Everyday Mysteries: Why most male birds are more colorful than females

Transcription

1 Everyday Mysteries: Why most male birds are more colorful than females By Scientific American, adapted by Newsela staff on Word Count 779 Mandarin ducks, a male (left) and a female, at WWT Martin Mere, a wetland nature reserve in Lancashire, England. Photo by: Francis C. Franklin Question: Why are male birds more colorful than female birds? Answer: Robert Heinsohn, professor of evolutionary biology at the Australian National University, has an answer. Darwin's Natural Selection Males birds are more colorful or ornamented than females in most bird species. This involves the evolutionary forces that shape animals' behavior. Charles Darwin developed much of the theory that helps explain this. Darwin proposed that traits promoting a specie's survival are favored by natural selection. Natural selection is a process in which creatures that are better suited for their environment survive. Those that are not as well suited for their environment do not. This article is available at 5 reading levels at 1

2 Darwin also found that traits that help the individuals of just one sex compete for mates are favored by sexual selection. Sexual selection is a process in which creatures that are more attractive to the opposite sex have more children. Thus, they pass on their genes. Sexual selection is responsible for many of the features unique to one sex in a given species. These features can be divided into two general categories. The first category is traits that allow males to fight for mates. The second category is traits that act as ornaments that attract the attention of females. Darwin concluded that color differences between sexes in birds result largely from female preference for bright colors in males. However, this can vary based on the particular species. Females of species that are exposed to predators while incubating tend to have dull colors. However, both sexes may be brightly colored in species that are less visible to predators. Competition For Mates Color is also a factor in competition between males for mates or territory. Bright colors can help show that an area is already occupied. It can also show that the occupant is ready to fight. The coverable red shoulder patch on red-winged blackbirds provides an excellent example. Males who had their patch covered in experiments tended to lose their territories more often than did uncovered birds. Studies have shown that females use the brightness of a male's color as an indicator of health and vitality. House finches provide one of the best examples of this. Males exhibit orange or red in their crowns and elsewhere. The extent and brightness of the color is directly related to the quality of their food. Females are often busier than males are. They have to incubate their eggs and raise their chicks. This means that males have to compete more for their attention. Invisible Ultraviolet Coloration Researchers realized only recently that birds see a much wider range of color than people do. Birds have four color cones in their eyes (compared to three in humans). This allows them to see the ultraviolet part of the color spectrum. Scientists have found that species who seem, through our eyes, to have similarly colored sexes are actually not so similar. Males in these species often sport bright ultraviolet colors. In an interesting twist, a handful of species are known to play reversed roles. In these scenarios, males incubate the eggs and females defend territories and fight amongst themselves to mate with the males. These species provide the exceptions that prove the rule, because they demonstrate that the competitive sex is the one most likely to have bright colors. Phalaropes, sandpipers and button quail are good examples of species in which the females are more colorful. This article is available at 5 reading levels at 2

3 A recent study of eclectus parrots showed that bright colors can evolve in both sexes at the same time. In this species the bright green males and red-and-blue females look so different that they were originally thought to be separate species. Our eight-year study in northern Australia, published in the July 22, 2005 issue of Science, demonstrated that the sex roles are not reversed. Females incubate eggs and protect the young. The sexes differ where the females do not join the males in foraging for food and instead defend the nest hollow for up to 11 months each year. Each female relies on up to five males to supply all the food she and her chicks need. Males face a higher predation risk from hawks while they are foraging, and their colors have evolved to blend in with the leafy foliage. Meanwhile, their shiny green stands out and appears very bright to other parrots against the wood at the nest hollow. In addition, the green is laced with ultraviolet pigments, which the parrots can see much better than hawks can. Their colors are therefore a clever compromise between camouflage and showiness. The females, however, are under less predation pressure, and their red and blue appears as a long range signal to other females of their presence at the hollow. This article is available at 5 reading levels at 3

4 Quiz 1 Which two sentences from the article include CENTRAL ideas of the article? Darwin concluded that color differences between sexes in birds result largely from female preference for bright colors in males. Researchers realized only recently that birds see a much wider range of color than people do. Darwin proposed that traits promoting a species' survival are favored by natural selection. Females are often busier than males are. Females of species that are exposed to predators while incubating tend to have dull colors. Males who had their patch covered in experiments tended to lose their territories more often than did uncovered birds. Studies have shown that females use the brightness of a male's color as an indicator of health and vitality. Phalaropes, sandpipers and button quail are good examples of species in which the females are more colorful. 2 Which statement would be MOST important to include in a summary of the article? Female birds are responsible for incubating the eggs after mating. The bright color of males signals health and a readiness to fight. Male birds are the same color as the food that they eat. Humans have a more limited range of color than parrots have. 3 Read the paragraph from the section "Competition For Mates." Studies have shown that females use the brightness of a male's color as an indicator of health and vitality. House finches provide one of the best examples of this. Males exhibit orange or red in their crowns and elsewhere. The extent and brightness of the color is directly related to the quality of their food. Which answer choice BEST explains why the author included this paragraph in the article? to indicate reasons for why females have less bright colors to give an example of why female birds prefer colorful male birds to suggest a reason for why bird crowns are either orange or red to explain why females prefer males with health and vitality This article is available at 5 reading levels at 4

5 4 How does the section "Darwin's Natural Selection" contribute to the development of the MAIN idea of the article? It establishes that the color differences in male and female birds have been studied for a long time. It establishes that Charles Darwin was the first person to recognize that male birds are more colorful than female birds. It provides a scientific explanation to the question posed about why male birds are more colorful than female birds. It highlights the events that led Charles Darwin to discover the link between natural selection and colorful male birds. This article is available at 5 reading levels at 5