Introduction
All living organisms live within complicated food and energy webs in which energy is transferred from organism to organism through consumption. Plants convert energy from sunlight to manufacture organic nutrients. Herbivorous animals eat plants to obtain this energy, and carnivorous animals eat herbivores and other carnivores to obtain the same energy. Within an animal species, individuals compete with one another for available food and natural resources to survive; in the process, they establish dominance hierarchies in which dominant individuals overpower subordinates. Among different species, interactions occur that lead to predation (an interspecific interaction in which the individuals of one animal species hunt and kill members of another species for food), disease, and parasitism; in each case, one species is feeding on another species.
Intraspecific competition and interspecific predation and parasitism represent principal animal behaviors important to the evolution of life. Each species possesses an innate (instinctive) drive to survive and to continue the transmission of its deoxyribonucleic acid (DNA) in space and time. Within the food webs of the living environment, each species evolves specific adaptations for survival that enable it to carve out a particular habitat, or place to live, and niche, or occupation, in the environment. Because the food webs of life on Earth are circular, the evolution of competition and predation are necessary for life to continue. All species will become predators (hunters) of certain other species and will simultaneously be prey (the hunted) for certain different species.
Learned and Genetic Adaptations
To survive, each species possesses specific adaptations for hunting its prey and for defending itself from predators. Defensive mechanisms come in many different varieties. Some very effective defense mechanisms are highly conserved between different species; other defenses are unique to only one or a few species. Of these defenses, some are instinctive, occurring automatically because of biochemical changes within individual animals, whereas others are learned from environmental experiences. Learned defensive mechanisms are prevalent in highly social mammal and bird species.
Intraspecific competition occurs among individuals of a given population or group and among different populations or groups. Within a population, the social structure is either genetically or behaviorally conditioned to construct castes ordominance hierarchies in which dominant individuals are superior to more subordinate individuals. The social insects (such as ants, termites, bees, and wasps), whose behavior is almost exclusively genetic in nature, construct their societies, or hives, around castes that have specific jobs to perform. Such societies revolve around a central fertile queen, sterile female workers, and male drones. Workers are subdivided into several specializations, such as hunting for food and defending the hive. Soldier workers have specialized body structures for attacking intruders; furthermore, they release pheromones (hormonal attractants) from their bodies at the sign of danger to attract other soldier workers to the region of intrusion.
Dominance Hierarchies
Within the highly social and intelligent mammal and bird species, populations either migrate in bands or groups or set up individual adjacent territories that are heavily defended by the owner. In either of these situations, learned dominance hierarchies are established in which stronger individuals outcompete weaker individuals, thereby establishing a “pecking order” (as it is called for chickens) of precisely ranked dominant individuals to progressively more subordinate individuals. The dominant individuals possess the best territory, the most food, and the most mates. The most dominant individuals also have the best protection from predators, because their territories are central and therefore are shielded by the territories of more subordinate individuals. Consequently, the most subordinate individuals have the worst territories, poor food, few if any mates, and poor protection from predators, which usually attack outskirt territories. These territories are maintained by constant fighting among males, especially during the breeding season. Males vocalize and present visual displays (visual dances or series of movements or gestures by individuals to communicate such things as dominance, aggression, and courtship to other individuals) to force their opponents to submit; submission is routine, and few encounters are fatal.
Predator-Prey Interactions
In interspecific predator-prey interactions, the prey utilize a variety of quick-response defenses. One of the most common defenses is the flocking or herding defense. When a predator approaches a group of prey and is identified, the discovering prey individual announces danger by a vocalization (an alarm call), specific movement, or the release of chemical pheromones to warn the other members of the group. The prey group response is instantaneous, with all members contracting into a dense mass. A predator is less likely to succeed in capturing a prey individual during an attack on a compact group than when the prey individuals are scattered. Furthermore, the predator may sustain personal physical damage in an attack on a compact group, which easily could turn on the predator. Most predators are far more successful at capturing very young, old, or sickly individuals that are isolated or located at the poorly defended outskirts of a prey group. Some predators, such as hawks, falcons, and wolves, do make repeated passes at compact groups in sometimes successful attempts at panicking individuals and thereby scattering the group. Flocking behavior is a very effective defense that is utilized by bees, fish, tadpoles, most bird species, and most mammal species. Prey species usually utilize excellent vision, hearing, and sense of smell.
The dynamics of predator-prey interactions can be complicated, although numerous mathematical models of such relationships have been developed that enable researchers to make predictions concerning future interactions in natural populations. Predator-prey interactions are important for the stability and survival of both predator and prey populations. Without prey, predators would die; however, without predators, prey populations would grow unchecked until they exceeded the available resources in the environment, ending in a massive population crash in which many individuals would die. Such occurrences have been thoroughly documented in many species, including moose, deer, rabbit, and even human populations. Defensive mechanisms are important to all species to ensure the survival of enough members of each species population to reproduce and continue the transmission of the species’ genetic information. Perfect defense, however, could be as detrimental to the population as no defense. Numerous mathematical ecologists have developed impressive models of animal population growth based on predator-prey interactions. Among the most famous models are those that were developed by Alfred Lotka and Vito Volterra, and they still are in use. Such models are of critical importance to the study of human overpopulation.
Animal Defense Mechanisms
Other species-specific defense mechanisms include camouflage, mimicry (an inherited or behavioral defense phenomenon in which an individual of a species either looks dangerous to its predators or can exaggerate its appearance to fool a predator), predator saturation (a defensive mechanism in some animal species in which a prey animal population synchronizes its growth so that it becomes too large for predators to consume any significant fraction of the population), and long-term incubation. Most species have adaptations in skin and fur coloration to blend in with their particular environment. For example, albino hares and squirrels predominate in areas that have snow for a good portion of the year, whereas the same species in more temperate climates usually have a grayish-brown coloration. Zebras have a striped pattern that enables them to blend with tall grass; many mammalian predators (lions, wild dogs) are color blind. Some species (the chameleon, for example) can alter their body color to their background through biochemical changes in their skin. Some lizard species simply discard body parts, such as their tails, when captured by predators.
Müllerian mimicry is a phenomenon used by moths and butterflies to defend themselves from bird predators. A few butterfly species with bright orange-black wings are poisonous; several dozen nonpoisonous species have coevolved bright black and orange or yellow wings and are therefore less likely to be eaten, since birds learn avoidance very quickly from only a few encounters with the poisonous varieties. Some species of fish, birds, and mammals have short, rapid bursts of reproduction, so that their predators are overwhelmed, or saturated, by the high prey densities. In both of these instances, some prey are eaten. The thirteen- and seventeen-year periodic locust species of North America combat predation by burrowing underground for many years before surfacing and reproducing within only a few weeks.
Implications for Human Society
The study of species-specific defensive reactions is of intense interest to animal behavior researchers. Research into such behavior enables them to understand how highly adaptive and elaborate defenses have evolved in animal species over the past five hundred million years. These studies also have potential impact on the psychology of human behavior. Humans are very territorial animals and exhibit considerable competitive behavior, including interpersonal conflicts and aggression. Consequently, defense mechanism studies are strongly applicable to the study of human conflict, social tensions, and warfare. Furthermore, species-specific defense mechanisms are also of interest to medicine, since humans, while being the apparently dominant species on Earth, are subject to predation, particularly from parasitic bacterial, fungal, and viral diseases.
The explosion of human technological growth during the past century has included medical advances that have eradicated many diseases that once were major killers of humans. Medicine has been a tremendous artificial defense mechanism developed by the intelligence of the human species. It has defeated dozens of bacterial, fungal, and viral predators and parasites of humans. Because of these advances, humans live longer and better lives, human birthrates have soared, and human death rates have declined; however, the elimination of human predators has produced some serious problems. One is overpopulation. The explosive human population growth is approaching the planet’s carrying capacity (the available food and resources). Some areas of the world, most notably Asia and Africa, have already seen human overpopulation far above the carrying capacity; the result has been devastating famines and millions of deaths. Furthermore, medical science is taxed by the appearance of new mutated viral and bacterial predators to replace the eradicated ones. The human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), is an example. Furthermore, certain diseases, such as cholera, are on the rise worldwide. Life on Earth is very homeostatic (self-regulatory); it contains mechanisms for keeping populations of all species in check.
Leaving the microscopic scale, humans and other primates defend themselves from competitors and very large predators in much the same fashion. Humans have territories that each territory-holder defends. Furthermore, several dominant males may group to attack intruders and other predators. There can be no doubt that such behavior has evolved into the large armies that different countries have amassed to defend their territorial borders. The structure of many such armies is somewhat reminiscent of mammalian dominance hierarchies. The frontline soldiers who face the brunt of an opponent’s attack usually are individuals who are less equipped and trained. They are sometimes called “cannon fodder.” Better-trained, more dominant individuals follow; they are more likely to survive hostile encounters with the enemy.
Species-specific defensive behaviors are also applicable to human behavior in terms of social and personal relationships. Complex human societies are rigidly structured along territorial lines, with laws to regulate the behaviors of individuals. Cross sections through American cities reveal the segregation of the poor from the middle class from the rich, and the segregation of black from white from Hispanic. Individuals in each of these groups construct physical, social, and legal barriers to defend themselves from competition from outsiders. With overpopulation and competition for resources, individuals and countries resort to mechanical weapons ranging from handguns to semiautomatic rifles to atomic bombs. Stress, inequality, and mistrust of others involve biological reactions that have evolved over hundreds of millions of years.
Each animal species on Earth has evolved through the endurance of predator-prey interactions. Understanding how species defend themselves can be of great importance in helping endangered species to survive and in controlling the overpopulations of species and the spread of disease. It also can help humankind to alleviate many of its own species’ social problems.
Understanding Ecosystems
The study of species-specific defense mechanisms is of considerable interest to animal behaviorists and psychologists because of their implications for human behavior. All animal behaviors can be influenced by endogenous (instinctive) or exogenous (environmental) factors. Endogenous behaviors
include imprinting and biochemical changes within the body of the individual that enable one to recognize events or situations instantaneously for survival. Behaviors such as recognizing danger are critical for survival and therefore must be instinctive. Within the intelligent and highly social mammals and birds, a period of learning during infancy enables the development of exogenous behaviors from experiences in one’s immediate environment. Such learned behaviors are of equal importance in such species.
The study of species-specific defensive reactions allows scientists to uncover the intricacies of species interactions within the environment. In any given ecosystem (such as a forest, grassland, ocean, or desert), populations of thousands of different species are linked by intricate food webs. The destruction of the environment or the extinction of any one species can have irreparable effects on all the other species within the ecosystem. The defensive mechanisms of most species can work only so well.
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