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What is caffeine? An alkaloid found in coffee, cocoa beans, tea, kola nuts and guarana. Also added to many fizzy drinks, energy drinks, pep pills and cold and flu remedies. For a single portion of espresso, 50 to 55 roasted coffee beans are required; a single imperfect bean will taint the whole sufficiently to be noticeable. This is because human olfaction and taste senses originated as defense mechanisms that protected our ancestors from rotten—hence, unhealthy—foods.
What does caffeine do? A stimulant of the central nervous system. Pure caffeine is a moderately powerful drug and is sometimes passed off as amphetamine. In small doses, such as the 150 milligrams in a typical cup of filter coffee, it increases alertness and promotes wakefulness. Caffeine also raises heart and respiration rate and promotes urine production. Higher doses induce jitteriness and anxiety. The fatal dose is about 10 grams.
How does caffeine work? Caffeine blocks receptors for the neurotransmitter adenosine, which is generally inhibitory and associated with the onset of sleep. Also raises dopamine levels, and stimulates the release of the fight-or-flight hormone adrenalin (From Newscientist.com).
What is coffee? Raw coffee beans are the seeds of plants belonging to the Rubiaceae family, which comprises at least 66 species of the genus Coffea. The two species that are commercially exploited are Coffea arabica, which accounts for two thirds of world production, and C. canephora, often called robusta coffee, with one third of global output. Robusta coffee plants and all wild coffee species have 22 chromosomes, whereas arabica has 44. Therefore, arabica and other coffee species cannot be crossed to produce a hybrid plant.
Robusta is a high-yielding and disease-resistant tree standing up to 12 meters tall that grows best in warm, humid climates. It produces a cup featuring substantial body, a relatively harsh, earthy aroma, and an elevated caffeine content that ranges from 2.4 to 2.8 percent by weight. Although robusta is sold by many purveyors, it does not give rise to the highest-quality coffee.
Arabica, which originated in the Ethiopian highlands, is a medium- to low-yielding, rather delicate tree from five to six meters tall that requires a temperate climate and considerable growing care. Commercially grown coffee bushes are pruned to a height of 1.5 to 2.0 meters. Coffee made from Arabica beans has an intense, intricate aroma that can be reminiscent of flowers, fruit, honey, chocolate, caramel or toasted bread. Its caffeine content never exceeds 1.5 percent by weight. Because of its superior quality and taste, arabica sells for a higher price than its hardy, rougher cousin.
A good rainfall induces coffee plants to blossom, and some 210 days afterward red or yellow fruit called cherries appear. Each cherry contains two oblong seeds—the coffee beans. The ultimate quality of the resulting coffee beans depends on the genetics of the plant, the soil in which it grows and the microclimate, which encompasses factors such as altitude, the amount of rainfall and sunlight, and daily temperature fluctuations. Along with the roasting processes that are applied, these agricultural and geographical considerations are responsible for the taste differences among the many varieties of coffee beans that suppliers combine to produce the various distinctive blends one can purchase (From Scientific American, June 2002).
Exercise 3. Now read detailed explanation of the effects of caffeine provided by biologist Neal J. Smatresk, Dean of the College of Science at the University of Texas at Arlington, and find answers to these questions:
1. What is a neurotransmitter? What is a second messenger?
2. How does caffeine affect heart?
3. Does caffeine affect all animals?
How does caffeine affect the body?
Caffeine--the drug that gives coffee and cola its kick--has a number of physiological effects. At the cellular level, caffeine blocks the action of a chemical called phosphodiesterase (PDE). Inside cells, PDE normally breaks down the second chemical messenger cyclic adenosine monophosphate (cAMP). Many hormones and neurotransmitters cannot cross the cell membrane, and so they exert their actions indirectly via such second messengers; when they bind to a receptor on the surface of a cell, it initiates a chemical chain reaction called an enzyme cascade that results in the formation of second messenger chemicals.
Historically, cAMP was the first second messenger ever described. Now, however, scientists have identified several major classes of second messengers, which are generally formed in similar ways through a set of molecules called G proteins. The advantage of such a complex system is that an extracellular signal can be greatly amplified in the process, and so have a massive intracellular effect.
Thus, when caffeine stops the breakdown of cAMP, its effects are prolonged, and the response throughout the body is effectively amplified. In the heart, this response prompts norepinephrine--also called noradrenalin--and a related neurotransmitter, epinephrine, to increase the rate and force of the muscle's contractions. Although the two act in concert, norepinephrine is released by sympathetic nerves near the pacemaker tissue of the heart, whereas epinephrine is released primarily by the adrenal glands. These chemical messages lead to "fight or flight" behavior. During stressful or emergency conditions, they raise the rate and force of the heart, thereby increasing the blood pressure and delivering more oxygen to the brain and other tissues.
Caffeine would be expected to have this effect on any animals that used these neurotransmitters to regulate their heartbeat. Generally speaking, the effects of caffeine are most pronounced in birds and mammals. Reptiles have some response, and lower vertebrates and invertebrates have rather small or no responses. From an evolutionary perspective, fish and amphibians don't show as strong a response to epinephrine and norepinephrine as the higher vertebrates, and they lack a well-developed sympathetic (that is, stimulatory) enervation to heart.
Exercise 4. What positive and negative effects produced by coffee do you know? Choose from the list below:
Exercise 5. Read the article about health effects of coffee to check your answers in Exercise 4.
Coffee as a Health Drink? Studies Find Some Benefits
By Nickolas Bakalar
Coffee is not usually thought of as health food, but a number of recent studies suggest that it can be a highly beneficial drink. Researchers have found strong evidence that coffee reduces the risk of several serious ailments, including diabetes, heart disease and cirrhosis of the liver. Among them is a systematic review of studies published last year in The Journal of the American Medical Association, which concluded that habitual coffee consumption was consistently associated with a lower risk of Type 2 diabetes. Exactly why is not known, but the authors offered several explanations.
Coffee contains antioxidants that help control the cell damage that can contribute to the development of the disease. It is also a source of chlorogenic acid, which has been shown in animal experiments to reduce glucose concentrations. Caffeine, perhaps coffee’s most famous component, seems to have little to do with it; studies that looked at decaffeinated coffee alone found the same degree of risk reduction.
Larger quantities of coffee seem to be especially helpful in diabetes prevention. In a report that combined statistical data from many studies, researchers found that people who drank four to six cups of coffee a day had a 28 percent reduced risk compared with people who drank two or fewer. Those who drank more than six had a 35 percent risk reduction.
Some studies show that cardiovascular risk also decreases with coffee consumption. Using data on more than 27,000 women ages 55 to 69 in the Iowa Women’s Health Study who were followed for 15 years, Norwegian researchers found that women who drank one to three cups a day reduced their risk of cardiovascular disease by 24 percent compared with those drinking no coffee at all. But as the quantity increased, the benefit decreased. At more than six cups a day, the risk was not significantly reduced. Still, after controlling for age, smoking and alcohol consumption, women who drank one to five cups a day — caffeinated or decaffeinated — reduced their risk of death from all causes during the study by 15 to 19 percent compared with those who drank none.
The findings, which appeared in May in The American Journal of Clinical Nutrition, suggest that antioxidants in coffee may dampen inflammation, reducing the risk of disorders related to it, like cardiovascular disease. Several compounds in coffee may contribute to its antioxidant capacity, including phenols, volatile aroma compounds and oxazoles that are efficiently absorbed. In another analysis, published in July in the same journal, researchers found that a typical serving of coffee contains more antioxidants than typical servings of grape juice, blueberries, raspberries and oranges. “We were surprised to learn that coffee quantitatively is the major contributor of antioxidants in the diet both in Norway and in the U.S.A.,” said Rune Blomhoff, the senior author of both studies and a professor of nutrition at the University of Oslo.
The same anti-inflammatory properties may explain why coffee appears to decrease the risk of alcohol-related cirrhosis and liver cancer. This effect was first observed in 1992. Recent studies, published in June in The Archives of Internal Medicine, confirmed the finding.
Still, some experts believe that coffee drinking, and particularly caffeine consumption, can have negative health consequences. A study published in January in The Journal of the American College of Cardiology, for example, suggests that the amount of caffeine in two cups of coffee significantly decreases blood flow to the heart, particularly during exercise at high altitude.
Rob van Dam, a Harvard scientist and the lead author of The Journal of the American Medical Association review, acknowledged that caffeine could increase blood pressure and slightly increase levels of the amino acid homocysteine, possibly raising the risk for heart disease. “I wouldn’t advise people to increase their consumption of coffee in order to lower their risk of disease,” Dr. van Dam said, “but the evidence is that for most people without specific conditions, coffee is not detrimental to health. If people enjoy drinking it, it’s comforting to know that they don’t have to be afraid of negative health effects.” (NY Times. August 15, 2006)
Exercise 6. In the following text the lines are mixed up. Put them in their proper order. The first and the last lines are in their correct places.
Decaf Coffee Plants Developed
By Sarah Graham
For many coffee lovers, their precious beverage comes with an unwanted ingredient: caffeine. As a result, which solvents flush caffeine from the beans. Their next step is to apply their technique to C. arabica plants, which produce the high-quality Arabica coffee that accounts for 70 percent of the world market. Of course, it remains to percent less caffeine than regular plants do.
Three enzymes are involved in making caffeine in coffee plants. Researchers at the Nara Institute of Science the gene controlling one of these enzymes--theobromine synthase, or CaMXMT1--was repressed. Compared with processes have been developed to remove the compound, although current methods are expensive and
regular plants, leaves from one-year-old GM plants exhibited a 50 to 70 percent reduction in caffeine content.
from the plant. Researchers report today in the journal Nature that their genetically modified coffee plants have 70 apart from their low caffeine content at maturity."
According to the report, "the transgenic plants described here should yield coffee beans that are essentially normal
The scientists note that their technique could sidestep some of the problems of industrial decaffeination, in and Technology in Japan led by Shinjiro Ogita engineered seedlings of Coffea canephora in which expression of sometimes compromise flavor. But scientists may have come up with a way to get decaffeinated coffee straight
be seen if java lovers will embrace "GM joe." (From Scientific American Online, June 19, 2003)
Exercise 7. Make up a list of the 10 key facts about coffee. Agree on the final list of facts with the whole group. Then summarize everything you now know about coffee into one report.
Unit 9. Human Genetics and Diversity
I am the family face;
Flesh perishes, I live on,
Projecting trait and trace
Through time to times anon,
And leaping from place to place
Over oblivion.
Thomas Hardy
From a drop of water... a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it.
Sir Arthur Conan Doyle A Study in Scarlet
Exercise 1. What do you know about genetics? Explain the following terms in English:
| DNA, RNA Nucleic acid Gene Genome Allele Dominant / recessive gene | Ribosome Replication Transcription Translation Processing Splicing | Phenotype Genotype Enhancer Promoter Silencer Terminator | Selective breeding Hybrid Breeding (crossing) Inbreeding Pure line |
Exercise 2. Discuss the following questions about genetic diversity of humans:
Exercise 3. Read the following two texts (Text A and Text B) about current genetic research of population diversity to check your answers in Exercise 2.
Text A. How Our Genomes Control Diversity
Two research efforts have determined DNA recombination mechanisms that underlie population diversity, how it happens and where in the genetic code it occurs
By Nikhil Swaminathan
Two recent discoveries have shed new light on the source of diversity in the human population. In one study, scientists examined patterns in DNA recombination, the process by which a person's genome is consolidated into one set of chromosomes to pass onto an offspring. In the other, a link was made between variants of a particular gene and the extent to which DNA recombination occurs.
In human testes and ovaries, where sperm cells and egg cells, respectively, are manufactured, sections of chromosomes inherited from a person's parents are shuffled together to create a collage of genetic material that is passed to offspring. This process by which a new, unique set of chromosomes is created (with a mix of roughly half the material coming from each parent) is called DNA recombination and is the source of variation in populations. "Recombination impacts population diversity," says George Coop, a postdoctoral fellow in human genetics at the University of Chicago and co-author of an article that details variation in the pattern in which genes are shuffled from individual to individual. "Recombination is the way that you generate novel haplotypes, novel combinations of mutations." (Haplotypes are combinations of different versions of genes on a single chromosome that are inherited as a unit.)
Coop and colleagues in Science reveal the results of a high-resolution study designed to map the locations where recombination occurs—where one parent's genes have been swapped out for another. Using a population of 725 Hutterites—communal farmers who settled in the Dakotas and Montana in the mid-19th century—the team scanned genomes for 500,000 single-nucleotide polymorphisms (SNPs). SNPs mark points of genetic variation to estimate where DNA shuffles occurred. Researchers can tell which part of a child's genetic code came from which of its four grandparents by comparing variants in both.
The researchers noted nearly 25,000 total recombination events in analyses of 364 offspring. Excluding the sex chromosomes, the team found that eggs typically showed 40 instances of recombination on each of their chromosomes, whereas the chromosomes in sperm are typically made with 26 recombinational occurrences. The University of Chicago team also noted that as women age, more recombination takes place during meiosis (the cellular process that produces an egg). In men, there is no age effect. Further, they noted that such incidents tended to focus on so-called "hot spots," locations where this crossover takes place often. Some turned out to be gender specific, with females utilizing some recombination regions more often than males (and vice versa). The usage of these zones of frequent recombination varied between individuals, but it seemed to be conserved among families, indicating that the extent and pattern of recombination may be inherited.
Interestingly, a finding out of the Icelandic biotech firm deCODE genetics, also appearing in Science, sheds light on that last observation. From a genome-wide analysis looking at 300,000 SNPs in 20,000 people, deCODE scientists were able to find two locations on a gene found on chromosome 4 and link variations at those two locales to the recombination rate. "What's interesting about the SNPs is that the variants have opposite effects on the sexes," says deCODE's chief executive officer Kari Stefansson. According to the new study, one of the locations on the gene, known as RNF212, is associated with high rates of recombination in men, but low rates in women; for the other marker, the gender effect is reversed. "If you were going to design a mechanism to keep rates within [certain] limits you would do exactly this," Stefansson explains about the gender paradigm. "For one generation, it leads to higher recombination rate; for the next generation, it would lead to a lower recombination rate." Overall, the two positions can account for 22 percent of the variability in a man's recombination rate and 6.5 percent of the variability in a female's, the study says.
Chicago's Coop lauded the deCODE efforts, noting that this was the first mapping of a gene that influences recombination in mammals. "I would imagine that the variation that we see in individuals is in part caused by these SNPs," he says. "I think this represents a big step forward in determining the events of human recombination." (From Scientific American Online, February 5, 2008)
Text B. Ethnic Differences Traced to Variable Gene Expression
Finding could explain why ethnic groups suffer from particular common diseases
By Nikhil Swaminathan
Tay-Sachs disease seems to favor Jews of Eastern European descent. Cystic fibrosis has an affinity for Caucasians. Type 2 diabetes strikes Latin Americans and people of African descent more often than it does those of other ethnic groups, appearing at rates of incidence that are 90 and 60 percent higher, respectively, than in Caucasians. Researchers have been conducting studies such as the International HapMap Project--a global effort to catalogue common single-nucleotide variations, such as the addition, deletion or substitution of a base in the code of a gene--to get to the bottom of long-observed correlations between ethnicity and common complex diseases.
But those efforts have borne little fruit, according to Vivian Cheung, a human geneticist at the University of Pennsylvania School of Medicine. So, rather than characterize these individual nucleotide changes in genes, Cheung and geneticist Richard Spielman employed microarray technology--essentially a genome chip that allows a researcher to analyze the expression of many genes at once--to study across Chinese, Japanese and European populations many different traits that are coded for in a type of white blood cell.
Their results, reported in this week's Nature Genetics, were that different ethnic groups not only carried different genes, but there were greater disparities than previously believed in the degrees to which genes that were the same among ethnic groups were expressed. Further, the genes themselves did not control the levels of their own expression, rather noncoding regions adjacent to them determined whether to ratchet up or down the proteins or other functional end products the genes encoded.
The authors of the new study note that large-scale changes to DNA--such as specific substitutions or deletions of genetic material--almost certainly also contribute to differences between ethnic groups. But Cheung says that expression levels likely can explain some of the ethnic underpinnings of Tay-Sachs and cystic fibrosis as well as hypertension, which plagues those of Afro-Caribbean descent at a higher rate than other populations.
From its microarray, the team measured 4,197 genes expressed by cells. (After measuring expression levels of those genes, Cheung, Spielman and their colleagues decided to lump the Japanese and Chinese groups together due to similar results.) When the researchers then compared the Asian populations with the Caucasian sampling, they noted that 1,097, more than 25 percent, of the genes had differing expression levels.
After analyzing some of the nearly 1,100 genes in detail, Cheung and Spielman believe that the expression level discrepancies were due to nucleotide differences in noncoding regions around the genes, and not the genes themselves. "We were able to pinpoint 11 genes where people have different forms of the regulator," Cheung reveals, providing an example: "Let's say that among the Caucasian population, maybe the regulator that turns on the gene more happens to be more frequent--overall the expression level of that gene will be higher. Whereas in the Asian population, more people have the regulator that causes the expression level to be lower."
Steve McCarroll, a population and medical geneticist at the Massachusetts Institute of Technology's Broad Institute says that with so many genetic variants out there, researchers need all the help they can get determining which ones actually will affect cell function. "One of the things that's exciting about this work is that identifying the genetic variants that account for gene expression differences could help the field to find those genetic variants that affect disease risk," he says. (From Scientific American Online, January 9, 2007)
Exercise 4. Who are the following scientists mentioned in the articles? What studies have they carried out?
• Kari Stefansson • George Coop
• Vivian Cheung and Richard Spielman • Steve McCarroll
Exercise 5. Using the information from the texts prove that:
Exercise 6. Put the sentences given below (a-e) into their correct place in the text (1-5).
Ancient Europeans More Diverse, Genetically Speaking, than Modern Ones
Bubonic plague may be responsible for reducing the genetic diversity of present-day Britons
By David Biello
Modern Britons are a cosmopolitan bunch. Peoples from across the globe now make the island home, bringing with them, theoretically, a diverse array of genes. (1) _____ Molecular ecologist Rus Hoelzel of Durham University in England and his European colleagues compared the genetic make up of six English ancestors from the Roman period, 25 from early in the Saxon conquest and 17 from the late Saxon period with the mitochondrial DNA sequences of more than 6,000 modern Europeans and Middle Easterners. "We found higher mitochondrial DNA diversity in ancient England (Roman to Saxon times) than in either modern England or in a combination of northern European countries," Hoelzel says. (2) _______
Even when present-day Europeans were broken down into 10 smaller samples of 48 individuals each, they still were less diverse than their ancestors. (3) ______; 6.3 percent of ancients carried it compared with nearly 22 percent of modern Britons and an average of nearly 19 percent of all Europeans.
The CRS haplotype imparts no known special traits in the humans who bear it, but the "black death" may have played a role in increasing its abundance in the modern population. (4) _____ The dread disease could either have increased the proportion of certain haplotypes somehow associated with increased survival or simply led to the extinction of rare haplotypes in families or villages who had a particular susceptibility, Hoelzel says.
Only three small groups—modern peoples from Belarus, Palestine and Turkey—showed similar levels of diversity to these early ancestors, though studies by others have shown that southern Europeans, such as Italians, seem to retain a broader pool of genetic material. (5) ______, according to researchers, the ancients possessed a more robust array of differing genetic stocks. (From Scientific American Online, August 1, 2007)
(a) The 48 ancients bore 36 different haplotypes—a set of variations in the genetic code. But their descendants more commonly carried one particular haplotype, known as Cambridge reference sequence (CRS)
(b) It is unclear exactly why contemporary Europeans are less genetically diverse despite a continuing influx of new populations but,
(c) This bubonic plague swept Britain (and all of Europe) in the 14th century, killing as much as half of the population, before recurring again in London in the 17th century.
(d) “Modern human populations are highly diverse, just less so in northern Europe, at least, than the ancient populations in England."
(e) But comparing the genetic material of more than 1,000 contemporary Englishmen with that of 48 of their ancient peers reveals that the ancients had even more diverse genetic codes.
Exercise 7. In the following text the lines are mixed up. Put them in the correct order.
Genetic Study Reveals Similarities between Diverse Populations
By Sarah Graham
genetically very similar, researchers say. A report published today in the journal Science suggests that 93 to 95 from differences in a very small proportion of genetic traits."
people from several continents, suggesting that only a tiny fraction of genetic traits are distinctive to specific percent of human genetic variation exists among individuals within populations, while differences among major Marcus W. Feldman of Stanford University and his colleagues analyzed samples from 1,056 people belonging to alone can provide enough information to group people by population.
52 populations. Specifically, they looked at 377 so-called microsatellites, short segments of DNA that occur in Though they may speak different languages and eat distinct foods, people from far-flung geographical locations are populations. This means that visible differences between human groups--such as skin color and skull shape--result
groups make up less than 5 percent of the variation. But the findings also reveal that even these tiny differences
specific patterns. "Each microsatellite had between four and 32 distinct types," Feldman says. "Most were found in
(From Scientific American Online, December 20, 2002)
Exercise 8. Summarize everything you know about population diversity and genetic mechanisms involved into one report.
Unit 10. Animal Diversity
Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation, nor on which side thereof an intermediate form should lie.
Aristotle
Exercise 1. What do you know about diversity of animals and their adaptations?
1. Why are animals classified into a separate Kingdom?
2. What are the basic taxonomic differences between the main classes of animals?
3. What basic adaptations have animals developed to different environments where animal live?
4. What environments can animals never adapt to? Why? Are there other organisms that can live there?
5. What is the role of species diversity in the stability of ecosystem?
6. What role do parasites and predators play in keeping biodiversity?
Exercise 2. You are going to read a text about bird migrations. The following figures will be used in the text. What do you think they refer to?
• 30,000 km • 100 hours • 500 beats • 9000 m • 21 grams
Exercise 3. Now read the text to check your answers.
Have Wings, Will Travel: Avian Adaptations to Migration
By Mary Deinlein
Avian Aeronautics
Flight affords the utmost in mobility and has made possible the evolution of avian migration as a means of exploiting distant food resources and avoiding the physiological stress associated with cold weather. Variations in the patterns of migration are nearly as numerous as the birds that migrate. While some species move only a few kilometers up and down mountain slopes, others will travel hundreds or even thousands of kilometers, often traversing vast bodies of water or tracts of inhospitable terrain.
One record holder in long-distance travel is the Arctic Tern (Sterna paradisaea), which makes an annual round-trip of about 30,000 kilometers between opposite ends of the globe, from Arctic breeding grounds to Antarctic seas. This feat is possible because terns are adapted for feeding at sea, allowing them to refuel en route. Even more amazing are the aerial voyages of the landbirds and shorebirds whose transoceanic flights must be accomplished non-stop. The Pacific Golden-Plover (Pluvialis fulva) flies continuously for more than 100 hours to travel the 5,000- to 7,000-kilometer distance from northern Siberia and Alaska to Hawaii and other islands in the Pacific Ocean.
The Blackpoll Warbler's (Dendroica striata) over-water flight from the coast of New England or southern Canada to South America keeps it aloft for 80 to 90 continuous hours over a distance of 3,000 to 4,000 kilometers, an effort which researchers Tim and Janet Williams conclude "requires a degree of exertion not matched by any other vertebrate; in man the metabolic equivalent would be to run a 4 minute mile for 80 hours. Even the tiny Ruby-throated Hummingbird (Archilochus colubris), weighing only about as much as a penny, makes the 1,000-kilometer, 24-hour spring flight across the Gulf of Mexico from the Yucatan Peninsula to the southern coast of the United States.
So how do they do it? What specialized adaptations allow birds to accomplish such prodigious feats of endurance?
Bird Basics
To understand how superbly adapted birds are to their highly mobile way of life, one must first consider the quintessential characteristics that distinguish birds from all other animals. Feathers, the trademark of the Class Aves, provide the insulation necessary to maintain a high "engine" (body) temperature, ranging from 107 to 113 degrees F across species. Additionally the long feathers of the wings act as airfoils which help generate the lift necessary for flight. Well-developed pectoral muscles power the flapping motion of the wings. A streamlined body shape and a lightweight skeleton composed of hollow bones minimize air resistance and reduce the amount of energy necessary to become and remain airborne.
Keeping the hard-running avian engine running smoothly requires super-efficient circulatory and respiratory systems. Birds have a large, four-chambered heart which proportionately weighs six times more than a human heart. This, combined with a rapid heartbeat (the resting heart rate of a small songbird is about 500 beats per minute; that of a hummingbird is about 1,000 beats per minute) satisfies the rigorous metabolic demands of flight. The avian respiratory system—the most efficient in the animal kingdom—consists of two lungs plus special air sacs, and takes up 20% of a bird's volume compared to 5% in a human. Unlike mammalian or reptilian lungs, the lungs of birds remain inflated at all times, with the air sacs acting as bellows to provide the lungs with a constant supply of fresh air.
Migratory Mania
In addition to these general avian characteristics, migratory birds exhibit a suite of specialized traits. Migrants generally have longer, more pointed wings than non-migratory species, a feature which further minimizes air resistance. Also, the pectoral muscles of migrants tend to be larger and composed of fibers which are more richly supplied with nutrient- and oxygen-carrying blood vessels and energy-producing mitochondria, making the pectoral muscles of migrants especially efficient at energy production and use.
Many migrants face the additional challenge of flying at high altitudes. Most songbirds migrate at 500 to 2,000 meters, but some fly as high as 6,800 meters; swans have been recorded at 8,000 meters and Bar-headed Geese (Anser indica) flying over the Himalayas at 9,000 meters. Accounting for their ability to withstand the low levels of oxygen available at such altitudes, the blood of migratory birds is characterized by two specialized adaptations. The oxygen-carrying capacity of the blood is enhanced by a high concentration of red blood cells. Secondly, instead of one form of hemoglobin in the red blood cells as is typical in non-migrants and other classes of vertebrates, some migratory birds possess two forms of hemoglobin which differ in their oxygen carrying and releasing capacities. This guarantees an adequate oxygen supply over a wide range of altitudes and allows birds to adapt rapidly to varying levels of oxygen availability.
Preparing for take-off
Migrants change rapidly into a "superbird state" in preparation for migration. This transformation is triggered by an internal annual "clock," which is set by day length and weather.
When it comes to fueling migration, fat is where it's at. Fat is not only lighter and less bulky than carbohydrates or protein, but also supplies twice as much energy. Not surprisingly, then, preparation for migration entails a rapid weight gain program geared to increasing fat reserves. This program combines both behavioral and physiological changes. A dramatic increase in appetite and food consumption, termed hyperphagia, begins about two to three weeks before migration and persists throughout the migratory period. Accompanying this veritable feeding frenzy is an increase in the efficiency of fat production and storage. As a result, a migratory bird can increase its body weight through fat deposition by as much as 10% per day (usually 1-3%). Additionally, in birds that are in migratory disposition, the pectoral muscles become larger and well supplied with enzymes necessary for the oxidation, or "burning," of fat.
Longer migration distances require greater amounts of fat. Non-migratory passerines maintain a "fat load" of about 3-5% of their lean body weight. In preparation for migration, short- and medium-distance migratory songbirds attain a fat load of between 10 and 25%, while long-distance migrants reach fat loads of 40 to 100%. Maximum fat loads are attained just prior to flights over major topographic barriers, such as deserts, high mountains, or large bodies of water. A typical Blackpoll Warbler at the end of its breeding season weighs about 11 grams, equivalent to the weight of four pennies. In preparing for its transatlantic trek, it may accumulate enough fat reserves to increase its body weight to 21 grams.
Readiness for migration entails other behavioral modifications. Before migrating in the fall, many migrants which ordinarily eat insects will switch to a diet of berries and other fruits. At this time when food intake needs are increasing and insect numbers are decreasing, fruits are abundant and high in carbohydrates and lipids which are readily converted to fat. Many migrants that typically are not gregarious will flock together prior to, or during, migration. This social behavior may result in improved predator avoidance, food finding, and orientation. Some species also fly in formation, a strategy that improves aerodynamics and reduces energy expenditure.
A radical shift from being active exclusively during the day to migrating at night occurs in many species during migration, including most shorebirds and songbirds. Possible advantages to flying at night include decreased vulnerability to predators, reduced threat of dehydration or overheating, a greater likelihood of encountering favorable winds and a stable air mass (rising hot air and more variable wind directions occur during the daytime), and time during the day to forage.
Migratory birds kept in captivity exhibit behavior termed Zugunruhe, or migratory restlessness. This behavior, characterized by rapid fluttering of the wings while perching, begins at the same time that conspecifics (individuals of the same species) in the wild are setting off on migration, and persists for the same length of time required for the wild counterparts to complete their migration. The captive birds even orient themselves in the appropriate direction in which they would be migrating. Over the past 15 years, this behavior has allowed researchers to demonstrate experimentally that many of the important physical and behavioral correlates to migration are under at least partial genetic control. For instance, when migratory Blackcaps (Sylvia atricapilla) were mated with non-migratory individuals of the same species, 30% of the offspring exhibited Zugunruhe. When individuals which displayed high levels of Zugunruhe, consistent with their long migratory routes, were bred with conspecifics with short migration routes, the offspring displayed intermediate levels of Zugunruhe. The results from these and other cross-breeding experiments support the hypothesis that migration and its associated patterns—such as distance and timing—are inherited traits, at least in some species. These experiments apply to species with relatively fixed migration routes. Many species have facultative migration patterns, moving only when food supply is low, or when weather turns bad. Research has shown that access to food for these species greatly affects Zugunruhe.
Despite this advanced understanding of some of the mechanisms behind avian migrations, the annual odysseys of billions of birds remain one of the most mysterious and amazing phenomena in the animal world.
Exercise 4. What bird species are mentioned in the text? What facts are given about each of them?
Exercise 5. Give detailed answers to the following questions using the information from the text:
1. Why do birds migrate?
2. What adaptations allow birds to accomplish their migrations? Describe each in detail.
3. How are migratory birds different from non-migratory species?
4. How do birds prepare for the hardships of migration?
5. What is Zugunruhe?
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| Unit 4. Bacteria | | | Exercise 6. In the following text the paragraphs are mixed. Put them in the correct logical order. The first paragraph is in its right place. |