哺乳动物体型处于趋减状态-凯发k8官网

哺乳动物体型处于趋减状态
文章来源:未知 文章作者:enread 发布时间:2012-01-31 05:12 字体: [ ] 
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just how big can mammals get and how fast can they get there? these are questions examined by an international team of researchers exploring increases in mammal size after the 1 became extinct 65 million years ago. research published in the journal 2 of the national academy of sciences shows it took about 10 million generations for terrestrial(陆地的) mammals to hit their maximum mass: that's about the size of a cat evolving into the size of an elephant. sea mammals, such as whales took about half the number of generations to hit their maximum.

the team, including dr. jessica theodor of the university of calgary, also discovered it took only about one hundred thousand generations for very large decreases, such as extreme dwarfism, to occur.

"our research demonstrates, for the first time, a large-scale history of mammal life in terms of the pace of growth. this is significant because most research focuses on microevolution, which are changes that occur within a specific species," says theodor, co-author of the study and an associate professor of biology at the university of calgary.

the research team looked at 28 different types of mammals from the four largest continents (africa, eurasia, and north and south america) and all ocean basins for during the last 70 million years. for example, one group would include the mammals related to an elephant, another group would include carnivorous(食肉的) mammals.

researchers were surprised to learn how quickly body-size decreased: the rate is more than 10 times faster than the increases.

"many of the species which shrunk, such as the 3 4, dwarf hippo and dwarf hominids, found in the indonesian island of flores, became extinct," says theodor, whose area of 5 are the artiodactyls, 6 mammals which include in the present day, cows, pigs, sheep, camels, hippos and whales.

"what caused their dwarfism? they may have needed to be small to survive in their environment or perhaps food was scarce and a small 7 would require less nutrients," adds theodor.

this research will help scientists to better understand mammal evolution: what conditions allow a certain mammals to thrive(繁荣,兴旺) and grow bigger and what conditions would slow the pace of growth and potentially contribute to 8.

researchers used generations instead of time in their study because species have different life spans. a mouse only lives for about two years and elephant for 80. they created a metric(度量标准) using the relationship between body size and generation time in living mammals and used it to reconstruct the life history of the extinct forms.



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1 87f9c39b9e3f358174d58a584c2727b4     
n.恐龙( dinosaur的名词复数 );守旧落伍的人,过时落后的东西
参考例句:
  • the brontosaurus was one of the largest of all dinosaurs. 雷龙是所有恐龙中最大的一种。 来自《简明英汉词典》
  • dinosaurs have been extinct for millions of years. 恐龙绝种已有几百万年了。 来自《简明英汉词典》
2 wk2zvx     
n.进程,过程,议程;诉讼(程序);公报
参考例句:
  • he was released on bail pending committal proceedings. 他交保获释正在候审。
  • to initiate legal proceedings against sb 对某人提起诉讼
3 ekjzh     
n.矮子,侏儒,矮小的动植物;vt.使…矮小
参考例句:
  • the dwarf's long arms were not proportional to his height.那侏儒的长臂与他的身高不成比例。
  • the dwarf shrugged his shoulders and shook his head. 矮子耸耸肩膀,摇摇头。
4 u2wy8     
n.长毛象;adj.长毛象似的,巨大的
参考例句:
  • you can only undertake mammoth changes if the finances are there.资金到位的情况下方可进行重大变革。
  • building the new railroad will be a mammoth job.修建那条新铁路将是一项巨大工程。
5 fmtx0     
n.专门知识(或技能等),专长
参考例句:
  • we were amazed at his expertise on the ski slopes.他斜坡滑雪的技能使我们赞叹不已。
  • you really have the technical expertise in a new breakthrough.让你真正在专业技术上有一个全新的突破。
6 a918239ee9bced8d1829f640a84a46d5     
adj.有蹄的,蹄形状的,装蹄的v.(兽的)蹄,马蹄( hoof的过去式和过去分词 )
参考例句:
  • he was hoofed out of the guards. 他被开除出警卫队。 来自互联网
  • he hoofed his way to town. 他步行去了城里。 来自互联网
7 rulw8     
n.(高度)水平,(高度)境界,身高,身材
参考例句:
  • he is five feet five inches in stature.他身高5英尺5英寸。
  • the dress models are tall of stature.时装模特儿的身材都较高。
8 spwzp     
n.熄灭,消亡,消灭,灭绝,绝种
参考例句:
  • the plant is now in danger of extinction.这种植物现在有绝种的危险。
  • the island's way of life is doomed to extinction.这个岛上的生活方式注定要消失。
tag标签: life size mammals
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