首页
登录
职称英语
Birds evolved during the great reptilian radiation of the Mesozoic era, A
Birds evolved during the great reptilian radiation of the Mesozoic era, A
游客
2023-10-14
26
管理
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 modem birds look quite different from modem reptiles because of their feathers and other distinctive flight equipment.
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, modem 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.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 adaptation.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.
In describing the characteristics of birds, almost every part of a typical bird’s anatomy is made different in some way that enhances【A1】________.Modem birds do not have 【A2】________, an adaptation that reduces the weight of the head.Birds use their own【A3】________to maintain the body temperature.Because birds’ eggs are shelled when laid, fertilization must be【A4】________.Most researchers believe that the 【A5】________was a feathered theropod that was a small, bipedal carnivorous dinosaur. [br] 【A1】
选项
答案
fight
解析
文章第二段第一句提到,典型的鸟类的解剖结构都以某种方式进行了修改是为了增强其飞行能力。因此,应填入flight。
转载请注明原文地址:https://tihaiku.com/zcyy/3098019.html
相关试题推荐
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Largeboycottsduring_____limitedparticipationinthe1980and1984OlympicG
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Malnutritionduringweaningage—whenbreastmilkisbeingreplacedbysemi-s
Itisessentialthatmisunderstanding______duringbusinessnegotiations.A、beavo
Ifyou’veeverdrivenonahighwayduringaheavyrain,youknowaboutthedang
Manylinguists______thatourhighlyevolvedbrainprovidesuswithinnatelangua
随机试题
RenownedChinesedirectorZhangYimou’slatestfilm,RidingAloneforThous
[originaltext]Predictingthejobsorskillsthatwillbeindemandyearsfro
企业()系统,是企业文化在意识形态领域中的再现。A.30 B.45 C.6
设A是3阶矩阵,P=(a1,a2,a3)是3阶可逆矩阵,且P-1AP=
有实验证明肩关节固定1周所致的挛缩需52天才能治愈,固定3周,治愈大约所需的天数
世界上最著名的围海造田的国家是荷兰。()
引起感觉需要的最小刺激量称为阈限。()
下列瞳孔的变化对诊断小脑幕切迹疝有意义的是A.患侧瞳孔先缩小,再散大 B.患侧
小柴胡汤与蒿芩清胆汤中均含有的药物是A:陈皮,甘草B:枳壳,大枣C:半夏,甘
上尿路结石大多数为A、草酸钙结石 B、磷酸镁铵结石 C、磷酸钙结石 D、尿
最新回复
(
0
)