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?

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.

How do deep-diving sea creatures withstand huge pressure changes?

Paul J. Ponganis and Gerald L. Kooyman of the Center for Marine Biotechnology and Biomedicine at Scripps Institution of Oceanography provide the following answer.

A sperm whale can dive down more than 2,000 meters and can stay submerged for up to an hour.

(A) Some sea creatures exploit great depths. The biggest physiological challenges in adapting to pressure are probably faced by those animals that must routinely travel from the surface to great depth. Two such animals are the sperm whale and the bottlenose whale. From the days of whaling, these animals have been recognized as exceptional divers, with reports of dives lasting as long as two hours after they were harpooned. Today, with the use of sonar tracking and attached time-depth recorders, dives as deep as 6,000 feet (more than a mile below the surface of the ocean) have been measured. Routine dive depths are usually in the 1,500- to 3,000-foot range, and dives can last between 20 minutes and an hour.

(B) Loss of gas exchange at depth has another important implication: the lungs of the deep diver cannot serve as a source of oxygen during the dive. Instead deep-diving whales and seals rely on large oxygen stores in their blood and muscle. Several adaptations enable this. First, these animals have mass specific blood volumes that are three to four times those found in terrestrial mammals (i.e., 200 to 250 milliliters of blood per kilogram body mass, in contrast to a human value of 70 milliliters blood per kilogram). Second, the concentration of hemoglobin (the oxygen-transport protein in blood) is also elevated to a level about twice that found in humans. Third, the concentration of myoglobin, the oxygen storage protein in muscle, is extremely elevated in these animals, measuring about 10 times that in human muscle.

(C) In summary, the primary anatomical adaptations for pressure of a deep-diving mammal such as the sperm whale center on air-containing spaces and the prevention of tissue barotrauma. Air cavities, when present, are lined with venous plexuses, which are thought to fill at depth, obliterate the air space, and prevent "the squeeze." The lungs collapse, which prevents lung rupture and (important with regard to physiology) blocks gas exchange in the lung. Lack of nitrogen absorption at depth prevents the development of nitrogen narcosis and decompression sickness. In addition, because the lungs do not serve as a source of oxygen at depth, deep divers rely on enhanced oxygen stores in their blood and muscle.

(D) Diving to depth can result in mechanical distortion and tissue compression, especially in gas-filled spaces in the body. Such spaces include the middle ear cavity, air sinuses in the head, and the lungs. Development of even small pressure differentials between an air cavity and its surrounding tissue can result in tissue distortion and disruption—a condition in human divers known as "the squeeze." In some species of cetaceans, the middle ear cavity is lined with an extensive venous plexus, which is postulated to become engorged at depth and thus reduce or obliterate the air space and prevent development of the squeeze. Cetaceans also have large Eustachian tubes communicating with the tympanic cavity of the ear and the large pterygoid sinuses of the head. These air sinuses of the head have an extensive vasculature, which is thought to function in a manner similar to that of the middle ear and facilitate equilibration of air pressure within these spaces. Lastly, most marine mammals lack frontal cranial sinuses like those present in terrestrial mammals.

(E) Collapse of the lungs forces air away from the alveoli, where gas exchange between the lungs and blood occurs. This blunting of gas exchange is important in the deep diver because it prevents the absorption of nitrogen into the blood and the subsequent development of high blood nitrogen levels. High blood nitrogen pressures can exert a narcotic effect (so-called nitrogen narcosis) on the diver. It may also lead to nitrogen bubble formation during ascent—a phenomenon known as decompression sickness or "the bends." Collapse of the lungs in the deep diver avoids these two problems.

(F) Another organ susceptible to compression damage is the lung. In deep-diving whales and seals, the peripheral airways are reinforced, and it is postulated that this allows the lungs to collapse during travel to depth. Such collapse has been observed radiographically and confirmed with blood nitrogen analyses in the deep-diving Weddell seal.

Exercise 7. Make up a list of the 10 key terms used in the text, then agree with the whole group on the final list. Retell the article using these terms.

Exercise 8. Prepare your own report about adaptations of other species to their habitat and lifestyle.