have ultraviolet reflective patches on their feathers. These patches are not seen by the human eye but they lure in female birds, and help the species reproduce. Birds also use ultraviolet light to find fruits that reflect the light. Ultraviolet vision is important to help the birds survive and reproduce.
ReplyDeletehttp://www.ncbi.nlm.nih.gov/pubmed/8023459
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1617148