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
32
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
The word " their" in the passage refers to______.
选项
A、feathers
B、species
C、layers
D、birds
答案
D
解析
代词指代题。根据代词指代上文名词或名词词组的基本原则,可找到前文的可能被指代的名词为feather(羽毛)和birds。根据搭配their metabolically generated heat(新陈代谢产生的热量)可判断,此处指代birds。同时,their只能指代复数名词,也可由此判断birds为被指代的名词。故选D。
转载请注明原文地址:https://kaotiyun.com/show/MVFO777K
0
考博英语
相关试题推荐
Thegovernmenthastodecidewhethercostorconcernfortheenvironmentshouldtake_____whenchoosingtherouteforthenewroa
Violenceisjustoneofthemanyproblems____incitylife.
Americansarehighly_____,andthereforemayfinditdifficulttobecomedeeplyinvolvedwithothers.
ThismonthSingaporepassedabillthatwouldgivelegalteethtothemoralobligationtosupportone’sparents.CalledtheMai
InEnglandduringthe17thand18thcenturies,Englishgentlemenwereexpectedtobepolite,pious,andcompassionateandtodevot
Writeashortcompositionofabout150wordsonthetopicgivenbelow.Topic:CommentontheDevelopmentoftheInternet
Duringthepasttenorfifteenyears,agreatdealofattentionhasbeen【C1】______to"tellingitlikeitis,"or"lettingitallh
Thefamousinventorwasawardedan______doctoratebytheuniversity.
MilankovitchproposedintheearlytwentiethcenturythattheiceageswerecausedbyvariationsintheEarth’sorbitaroundthe
随机试题
不属于肝门的结构是()
30岁妇女,不孕症,月经规律,痛经2年,每次需服止痛药。盆腔检查:子宫后位,稍活动,双侧卵巢增大6cm×4cm×4cm大小,右骶骨韧带处有触痛硬结。可能的阳性检查是
经穴大多位于
有一60岁老人,近日出现喘息、咳嗽、胸闷等症状,夜间及凌晨发作加重,呼吸较困难,并伴有哮喘鸣音。根据诊断结果,可选用的治疗药物是()
某市拟在1栋地上6层,地下1层,建筑高度为25.1m(建筑室外设计地面标高-0.1m,建筑首层室内地面标高±0.0m),每层建筑面积均为960m2,除首层层高为5m外,其他各层层高均为4m的已建医疗建筑的东侧建设1栋建筑高度为32m(建筑室外设计地面标高-
下列关于包装物的核算,表述不正确的有()。
乙且的积的个位上的数字是几?
近视:眼镜
教师:编辑
法律通过对利益的调控实现对社会的功能,需要处理的几种关系包括()。
最新回复
(
0
)