首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
"The Evolution of Birds" Birds Began as Feathered Reptiles Birds evolved during the great reptilian radiation of the M
"The Evolution of Birds" Birds Began as Feathered Reptiles Birds evolved during the great reptilian radiation of the M
admin
2012-01-14
38
问题
"The Evolution of Birds"
Birds Began as Feathered Reptiles
Birds evolved during the great reptilian radiation of the Mesozoic era. Amniotic eggs and scales on the legs are just two of the reptilian features we see in birds. But modern birds look quite different from modern reptiles because of their feathers and other distinctive flight equipment.
Characteristics of Birds
→ Almost every part of a typical bird’s anatomy is
modified
in some way that enhances flight. The bones have an internal structure that is honeycombed, making them strong but light. The skeleton of a frigate bird, for instance, has a wingspan of more than 2 meters but weighs only about 113 grams. Another adaptation reducing the weight of birds is the absence of some organs. Females, for instance, have only one ovary. Also, modern birds are toothless, an adaptation that trims the weight of the head. Food is not chewed in the mouth but ground in the gizzard, a digestive organ near the stomach. (Crocodiles also have gizzards, as did some dinosaurs.) The bird’s beak, made of ker atin, has proven to be very adaptable during avian evolution, taking on a great variety of shapes suitable for different diets.
Flying requires a great expenditure of energy from an active metabolism. Birds are endothermic; they use their own metabolic heat to maintain a warm, constant body temperature. Feathers and, in some species, layers of fat provide insulation that enables birds to retain
their
metabolically generated heat. An efficient respiratory system and a circulatory system with a four-chambered heart keep tissues well supplied with oxygen and nutrients, supporting a high rate of metabolism. The lungs have tiny tubes leading to and from elastic air sacs that help dissipate heat and reduce the density of the body.
For safe flight, senses, especially vision, must be acute. Birds have excellent eyes, perhaps the best of all the vertebrates. The visual areas of the brains are well developed, as are the motor areas; flight also requires excellent coordination.
With brains proportionately larger than those of reptiles and amphibians, birds generally display very complex behavior. Avian behavior is particularly intricate during breeding season, when birds engage in
elaborate
rituals of courtship. Because eggs are shelled when laid, fertilization must be internal. Copulation involves contact between the mates’ vents, the openings to their cloacas. After eggs are laid, the avian embryo must be kept warm through brooding by the mother, father, or both, depending on the species.
→ A bird’s most obvious adaptation for flight is its wings. Bird wings are airfoils that illustrate the same principles of aerodynamics as the wings of an airplane. Providing power for flight, birds flap their wings by contractions of large pectoral (breast) muscles anchored to a keel on the sternum (breastbone). Some birds, such as eagles and hawks, have wings adapted for soaring on air currents and flap their wings only occasionally; other birds, including hummingbirds, must flap continuously to stay aloft. In either case, it is the shape and arrangement of the feathers that form the wings into an airfoil. The fastest birds are the appropriately named swifts, which can fly 170 km/hr.
→ In being both extremely light and strong, feathers are among the most remarkable of vertebrate adaptations. Feathers are made of keratin, the same protein that forms our hair and fingernails and the scales of reptiles. Feathers may have functioned first as insulation during the evolution of endothermy, only later being co-opted as flight equipment.
The evolution of flight required
radical
alteration in body form, but flight provides many benefits.A It enhances hunting and scavenging: many birds exploit flying insects, an abundant, highly nutritious food resource.B Flight also provides ready escape from earthbound predators and enables some birds to migrate great distances to utilize different food resources and seasonal breeding areas.C The bird that travels farthest in its annual migration is the arctic tern, which flies round-trip between the North Pole and South Pole each year. D
→ Analyses of fossilized skeletons support the hypothesis that the closest reptilian relatives of birds were the
theropods
, a group of relatively small, bipedal carnivorous dinosaurs. Most researchers agree that the ancestor of birds was a feathered theropod.
However, some soientists place the origin of birds much earlier, from an ancestor common to both birds and dinosaurs.
The
intense
current interest in the origin of birds will undoubtedly bring us closer to understanding how these masters of the sky evolved from nonflying reptiles.
Glossary
brooding: to hatch eggs by sitting on the nest
cloacas: the opening to the reproductive and intestinal tract
Look at the four squares [■] that show where the following sentence could be inserted in the passage. Furthermore, migration allows birds to avoid climates that are too hot or too cold during certain seasons. Where could the sentence best be added? Click on a square [■] to insert the sentence in the passage.
选项
A、Square A.
B、Square B.
C、Square C.
D、Square D.
答案
C
解析
Addition is a transitional device that connects the insert sentence with previous sentences. Several benefits of flight are explained before the insert sentence. The sentence after the insert expands on the benefit of migrating to avoid extreme climates.
转载请注明原文地址:https://kaotiyun.com/show/xPyO777K
0
托福(TOEFL)
相关试题推荐
Inthelastdecadearevolutionhasoccurredinthewaythatscientiststhinkaboutthebrain.Wenowknowthatthedecisionshu
Inthelastdecadearevolutionhasoccurredinthewaythatscientiststhinkaboutthebrain.Wenowknowthatthedecisionshu
TheEvolutionaryMystery:CrocodileSurvivesA.Eventhoughcrocodileshaveexistedfor200millionyears,they’reanything
TheEvolutionaryMystery:CrocodileSurvivesA.Eventhoughcrocodileshaveexistedfor200millionyears,they’reanything
TheEvolutionaryMystery:CrocodileSurvivesA.Eventhoughcrocodileshaveexistedfor200millionyears,they’reanything
TheEvolutionaryMystery:CrocodileSurvivesA.Eventhoughcrocodileshaveexistedfor200millionyears,they’reanything
Tiktaalikrosaeisconsideredtobeanevolutionary______.【38】
Thereisa______at13WardrockPlace.______mightofferdiscountedaccommodationduringlowseasons.
随机试题
中华民族伟大复兴的中国梦,包含着丰富的思想内涵。其中,作为根本出发点和落脚点的是()
牛为多室胃动物,成年牛容积最大的胃是()。
一病人左颞颌关节疼痛半年,咀嚼开口时疼痛加剧,关节区压痛,开闭口时,闻及摩擦音,开口型偏左侧,其诊断应为
下列属于氯化副产物的是()
根据《建设工程工程量清单计价规范》,分部分项工程量清单的十二位编码中最后三位表示()。
下列各项中,按件数计征印花税的是()。
教师的人格威信不仅是一种重要的教育力量,而且还可以弥补教师自身能力的某些不足。()
企业在管理市场营销活动时,对于市场营销计划中的效率控制,主要包括对()效率控制。
下列关于Winmail邮件服务器的描述中,正确的是()。
Peopletravelinglongdistancesfrequentlyhavetodecideif【M1】______theywouldprefertogobylandorsea.Hardlycananyon
最新回复
(
0
)