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
Birds evolved during the great reptilian radiation of the Mesozoic era. Amniotic eggs and scales on the legs are just two of the
Birds evolved during the great reptilian radiation of the Mesozoic era. Amniotic eggs and scales on the legs are just two of the
admin
2017-06-11
18
问题
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 keratin, 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 heat and reduce the density of the body metabolism. The lungs have tiny tubes leading to and from elastic air sacs that help dissipate.
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(breast-bone). 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 endotherm, only later being so-opted as flight equipment.
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 scientists place the origin of birds much earlier, from an ancestor common to both birds and dinosaur. The intense current interest in the origin of birds will undoubtedly bring us closer to understanding how these masters of the sky evolved from non-flying reptiles
According to paragraph 6, which of the following is true about the wings of birds?
选项
A、All birds flap their wings constantly by using breast muscles.
B、Eagles and hawks have wings that propel them at 170 km/hr.
C、The airfoils of birds function like the wings on airplanes.
D、Wings are attached to airfoils in the bird’s skeletal structure.
答案
C
解析
细节题。第6段第2句讲到“鸟类翅膀就是airfoils(机翼),展示了与飞机机翼相同的空气动力学(aerodynamics)原理”。故选c。
转载请注明原文地址:https://jikaoti.com/ti/2LmYFFFM
0
考博英语
相关试题推荐
Thereisnodenyingthatstudentsshouldlearnsomethingabouthowcomputerswork,justasweexpectthematleasttounderstand
Theriverisalready_____itsbanksbecauseofexcessiverainfallandthecityisthreatenedwithalikelyflood.
InTheBirthOrderBook:WhyYouAretheWayYouAre(2004),Dr.KevinLemannotesthat21ofthefirst23Americansinspacewer
Oneofthemajordifferencesbetweenmanandhisclosestlivingrelativeis,ofcourse,thatthechimpanzeehasnotdevelopedthe
Duringthepastgeneration,theAmericanmiddle-classfamilythatoncecouldcountonhardworkandfairplaytokeepitselffin
CharlesIIwasanothermonarch_____malignedbyVictorianhistorians.
IntheopinionofmanyAmericansandEuropeans,weonlybegantoreallyexploreourworldinthesixteenthcentury.Accordingto
MilankovitchproposedintheearlytwentiethcenturythattheiceageswerecausedbyvariationsintheEarth’sorbitaroundthe
随机试题
女性,53岁,阴道不规则流血,超声显像见子宫浆膜下、肌壁和黏膜有多个大小不等结节,最可能诊断是()
A、治热以寒B、治寒以热C、阳病治阴D、阴病治阳E、阴中求阳阴虚发热证的治则是()
有关资金的时间价值推论不正确的一项为()。
住房公积金财务管理的主要任务:编制住房公积金和管理中心管理费用年度预决算,建立职工住房公积金明细账,记载职工个人住房公积金的缴存、提取等情况;建立健全内部财务制度,做好财务管理工作;降低运作风险,保证住房公积金保值增值,确保住房公积金所有者的合法权益不受侵
由一个力和一个力偶组成的平面力系()。
自理报关单位没有进出口经营权,但有报关权。()
2017年联合国迎来的新任秘书长是()。
“就业至上”是针对“就业难”提出来的。面对国际金融危机,今年高校毕业生就业遇到了前所未有的困难,各高校都把解决就业难问题作为“一把手工程”来抓,这本来无可厚非。但重视就业,为了走出困境,就把“就业至上”上升到办学理念的高度,有欠妥当。这段文字的主要
[2014年第42题]这两个《通知》或者属于规章或者属于规范性文件,任何人均无权依据这两个《通知》将本来属于当事人选择公证的事项规定为强制公证的事项。根据以上信息,可以得出以下哪项?
What’stheaverageincreaseperyearofforeignstudentpopulationintheperiodbetween1985and1990intermsofpercentage?
最新回复
(
0
)