Why Do Animals Migrate?
Pronghorn (Antilocapra americana) running in snow.
Birds do it. Fish do it. Mammals, insects and reptiles do it. Migrate, that is.
Migration is nearly universal withi in fact,
even sponge and jellyfish larvae make use of that defining animal trait,
mobility, to adapt to imperfect habitats. Animals across the globe fly,
swim, walk or drift in their effort to find , a more hospitable climate or places to breed.
These primal drives lead to arduous and often treacherous treks, some spanning thousands of miles.
Humpback whales
of lengthy mammal migrators, traveling as much as 5,000 miles (8,047
kilometers) one way. One generation of monarch butterflies flutters some
2,000 miles (3,219 km) between southern Canada and central Mexico. A
shorebird called the bar-tailed godwit holds the record for the longest
nonstop flight: 6,835 miles (11,000 km) in eight days.
Then there are the enormous effort a salmon displays to reach its
breeding grounds, the three-mile (five-kilometer)
crawl through numerous obstacles undertaken by red crabs on Christmas
I the 300-mile loop taken by zebras and wildebeest in Africa,
following seasonal rains, and many more.
So why do they do it? Migration is driven by a simple fact: Resources
on Earth fluctuate. Warm summer months may be followed by inhospitable
cold. Plants & or other meals & may be abundant, but only for a short
time. The best place to give birth or hatch young may not be a good
place to find food.
And why not find a shorter, simpler journey or stop altogether? "The
simple answer is that the benefits of long-distance migration outweigh
its cost and the benefits of shorter distances," Hugh Dingle, a
professor emeritus at the
of California, Davis, told Life's Little Mysteries.
There is incredible diversity in animal migrations, and while science
has pinned down certain aspects, much about them remains a mystery.&
Many paths
While migrations can be complex and varied, the motivations behind
them are simple. Food, breeding and climate are recurring themes.
For humpback whales, the best place to feed isn't the best place to
breed. During the warm months, these whales feed in nutrient-rich waters
of the high latitudes. However, they mate and calve in warmer waters
that contain little food, so after these whales have journeyed as much
as 5,000 miles (8,047 km) to their breeding grounds, they will fast
throughout the winter.
In North America, pronghorn antelope follow new greenery as they
travel north from their overwintering grounds in the Upper Green River
Basin in Wyoming to their summer territory in Grand Teton National Park.
Their return trip, in autumn, is a necessity because the antelope would
not survive the deep snows in the north & and their journey must begin
before snow blocks their return route through the Gros Ventre Mountains.
In autumn, one generation of monarch butterflies, the migrant
generation, flies to central Mexico for the winter from as far north as
Canada. The same insects start the return trip but stop in the southern
United States, where the females lay eggs and pass the baton .
This happens at least once more before the butterflies reach their
summer destination. While it is believed that the butterflies travel
north following the emergence of milkweed, which is the plant that hosts
their young, the reasons for their trip back south aren't fully
understood, according to Steven Reppert, a neurobiologist at the
University of Massachusetts Medical School.
Aphids' annual migrations & from an overwintering site and between
plants as they search for fresh greenery & also spans multiple
generations. Their trips can be as short as a few hundred meters or as
long as several thousand up the Mississippi Valley from Texas to
Illinois on prevailing springtime winds, according to Dingle.
For the Pacific salmon, migration defines its lifespan. After
spending their adult lives in the ocean, these fish journey against the
current of the freshwater rivers where they hatched. Once home, they
spawn and die. It's a difficult and dangerous task: Only one out of a
thousand salmon may live to return to the stream where it was hatched,
according to the U.S. Fish & Wildlife Service.
Red-billed queleas, an avian agricultural pest in Africa, have a
migration pattern that appears erratic. The birds feed on seeds, so once
a rainy season has started and seeds germinate, the birds are forced to
move. Eventually, the rain fronts catch up with the birds, and they fly
back to places where the rains began, where they breed. They may later
move and breed again in the same season, according to Robert Cheke, a
researcher with the Natural Resources Institute.
Desert locust invasions are also complex. Under normal conditions,
desert locusts behave like grasshoppers and do not swarm & but after a
period of good breeding conditions, once populations reach a critical
density, their color and behavior change. The
and migrate downwind to find green vegetation to eat and damp
environments in which to lay their eggs, a phenomenon that earned them a
place in the Bible.
Programmed to migrate
Natural selection shapes these journeys, and it has programmed this
traveling nature into many animals, putting a destination in their
genes. But the innate sense that points these migratory animals in the
right direction has remained something of a mystery.
For instance, direction and distance must be encoded in a songbird's
genes, because young birds make their first flight alone, according to
Susanne &Akesson of Lund University in Sweden. The birds also have a
genetically based internal clock that allows them to synchronize their
behavior & whether preparing for migration or becoming sexually
& with the seasons.
The genetic structure of these abilities is not well understood, &Akesson said.
Scientists do know, however, that migrants can use the sun, the
stars, reflected light, the Earth's magnetic field and their sense of
smell to find their way.
Regardless of its genetic components, migration is not mandatory. In the early 1900s, the
population, for example, stopped migrating as their numbers dwindled.
After roughly 50 years, the pronghorn population in Wyoming returned a
certain level and the migration resumed, according to Jon Beckmann, an
associate conservation ecologist with the Wildlife Conservation Society.
Closely related populations of a European bird, the black cap
warbler, display a full range of migratory behavior, from traveling long
distances to staying put, &Akesson said.
"If you don't have any need to change environments, you might not
just activate this program encoding the activity to move from a certain
area," she said. "But maybe the capability is present in most birds."
Unique journeys
Migrants have evolved to respond to cues, such as changes in day
length, that tell them it's time to move. Waiting too long brings a
number of risks, including losing the opportunity to stock up on food
before departure or losing the competitive edge that comes with early
arrival at your next habitat.
In addition to being pre-emptive, the travelers are focused. They
don't meander, and they aren't distracted by food or mates along the
way. The migrant generation of monarch butterflies is unable to
reproduce until after it has reached its southern destination. And
are initially drawn to the blue light of the sky, then, as their
journey progresses, to the yellow light reflected by their preferred
food, young leaves.
Single-mindedness makes sense, Dingle said. "If you get distracted, you are not going to get where you need to go."
How do they do it?
In addition to guiding migrants to a tried-and-true destination,
natural selection has taught many migrants how to take advantage of
winds or currents that will aid or even carry them to their
destinations.
"You evolve to take advantage of abilities that already exist,"
Dingle said. "Birds already can fly, to take the simplest case, so
instead of just restricting flights to short distances, you evolve a
mechanism to take flights to long distances."
Research on migrating birds showed that, the more rounded their wings
were and the more weight they carried per wing area, the greater their
heart rates and energy expenditures were. The birds could make their
flights more energy-efficient by avoiding high winds and turbulence.
Meanwhile, migratory butterflies appear to have developed larger and more elongated wings.
Migrating animals also have to prepare. The bar-tailed godwits stuff
themselves, more than doubling their weight before they begin their
autumn flight, according to Anders Hedenstr&m from Lund University. They
also increase the mass of their flight muscle slightly, but over the
course of the flight, it is partially consumed, and they lose weight
elsewhere by shrinking their intestines and liver. During a long,
snackless flight, these organs make for unnecessary payload, Hedenstr&m
said. Similar physiological changes have been observed in other
migrating birds.
Their journeys aren' migrants fast, swim upstream, fly
nonstop, and face hungry predators and barriers built by humans. The
however, mortality data is difficult to
obtain, according to Dingle.
"My own suspicion is that it's a lot less than people think. They just seem able to do it well," he said.
Wynne Parry is a Senior Writer for , a sister site of Life's Little Mysteries.
Author Bio
Copyright & 2015 All Rights Reserved.CCC1.见第一段第三句。2.见第二段倒数二、三句。3.见第三段第一、二句。
请选择年级高一高二高三请输入相应的习题集名称(选填)：
科目：高中英语
题型：阅读理解
THE BRONTE FAMILY
Yorkshire, England was the setting for two great novels (小说) of the 19th century. These were Charlotte Bronte’s Jane Eyre and Emily Bronte’s Wuthering Heights. The youngest sister, Anne, was also a gifted novelist, and her books have the same extraordinary quality as her sisters’.
Their father was Patrick Bronte, born in Ireland. He moved with his wife, Maria Bronte, and their six small children to Haworth in Yorkshire in 1820. Soon after, Mrs. Bronte and the two eldest children died, leaving the father to care of the remaining three girls and a boy.
Charlotte was born in 1816. Emily was born in 1818 and Anne in 1820. Their brother Branwell was born in 1817. Left to themselves, the children wrote and told stories and walked over the hills. They grew up largely self-educated. Branwell showed a great interest in drawing. The girls were determined to earn money for his art education. They took positions as teachers or taught children in their homes.
As children they had all written many stories. Charlotte, as a young girl, alone wrote 22 books, each with 60 to 100 pages of small handwriting. Therefore, they turned to writing for income. By 1847, Charlotte had written The P Emily, Wuthering H and Anne, Agnes Grey. After much difficulty Anne and Emily found a publisher(出版商), but there was no interest shown in Charlotte’s book. (It was not published until 1859.) However, one publisher expressed an interest in seeing more of her works. Jane Eyre was already started, and she hurriedly finished it. It
thus each of the sisters had a book published in 1847.
Jane Eyre was im the other two, however, did not do so well. People did not like Wuthering Heights. They said it was too wild, too animal-like. But gradually it came to be considered one of the finest novels in the English language. Emily lived only a short while after the publication of the book, and Anne died in 1849.
Charlotte published Shirley in 1849, and Villette in 1853. In 1854 she married Arthur Bell Nicholls. But only a year later, she died of tuberculosis(肺结核) as her sisters had.
We know from the text that&&&&& .
A. Jane Eyre was published in 1847&&&&
B. Charlotte Bronte wrote 22 books in all
C. the Bronte sisters received good education&&&&&
D. Patrick Bronte helped his daughters with their writing
The underlined words “the other two” in the 5 th paragraph refer to&&&&& .
&&&&&& A. Shirley and Villette&& B. The Professor and Agnes Grey
&&&&&& C. Agnes Grey and Wutheriing Heights&& D. The Professor and wuthering Heights
What do we know about the Bronte sisters from the text?
&&&&&& A. Their novels interested few publishers.
&&&&&& B. None of them had more than two books published.
&&&&&& C. None of them lived longer than 40 years old.
&&&&&& D. Emily was the least successful of the three.
科目：高中英语
来源：2012届北京市高三高考预测英语试卷（带解析）
题型：阅读理解
Obama Still Smokes in SecretUS President Barack Obama has just made life more difficult for cigarette makers. He has just signed a law that will set tough new rules for the tobacco industry. The new law gives the US Food and Drug Administration the power to strictly limit the making and marketing of tobacco products.At a White House signing ceremony Monday, Obama said that he was among the nearly 90% of smokers who took up the habit before their 18 th birthday.Obama, who has publicly struggled to give up smoking, said he still hadn’t completely kicked the habit. Every now and then he still smokes in secret.“As a former smoker I struggle with it all the time. Do I still smoke sometimes? Yes. Am I a daily smoker, a constant smoker? No.” Obama said at a news conference.“I don' t do it in front of my lads．I don ?t do it in front of my family．I would say that I am 95% cured, but there are times when I mess up, " he said．"Once you go down this path, it' s something you continually struggle with, which is exactly why the law is so important．The new law is not about me, it' s about the next generation of kids coming up．What we don ' t want is kids going down that path," he said． Nearly 20% of Americans smoke and tobacco use kills about 440,000 people a year in the United States due to cancer, heart disease, and other serious diseases．【小题1】The new law makes life difficult for&&&&&&&&&&&&&.A．Obama B．tobacco industryC．White House D．US Food and Drug Administration【小题2】What do we know about Obama?A．He no longer smokes．B．He still smokes as usual．C．He began to smoke at eighteen．D．He is trying hard to give up smoking．【小题3】According to the passage, Obama is most concerned about &&&&&&&&&&．A．childrenB．officialsC．his familyD．Businessmen
科目：高中英语
来源：学年河南省扶沟县高级中学高一第三次考试英语试卷（带解析）
题型：阅读理解
Obama Still Smokes in Secret US President Barack Obama has just made life more difficult for cigarette makers．He has just signed a law that will set tough new rules for the tobacco industry．The new law gives the US Food and Drug Administration the power to strictly limit the making and marketing of tobacco products． At a White House signing ceremony Monday, Obama said that he was among the nearly 90% of smokers who took up the habit before their 18 th birthday． Obama, who has publicly struggled to give up smoking, said he still hadn’t completely kicked the habit．Every now and then he still smokes in secret． “As a former smoker I struggle with it all the time．Do I still smoke sometimes? Yes．Am I a daily smoker, a constant smoker? No．” Obama said at a news conference． “I don' t do it in front of my lads．I don’t do it in front of my family．I would say that I am 95% cured, but there are times when I mess up, " he said． "Once you go down this path, it' s something you continually struggle with, which is exactly why the law is so important．The new law is not about me, it' s about the next generation of kids coming up．What we don ' t want is kids going down that path," he said． Nearly 20% of Americans smoke and tobacco use kills about 440,000 people a year in the United States due to cancer, heart disease, and other serious diseases．【小题1】The new law makes life difficult for&&&&&&&&&&&&&．A．ObamaB．tobacco industryC．White HouseD．US Food and Drug Administration【小题2】What do we know about Obama?A．He no longer smokes．B．He still smokes as usual．C．He began to smoke at eighteen．D．He is trying hard to give up smoking．【小题3】According to the passage, Obama is most concerned about &&&&&&&&&&．A．childrenB．officialsC．his familyD．businessmen
科目：高中英语
来源：学年内蒙古巴市高三第一次模拟考试英语卷（解析版）
题型：阅读理解
Obama Still Smokes in Secret
US President Barack Obama has just made life more
difficult for cigarette makers. He has just signed a law that will set tough
new rules for the tobacco industry. The new law gives the US Food and Drug
Administration the power to strictly limit the making and marketing of tobacco
products.
At a White House signing ceremony Monday, Obama said
that he was among the nearly 90% of smokers who took up the habit before their
18 th birthday.
Obama, who has publicly struggled to give up smoking,
said he still hadn’t completely kicked the habit. Every now and then he still
smokes in secret.
“As a former smoker I struggle with it all the time.
Do I still smoke sometimes? Yes. Am I a daily smoker, a constant smoker? No.”
Obama said at a news conference.
“I don' t do it in front of my lads．I don ?t do it in front of my family．I would say that I am 95% cured, but there are times
when I mess up, & he said．
&Once you go down this path, it' s something you
continually struggle with, which is exactly why the law is so important．The new law is not about me, it' s about the next
generation of kids coming up．What we don ' t want is
kids going down that path,& he said．
&&&&&&&& Nearly
20% of Americans smoke and tobacco use kills about 440,000 people a year in the
United States due to cancer, heart disease, and other serious diseases．
1.The
new law makes life difficult for&&&&&&&&&&&&&
.
B．tobacco industry
C．White House
D．US Food and Drug
Administration
2.What
do we know about Obama?
A．He no longer smokes
B．He still smokes as usual
C．He began to smoke at
eighteen
D．He is trying hard to give
up smoking
3.According
to the passage, Obama is most concerned about &&&&&&&&&&．
A．children&&&&&&
B．officials
C．his
family&&&&& & D．businessmen
科目：高中英语
来源：学年陕西省咸阳市高三上学期第一次月考英语题
题型：阅读理解
Obama Still Smokes in Secret
US President Barack Obama has just made
life more difficult for cigarette makers．He has just signed a law that will set tough new rules for the
tobacco industry．The
new law gives the US Food and Drug Administration the power to strictly limit
the making and marketing of tobacco products．
At a White House signing ceremony Monday,
Obama said that he was among the nearly 90% of smokers who took up the habit
before their 18 th birthday．
Obama, who has publicly struggled to give
up smoking, said he still hadn’t completely kicked the habit．Every now and then he still
smokes in secret．
“As a former smoker I struggle with it all
the time．Do I still
smoke sometimes? Yes．Am I a daily smoker, a constant smoker? No．” Obama said at a news
conference．
“I don' t do it in front of my lads．I don 't do it in front of my
family．I would say
that I am 95% cured, but there are times when I mess up, & he said．
&Once you go down this path, it' s
something you continually struggle with, which is exactly why the law is so
important．The new law
is not about me, it' s about the next generation of kids coming up．What we don ' t want is kids
going down that path,& he said．
Nearly 20% of Americans smoke and tobacco
use kills about 440,000 people a year in the United States due to cancer, heart
disease, and other serious diseases．
1.The new law makes life difficult
for&&&&&&&&&&&&&．
B．tobacco industry
C．White House
D．US Food and Drug Administration
2.What do we know about Obama?
A．He no longer smokes．
B．He still smokes as usual．
C．He began to smoke at eighteen．
D．He is trying hard to give up smoking．
3.According to the passage, Obama is most
concerned about &&&&&&&&&&．
A．children
B．officials
C．his family
D．businessmen