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
44
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
Which of the sentences below best expresses the information in the highlighted statement in the passage?
选项
A、Very early birds appeared after the dinosaurs.
B、Birds and dinosaurs may share the same ancestor.
C、Birds and dinosaurs were probably very common.
D、The origin of birds and dinosaurs is very early.
答案
B
解析
句意解释题。选项B兼用了同义替换(mutual与common同义)和变换句型结构对原句进行解释。原句包含两个对立信息:“an ancestor common”“to both birds and dinosaur”,而选项B完整地展现了原句内涵。
转载请注明原文地址:https://kaotiyun.com/show/uVFO777K
0
考博英语
相关试题推荐
Overthepasttwodecades,thelivesofAmericanwomenhaveundergoneunparalleledchange.TheVirginiaSlimsOpinionPollhasc
Thereisnoquestionthattheoldstyleofairpollutioncouldkillpeople.Inoneweekfollowingtheinfamous"peasouper"fogi
Can’tyouspeakmore____toyourparents?
Futuristshaveidentifiedtwochangesthatseemtobecentraltocontemporarysociallife.First,theU-nitedStatesisbeingre
Duringthepasttenorfifteenyears,agreatdealofattentionhasbeen【C1】______to"tellingitlikeitis,"or"lettingitallh
Duringthepasttenorfifteenyears,agreatdealofattentionhasbeen【C1】______to"tellingitlikeitis,"or"lettingitallh
Oneofthemajordifferencesbetweenmanandhisclosestlivingrelativeis,ofcourse,thatthechimpanzeehasnotdevelopedthe
Duringthepastgeneration,theAmericanmiddle-classfamilythatoncecouldcountonhardworkandfairplaytokeepitselffin
Nowadays,ourgovernmentadvocatescredittowhateverwedoorwhoeverwecontactwith.Onceyou____yourwords,youwillloseyo
MilankovitchproposedintheearlytwentiethcenturythattheiceageswerecausedbyvariationsintheEarth’sorbitaroundthe
随机试题
甲公司与A市自然资源局签订建设用地使用权出让合同,甲公司按照约定支付了土地使用权出让金,取得B地块作为建设用地,但并未办理权属登记。不久,A市自然资源局又将B地块批给乙公司,乙公司支付了出让金并办理了权属登记。对此,下列表述正确的是()
下颌做侧方咀嚼时,工作侧上下后牙的接触关系为
磺酰脲类药物可用于治疗
可用于耐青霉素的金葡菌引起的轻、中度感染或青霉素过敏者的药物是
证券交易风险就是指证券公司在代理买卖证券的过程中由于各种主观原因而造成损失的可能性。()
操作风险可以分为由人员、系统、流程和外部事件所引发的四类风险,并由此分为()表现形式。
对下列对联依次所指的文学家,判断正确的一项是()。①哀怨托离骚,生而独开诗赋立;孤忠报楚国,余风波及汉湘人。②兵甲富于胸中,一代功名高宋室;忧乐关乎天下,千秋俎豆重苏台。③质而绮,真而醇,自可传之千古;樽中酒,篱下诗,岂甘了此一生。④笔诤
设L1:,L2:,则过L1平行于L2的平面方程为_________.
软件测试的目的是()。
下面对静态数据成员的描述中,正确的是()。
最新回复
(
0
)