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托福阅读背景知识之动物发声

2015-01-09 19:46:57来源：网络

patterned wings in the bright spotlights. Theyalso compete with each other for the best spot of sunlight.

　　Very little light filters through the canopy of leaves and branches in a rain forest to reachground level—or close to the ground—and at those levels the yellow-to-green wavelengthpredominate. A signal might be most easily seen if it is maximally bright. In the green-to-yellow lighting conditions of the lowest levels of the forest, yellow and green would be thebrightest colors, but when an animal is signaling, these colors would not be very visible if theanimal was sitting in an area with a yellowish or greenish background. The best signal dependsnot only on its brightness but also on how well it contrasts with the background against whichit must be seen. In this part of the rain forest, therefore, red and orange are the best colors forsignaling, and they are the colors used in signals by the ground-walking Australian brushturkey. This species, which lives in the rain forests and scrublands of the east coast ofAustralia, has a brown-to-black plumage with bare, bright-red skin on the head and a neckcollar of orange-yellow loosely hanging skin. During courtship and aggressive displays, theturkey enlarges its colored neck collar by inflating sacs in the neck region and then flings abouta pendulous part of the colored signaling apparatus as it utters calls designed to attract orrepel. This impressive display is clearly visible in the light spectrum illuminating the forestfloor.

　　Less colorful birds and animals that inhabit the rain forest tend to rely on forms ofsignaling other than the visual, particularly over long distances. The piercing cries of therhinoceros hornbill characterize the Southeast Asian rain forest, as do the unmistakable callsof the gibbons. In densely wooded environments, sound is the best means of communicationover distance because in comparison with light, it travels with little impediment from trees andother vegetation. In forests, visual signals can be seen only at short distances, where they arenot obstructed by trees. The male riflebird exploits both of these modes of signalingsimultaneously in his courtship display. The sounds made as each wing is opened carryextremely well over distance and advertise his presence widely. The ritualized visual displaycommunicates in close quarters when a female has approached.

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