A disease is a condition that impairs the proper function of the body or of one of its parts. Every living thing, both plants and animals, can succumb to disease. People, for example, are often infected by tiny bacteria, but bacteria, in turn, can be infected by even more minute viruses.

Hundreds of different diseases exist. Each has its own particular set of symptoms and signs, clues that enable a physician to diagnose the problem. A symptom is something a patient can detect, such as fever, bleeding, or pain. A sign is something a doctor can detect, such as a swollen blood vessel or an enlarged internal body organ.

Every disease has a cause, although the causes of some remain to be discovered. Every disease also displays a cycle of onset, or beginning, course, or time span of affliction, and end, when it disappears or it partially disables or kills its victim.

Endemic disease (also called childhood disease), disease continually prevalent in a region.

An epidemic disease is one that strikes many persons in a community. When it strikes the same region year after year it is an endemic disease.

An acute disease has a quick onset and runs a short course. An acute heart attack, for example, often hits without warning and can be quickly fatal. A chronic disease has a slow onset and runs a sometimes years-long course. The gradual onset and long course of rheumatic fever makes it a chronic ailment.

How Germs Invade the Body

Humans live in a world where many other living things compete for food and places to breed. The pathogenic organisms, or pathogens, often broadly called germs, that cause many diseases are able to invade the human body and use its cells and fluids for their own needs. Ordinarily, the body’s defence system can ward off these invaders.

Pathogenic organisms can enter the body in various ways. Some such as those that cause the common cold, pneumonia, and tuberculosis are breathed in. Others such as those that cause venereal diseases enter through sexual contact of human bodies. Still others such as those that cause bacillary dysentery, cholera, and typhoid fever get in the body through contaminated food, water, or utensils.

Insects can spread disease by acting as vectors, or carriers. Flies can carry germs from human waste or other tainted materials to food and beverages. Germs may also enter the body through the bite of a mosquito, louse, or other insect vector.

Kinds of Disease

Infectious, or communicable, diseases are those that can be passed between persons such as by means of airborne droplets from a cough or sneeze. Tiny organisms such as bacteria and fungi can produce infectious diseases. So can viruses. So can tiny worms. Whatever the causative agent might be, it survives in the person it infects and is passed on to another. Or, its eggs are passed on. Sometimes, a disease-producing organism gets into a person who shows no symptoms of the disease. The asymptomatic carrier can then pass the disease on to someone else without even knowing he has it.

Non-infectious, or non-communicable, diseases are caused by malfunctions of the body. These include organ or tissue degeneration, erratic cell growth, and faulty blood formation and flow. Also included are disturbances of the stomach and intestine, the endocrine system, and the urinary and reproductive systems. Some diseases can be caused by diet deficiencies, lapses in the body’s defence system, or a poorly operating nervous system.

Disability and illnesses can also be provoked by psychological and social factors. These ailments include drug addiction, obesity, malnutrition, and pollution-caused health problems.

Furthermore, a thousand or more inheritable birth defects result from alternations in gene patterns. Since tiny genes are responsible for producing the many chemicals needed by the body, missing or improperly operating genes can seriously impair health. Genetic disorders that affect body chemistry are called inborn errors of metabolism. Some forms of mental retardation are hereditary.

HOW THE BODY FIGHTS DISEASE

Mucous membrane (or mucosa), membrane that secretes mucus and lines the mouth, nose, throat, windpipe, lungs, eyelids, and the alimentary canal.

As a first line of defence, a healthy body has a number of physical barriers against infection. The skin and mucous membranes covering the body or lining its openings offer considerable resistance to invasion by bacteria and other infectious organisms. If these physical barriers are injured or burned, infection resistance drops. In minor cases, only boils or pimples may develop. In major cases, however, large areas of the body might become infected.

Cilia (plural of cilium), hairlike, vibratory appendages found in some plants and animals.

Breathing passages are especially vulnerable to infection. Fortunately, they are lined with mucus-secreting cells that trap tiny organisms and dust particles. Also, minute hairs called cilia line the breathing passages, wave like a field of wheat, and gently sweep matter out of the respiratory tract. In addition, foreign matter in the breathing passages can often be ejected by nose blowing, coughing, sneezing, and throat clearing. Unfortunately, repeated infection, smoking, and the repeated use of strong chemicals (including alcohol and drugs) can damage the respiratory passageways and make them more susceptible to infection.

Scavenger cells are present too in the walls of the bronchi, the branched air tubes to the lungs. Foreign matter reaching the bronchi after evading the other defences can be “eaten” by the scavengers and disposed of in the lymph glands of the lungs.

Many potential invaders cannot stand body temperature (98.6° F or 37° C). Even those that thrive at that temperature may be destroyed when the body assumes higher, fever temperatures.

Wax in the outer ear canals and tears from eye ducts can slow the growth of some bacteria. And stomach acid can destroy certain swallowed germs.

Lymph, a colourless liquid exuded through capillaries to nourish tissues of the body.

The body’s second line of defence is in the blood and lymph. Certain white blood cells flock to infected areas and try to localize the infection by forming pus-filled abscesses. Unless the abscess breaks and allows the pus to drain, the infection is likely to spread. When this happens, the infection is first blocked by local lymph glands. For example, an infection in the hand travels up the arm, producing red streaks and swollen, tender lymph glands in the armpit. Unless the infection is brought under control, it will result in blood poisoning.

Scavenger cells, or phagocytes, are located at various sites to minimize infection. One type in the spleen and liver keeps the blood clean. Others in such high-risk areas as the walls of the bronchi and the intestines remove certain bacteria and shattered cells.

How We Become Immune to Disease

The body has a special way of handling infection. It has a system that fends off the first traces of an infectious substance and then, through a “memory,” gives the body a long-lasting immunity against future attacks by the same kind of invader.

Antigen, a substance in blood that causes production of antibodies against itself.

Antibody, the protective substance produced in body fluids in response to exposure to foreign antigen in blood.

Many substances could harm the body if they ever entered it. These substances, or antigens, range from bacteria and pollen to a transplanted organ (viewed by the body as an invader). To fight them the body makes special chemicals known as antibodies.

Antibodies are a class of proteins called immuno-globulins. Each antibody is made of a heavy chain of chemical subunits, or amino acids, and a light chain of them. The light chain has special sites where the amino acids can link with their complements on the antigen molecule. When an antibody hooks up with an antigen, it often puts the antigen out of action by inactivating or covering a key portion of the harmful substance. In some cases, through the process of opsonization, antibodies “butter” the surface of some antigens and make them “tastier” to phagocytes, which engulf the antigens. Sometimes an antibody hooks to a bacterial antigen but needs an intermediate, or complement, to actually destroy the bacterium. As the antibody-antigen complex circulates in the blood, the complex “fixes” complement to it. In turn, the complement causes powerful enzymes to eat through the bacterial cell wall and make the organism burst.

There are several kinds of immuno-globulins IgM, the largest; IgG, the most plentiful and versatile; and IgA, the next most plentiful and specially adapted to work in areas where body secretions could damage other antibodies. Other immuno-globulins are tied in with allergic reactions. IgM is made at the first signs of an antigen. It is later supplanted by the more effective IgG.

When infection first strikes, the immunity system does not seem to be working. During the first day or so, antibodies against the infection cannot be found in the blood. But this is only because the basic cells involved in antibody production have been triggered by the presence of antigen to multiply themselves. The antibody level starts to rise on about the second day of infection and then zooms upward. By the fifth day the antibody level has risen a thousandfold.

The first antibodies, the large IgM type, are not the best qualified to fight a wide range of antigens, but they are particularly effective against bacteria. The more versatile IgG is circulating in the blood on about the fourth day of infection. Its production is stimulated by the rising level of IgM in the blood. At this time, IgM production drops off and the immunity system concentrates on making IgG. The IgG type of antibody sticks well to antigens and eventually covers them so that the antigens can no longer stimulate the immune response and IgG production is switched off. This is an example of negative feedback control.

Antibody Production

Thymus, organ, located behind the breastbone and above the heart; participates in the production of white blood cells or lymphocytes; because it attains maximum size at puberty and becomes smaller in adults, researchers feel it may be an endocrine gland that affects growth and sexual maturation.

Antibodies are made by two kinds of cells plasma cells and a class of white blood cells, lymphocytes. Plasma cells actually originate from lymphocytes and are found throughout the lymphatic tissue. Lymphocytes stem from cells in the blood-forming sections of bone marrow. When the bone-marrow cells circulate to the thymus, a lymphatic structure in the chest, they receive “orders” to become lymphocytes and make antibodies. Most lymphocytes last for only a few hours, but a few wander through the blood and body tissues for years. These lymphocytes are responsible for “remembering” old antigens and for inducing the immunity system to produce antibodies against those or similar antigens if they ever again enter the body.

When people develop antibodies against a disease by the action of their own immunity system, they have active immunity. When they are given someone else’s antibodies, however, they just have passive immunity to a disease.

Passive immunity is only temporary. Some people may also get temporary relief from a disease through injections of serum containing gamma globulin, a portion of the blood rich in antibodies.

Without protective antibodies, we could die of the first disease that struck us. This would be true, too, of newborn babies, except that they receive passive immunity from their mothers. During her lifetime, a mother accumulates a wide variety of antibodies against a host of diseases. Enough of them are passed to the developing baby in her womb to give it a temporary immunity to many diseases during the early months of its life, until it can develop its own set of antibodies.

Other Metabolic Diseases

Gout is faulty metabolism of purine, an amino acid, resulting in the accumulation of uric acid in the blood and urate salts in the tissues, especially the joints where they cause painful arthritis. It may stem from an inborn error of metabolism or from other diseases. It usually strikes middle-aged men. The joint at the base of the big toe is the typical site of a sudden acute attack of gout. The affected joint becomes red, hot, swollen, and painful. Fever accompanies the attack. Joints of other limbs might become similarly affected. Attacks of gout recur, but the sufferer enjoys complete relief in between them. Some patients develop chronic arthritis from gout. Gout is treated with low-purine foods and such drugs as allopurinol that lower the uric acid level of the blood.

Cystic fibrosis, an inherited disease in which the pancreas fails to provide secretions necessary for normal digestion of food; commonly associated with chronic lung disease.

Cystic fibrosis is a genetic disorder involving the pancreas and the lungs. It appears during the first 10 years of life, although sometimes it is not discovered until later. Certain glands of the pancreas become plugged by thick mucus, which bottles important digestive enzymes. Intestinal troubles result.

Furthermore, the lungs suffer scarring, infection, and eventual emphysema. Cystic fibrosis is treated with substitute pancreatic enzymes, vitamins, and a high-calorie diet. Antibiotics are given to fight the lung troubles.

Other metabolic disorders include phenylketonuria (PKU) and galactosemia. PKU is an inherited inability to metabolize phenylalanine, an amino acid. Galactosemia is an inherited inability to change galactose, one type of sugar, into sucrose, another, because a necessary enzyme is missing. Both diseases can result in mental retardation of children if not corrected in time.

Arthritis and Lupus

Rheumatoid arthritis (RA), chronic disease of the connective tissue, causing painful sensations in joints and muscles.

When the body fails to recognize itself, it makes antibodies against its own tissues. Rheumatoid arthritis and systemic lupus erythematosus are two among a rising number of such autoimmune diseases.

Rheumatoid arthritis is a chronic crippling disease that deforms bone joints and their adjacent tissues. It can strike nearly anyone. Although arthritis is not especially prevalent in damp climates, its symptoms are more bothersome there. It is marked by inflammation of an entire joint, including its synovial lining. Tendon coverings and bursas, or fluid-filled cushions, can become inflamed too. Cartilage in the joint and adjacent bone are destroyed, causing painful stiffness and eventual ankylosis, or “freezing,” of the joint. Skin over the joint is taut, shiny, and clammy. Arthritics often suffer aches and pains. The rheumatoid factor, a large protein molecule, is present in the blood of so many adult patients that it aids in the diagnosis of the disease. Rheumatoid arthritis is usually treated with rest, physical therapy, and aspirin and other salicylates.

Systemic lupus erythematosus (SLE), or lupus, is a serious degenerative disease that can strike one or many body systems over a period of years. The blood serum of afflicted persons contains a number of peculiar proteins, including the so-called L.E. factor, the antibody characteristic of the disease. Symptoms of SLE resemble other diseases, including cancer and tuberculosis, but lesions around the nail beds and fingertips that destroy the skin in those areas earmark lupus. In addition, the spleen and lymph glands of the neck and armpits may enlarge. The pericardium and heart valves are affected too. The kidneys and portions of the central nervous system may also become damaged. Although anyone may be affected, females between the ages of 20 and 40 years most often develop this incurable disease.

Osteoarthritis is a painfully disabling disease of the spine and other weight-bearing joints. Cartilage in the joint is destroyed, followed by overgrowth of nearby bone. The incurable but non deforming disease develops with advancing age.

Ankylosing spondylitis is a disabling and deforming disease of the spine, sacroiliac joints, and sometimes the shoulders, hips, and knees. The synovial lining of the affected joint becomes inflamed, the bone is weakened by loss of calcium, and the spine is bent forward. Eventually, the spinal vertebrae fuse and the spine becomes locked in the deformed position.

KIDNEY AND GENITAL DISEASES

Disease can affect any of the parts of the closely related urinary and genital systems. Both can be infected or malfunction because of a shortcoming in development.

Kidney Inflammations

Kidney disease, commonly result of inflammation or damage to blood vessels of kidneys; severe forms lead to breakdown of normal elimination of waste products.

Glomerulonephritis is a serious inflammatory disease of the kidneys. It usually is triggered by a prior infection, often by streptococcal bacteria, which inflames the glomeruli, the tiny tufts through which blood is filtered. The inflammation may go away after a few weeks or may slowly destroy all the glomeruli. In the early stages, the inflammation may reduce filtration enough to cause blood to retain some excess fluid, salts, and wastes. Blood pressure might also rise. If the inflammation persists, the glomeruli are destroyed, blood pressure soars, and urine formation may stop. Mechanical means must be taken to cleanse the blood.

Pyelonephritis, bacterial infection of the inner portions of the kidneys and the urine.

Pyelonephritis is a bacterial infection of the inner portions of the kidneys and the urine. If quickly treated, the infection can be cured. If untreated, however, the infection may scar and eventually destroy kidney tubules, resulting in a need for mechanical cleansing of the blood. Once damaged by a bout of pyelonephritis, the kidneys are easily reinfected.

Toxaemia of pregnancy is a disorder stemming from other kidney problems experienced by some women in the last half of pregnancy. During a pregnancy, the kidneys must work more than usual. However, a woman entering pregnancy with a kidney disease such as Glomerulonephritis may not be able to step up kidney function enough to meet the new demands. In severe cases of toxaemia, the foetus may die or have to be aborted to save the mother’s life. In lesser cases, however, medical treatment poses little risk to either life. Once a woman develops toxaemia, she is likely to develop it again in later pregnancies.

Calculi and Other Urinary Disorders

Calculus disease, condition that occurs when certain substances in urine crystallize into compact stones.

Calculus disease occurs when certain substances in urine crystallize into compact stones called calculi. A stone may be formed within a kidney and become swept by urine into the ureters and the bladder. It may cause pain, obstruct urine flow, or grow large enough to damage the kidney or bladder. Small calculi may be passed in urine, and large ones can be pulverized without surgery by means of energetic sound waves. Calculi can consist of calcium, urates, cystine, or other crystals. The tendency to form kidney stones sometimes runs in families.

Polycystic disease, an inherited failure of normal kidney development, strikes infants as well as adults. Many fluid-filled cysts spring up throughout the kidneys and cause them to malfunction. Polycystic disease sufferers eventually become uraemic

Uraemia means “urine in blood.” It describes the condition in which the kidneys almost totally fail to operate. The blood then retains the nitrogenous products of protein metabolism instead of having them removed by the kidneys. Also, the concentration of many of the electrolytes, or salts, in the blood rises too high. The breath or perspiration of affected persons smells of urine. Each of the previously mentioned kidney ailments could cause uraemia Artificial kidneys have been developed to cleanse the blood of uraemic patients. In some cases, patients with destroyed kidneys can receive a human kidney transplant.

Genital Disorders

Sometimes portions of the genital system fail to develop normally. In some rare cases, the gonads male testes and female ovaries or other sex structures fail to develop at all. Without gonads, a person neither achieves puberty nor develops secondary sexual characteristics, such as breasts and uterus growth in females and penis growth and muscle development in males.

Infections such as gonorrhoea can cause sterility by blocking the oviducts, or egg passages, of females or the vas deferens, or sperm passages, of males. In males, gonorrhoea may also interfere with urination.

The prostate gland at the neck of the bladder in males enlarges slowly with age. It eventually may hamper urination and need surgical correction.

REHABILITATION

Rehabilitation is a fairly new medical speciality, although the notion of helping someone cope with a disabling disease or disorder is an old one. As an increasing number of people become disabled by stroke, paraplegia (paralysis of the lower body), limb losses, and many chronic nervous system and other physical disorders, it has been shown that medical rehabilitation can help many of them live a reasonably normal life. This is true even when the handicapping problem is not medically correctable.

Rehabilitation means getting utmost use from the limbs, senses, or other body systems that remain in operation after a chronic disability. Its goal is to help the patient become as independently active as possible. The disabling condition might result from a disease, birth defect, or severe accident. Sometimes rehabilitation involves fitting an amputee with an artificial limb, fitting a lame person with a brace, or teaching a paraplegic how to manoeuvre a wheelchair. Sometimes it only involves counselling and other psychological techniques for persons who are mentally disabled.

In its early days medical rehabilitation concentrated on helping people who had walking and other movement problems. The advances made in rehabilitating them sparked efforts to aid people who were stricken by stroke, chronic arthritis, and spinal cord disease and other chronic nervous system disorders. Afterwards, it was learned that rehabilitation could also help patients with heart disease, chronic lung disease, and a variety of conditions that slowed recovery from surgery. Bio engineers have been successful in devising artificial limbs and other life support structures that function so much like natural ones that recipients no longer suffer a disabling handicap.

Rehabilitation is a team effort. It requires the work and dedication of physicians, physical therapists, occupational therapists, psychologists, social workers, and vocational counsellors The doctor and the physical and occupational therapists work to restore those body functions impaired by the disability. The psychologist, social workers, and vocational counsellor help the patient get a mental grip on himself to better deal with the emotional and social problems brought on by the disability. Members of a coordinated rehabilitative team can do wonders in restoring a handicapped person to a functional life.

In addition, the rehabilitative team works with a disabled person to prevent the physical deterioration that takes place when muscles are not used. Furthermore, the team aims at getting the maximum output from the patient’s remaining body functions. Exercises and other means are used to develop fully the remaining physical reserves because disabled persons expend more energy and need more stamina to do ordinary things than do non disabled persons.

Teaching new tasks to the disabled is an integral part of physical rehabilitation. For example, a crippled person may be trained to use a wheelchair or other motive device well enough to manage into the driver’s seat of an auto mobile and thus achieve a measure of independence, the important goal of the entire rehabilitative process.

A chronically disabled person often suffers mental depression. The rehabilitation team tries to restore that person’s confidence so that he can take an optimistic view on resuming daily activities. Positive attitudes of patient, friends, and family toward any disability are important factors in the success of rehabilitation. During rehabilitation the patient is encouraged to find meaning in life, overcome feelings of being a “permanent patient,” and resume his place as an active member of society. Counselling is also important in rehabilitating alcoholics and the mentally ill.

Treatment for the disabled is given at special rehabilitation centres or in the rehabilitation departments of some hospitals. Rehabilitation units are designed so that patients can do many things by themselves; the quarters are built to simulate conditions the patients will encounter when discharged. As a consequence, patients get practice in dealing with such problems as opening and closing doors, going up and down stairs, and a host of other environmental situations that they will face when the rehabilitation program is over.

Orienting Behaviour

An animal orients by adjusting its posture and position in space. It does so in relation to the source of different forms of energy in its environment. These forms include light, heat, and chemicals in the air or water, pressure, electric current, air or water currents, gravity, radiation, and magnetic fields.

Tropism, involuntary turning of a cell or organism in response to a stimulus.

Orienting behaviour may take the form of a tropism an action in which the animal simply orients its body toward or away from the source of energy without changing location. Plants can also respond in this way. However, the orienting response may take the form of a taxis a movement toward or away from the source of energy by swimming, flying, or locomotion. As a rule, only animals are capable of such responses. Still another type of orienting response is called a kinesics an increase or decrease in an animal’s activity, but in no particular direction.

Prefixes are usually added to the root words tropism, taxis, and kinesics to indicate the kind of energy to which the organism is responding. For example, geotropism is response to gravity; photo taxis, response to light. Prefixes may also indicate the type of response made. Thus klinokinesis refers to turning activities. In addition, the direction or intensity of a response may be described as positive, if directed toward a stimulus, or negative, if directed away from it.

Orientation makes it possible for an animal to feed, to exhibit social behaviour, and to avoid obstacles and barriers. Some organisms, such as the bat, use sonar reflected sound to locate prey and to avoid obstacles. Some fish can navigate through tight crevices by detecting changes in their electric field. Electronic instruments enable researchers to detect and record the sound frequencies and electricity emitted by different species.

When foraging for food, the honeybee orients to the odour of flowers and the polarization of light. It also responds to cues from the sun’s position off the horizon. This type of activity is called sun compass orientation. On returning to the hive the bee performs certain “dances” a variety of motor patterns that vary with the distance and direction of the food. These dances stimulate the other bees to travel the path of the returning bee.

Fish and birds also exhibit compass orientation when homing or migrating. However, scientists are not sure that animals navigate in the same way as man. When humans navigate, they use such instruments as the sextant to find the altitude of the sun and stars and a chronometer for timekeeping. It has not yet been demonstrated that homing and migrating animals can “shoot an azimuth” and “tell time.”

Some animals are known to return to the areas where they were born or spawned. The salmon, for example, upon reaching sexual maturity responds to the chemical characteristics of the stream in which it was spawned. The hormonal changes associated with sexual maturity are a cause of this new sensitivity. The stickleback moves from salty to brackish water to reproduce. Its behaviour is related to endocrine gland responses to seasonal fluctuations in light. Similar hormonal changes in birds lead to migration and reproduction. These cyclic changes in behaviour due to hormonal regulation are considered evidence of a chronometer that might enable migrating or homing animals to correlate changes in visual cues during compass orientation with changes in internal rhythms and thus make navigation possible.

Social Behaviour

All living things relate to other members of their species. In an amoeba, the relationship occurs only during the short time it takes the animal to split into two animals. In other species, such as the social insects, the relationship is so necessary that they cannot survive as individuals. This is true also of humans, who are dependent on others until they reach maturity. Social organization of some kind is common to all animals. However, the type of organization varies with the nervous system of the species. And in true social organization, animals of the same species react to each other.

Con-specifics, or animals of the same species, may at times be close to each other without exhibiting social behaviour For example, mollusc larvae may respond to changes in the intensity of light by swimming to the water surface. The resultant grouping, called an aggregation, stems from a common response to a physical aspect of the environment. But a response is truly social only when it is a response to visual, chemical, auditory, or other stimuli emanating from a con-specific As a result of such stimuli, animals may approach each other to form a bond or to fight. Although dissimilar, both reactions are examples of social behaviour

Goby, any of numerous, widely distributed, spiny-finned fishes constituting family Gobiidae, having wide, flat head, large mouth, and ventral fins often united in funnel-shaped disk.

The type of bond formed by con-specifics is a measure of their nervous and hormonal systems. Organisms with relatively simple systems may respond to each other only as long as they give off attractive or offensive stimuli. For example, a worm will approach another worm during the reproductive state because certain chemicals are released. Once mating has occurred, they have nothing further to do with each other. A goby will remain near its eggs only as long as the hormonal state of the fish and the chemical and visual features of the eggs remain the same. Once the fry, or young, hatch, the fish responds to them as it would toward any small fish and tries to eat them. The goby does not recognize the fry as its own offspring.

Although orientation, changing hormonal levels, and other processes play a part, social bonding depends primarily on a mutual exchange of stimulation and food between animals. The give-and-take stimulation of a pair or a group is fundamental to the organization of social groups.

The Army Ant Colony – An Example of a Social Group

An army ant colony consists of many thousands of workers and a queen. The queen is capable of laying large batches of infertile eggs when she is fed sufficiently. These eggs hatch into workers, females incapable of sexual reproduction. However, at a certain stage of the queen’s development she produces a brood of males and females capable of reproducing and starting new colonies.

The colony has a two-phase cycle of activity. The nomadic phase lasts about 18 days. By late afternoon or early evening, the larger workers cluster and leave the bivouac area where they spent the previous night. They move out over many yards in the area around the bivouac. As they crawl, they lay a chemical trail. Other ants in the colony travel over the trail, and as the trail becomes more frequently travelled the concentration of chemical stimuli on it becomes stronger. The entire colony, queen and all, eventually move out from the bivouac along the trail. The ants range over large areas, preying on other insects and their young.

Army ants take in considerable food during the nomadic phase. The queen receives a good deal of it. She does not usually forage but is able to feed on the food brought back by medium-size workers. They return to the bivouac to lick the queen for the highly attractive chemicals she exudes. Chemicals that attract or repel con-specifics and heterospecifics (members of other species) are called pheromones. The exchanges of food and secretions between the queen and the workers produce a strong social bond that aids in keeping the colony together. The queen’s increased food intake enables her to lay a batch of eggs. However, this affects her relationship with the workers. She becomes less stimulating to them, and their foraging, therefore, begins to decrease. Now the colony enters the other phase of its cycle the statuary phase. The number, frequency, distance, and area of foraging decreases considerably. The level of the entire colony’s activity drops to a minimum.

After about 21 days the eggs hatch, and the larvae emerge. These squirming, active young are an intense source of stimulation to the workers. The workers are driven out of the bivouac and the nomadic phase starts again. They are now attracted by the pheromones of the larvae and the queen. When the workers return from foraging, they drop their food and feel and handle the larvae with their antennae and legs. As a result of this excitation, the number and frequency of raids again increase. The colony travels great distances, the larvae are fed, and the queen is overfed. At this point, the colony consists of the queen, workers, and larvae.

About 18 days after the eggs have hatched, the larvae enclose themselves in cocoons and become pupae. At about the same time, the queen lays her next batch of eggs. Now the colony consists of the queen, workers, pupae, and developing eggs. However, the pupae and the eggs offer little stimulation for the workers, and the statuary slowdown begins. But the queen continues to secrete pheromones that socially bind the colony.

Dominance

In communities of certain animals the ruling, or dominant, animal is the largest, strongest, or most aggressive and thereby exerts the most influence on the other animals in the group. The dominant animal enjoys the greatest and most preferential access to members of the opposite sex and control of the best territory for feeding and breeding. Many groups of animals, most notably baboons, birds, foxes, lions, and crocodiles, establish dominance hierarchies. The best-known example is the pecking order of chickens. Flock members are arranged on the “rungs” of a social ladder, with each chicken superior to those below and subordinate to those above. The top animal has primary access to the necessities of life, such as the best food, mates, and living quarters. Submissive animals are left with less-desirable food, mates, and living quarters. Such animals may even be expected to groom dominant members and to help care for the offspring of more dominant animals, because subordinates are often prevented from having offspring of their own.

In other animal groups, dominance hierarchies are more complicated. Wolf packs, for example, are led by two dominants who have three subclasses of subordinates below them. Other animals have only one dominant leader with all other animals below him or her being exactly equal. Once an animal has established dominance, challenges to the order are rarely made from within the group, since animals are reluctant to fight other animals that are bigger, stronger, or more aggressive than they are themselves. Sometimes, however, animals from outside the group can successfully challenge and overthrow a long-time leader, but this is rare.

In more intelligent species, such as baboons, factors beyond mere size and strength determine the dominance hierarchy. Age seniority, hormonal condition, maternal lineage, and personality are sometimes factors that affect dominance in more intelligent animals. In baboon groups, furthermore, hierarchies are often elaborate. Adult males are dominant over less mature males and females; yet a fully mature female can be dominant over a less mature male. A dominant baboon displays its superiority with rapid “fencing” manoeuvres, open-jaw displays, and hitting.

Close Bonds Among Animals

Animals with complex nervous systems, ranging from some fish to mammals, may form monogamous bonds. The mates of such species stay together for a breeding season or even for a lifetime. Their social ties are not restricted by the time-bound, immediate stimulation that simpler animals need. However, monogamous pairs must be able to identify their mates from other con-specifics This requires the intricate action of an advanced nervous system.

Some birds and many mammals band in large groups, such as herds and families. These groups include adult males and females and offspring of different ages. The offspring in most mammalian groups remain with the group until they reach sexual maturity. The females frequently remain until the group splits up. Some socially bonded groups of mammals consist of an older male, a number of younger males, many females, and immature offspring. Among the howler monkeys and some other mammals, the younger males band together into a marginal bachelor group until each establishes himself as the older male in a new social group.

Not all mammals maintain elaborate group arrangements. Many live fairly solitary lives, coming together only for mating. Afterwards, the female remains with the litter until the young become juveniles or are sexually mature. In some instances, the mating pair stay together until the young are born. Beavers behave in this way. In other instances, the male and the female separate immediately after mating. This is true of many other rodents.