As noted earlier, the term behavior would seem at first glance to apply only to animals and not to plants. Certainly the majority of attention in behavioral studies, outside the realm of humans, is devoted to ethology, but plants are not without their observable behavioral characteristics. These features primarily manifest in the form of tropism, a response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. Tropism primarily affects members of the plant kingdom, though it has been observed in algae and fungi as well.
Though the word tropism itself may be unfamiliar to most people, the phenomenon itself is not. There are plenty of opportunities in daily life to observe the response of plants to energy, substances, or forms of stimulation. For example, perhaps you have noticed the way that trees or flowers grow toward sunlight, even bending in their growth if it is necessary to reach the energy source. Similarly, plants in a parched region are likely to develop roots directed laterally toward a water source.
Among the various forms of tropism are phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular chemical substances), hydrotropism (response to water), thigmotropism (response to mechanical stimulation), traumatropism (response to wounds), and galvanotropism or electrotropism (response to electric current). Most of these types involve growth toward a stimulus, a phenomenon known as positive growth, or orthotropism. Plants tend to grow
Tropism, along with movement due to changes in water content, is one of the two principal forms of innate behavior on the part of plants. In general, stems and leaves experience positive phototropism, as they grow in the direction of a light source, the Sun. At the same time, roots exhibit positive gravitropism, or growth toward the gravitational force of Earth, as well as positive hydrotropism, since they grow toward water sources below ground. On the other hand, a plant may move in a specific way regardless of the direction of the stimulus. Such movements are temporary, reversible, and result from changes in the water pressure inside the plant.
An excellent example of an innate animal behavior, and one in which humans also take part, is
More complex reflexes require processing interneurons between the sensory and motor neurons as well as specialized receptors. These neurons send signals across the body, or to various parts of the body, as, for example, when food in the mouth stimulates the salivary glands to produce saliva or when a hand is pulled away rapidly from a hot object.
Reflexes help animals respond quickly to a stimulus, thus protecting them from harm. By contrast, learned behavior results from experience and enables animals to adjust to new situations. If an animal exhibits a behavior at birth, it is a near certainty that it is innate and not learned. Sometimes later in life, however, a behavior may appear to be learned when, in fact, it is a form of innate behavior that has undergone improvement as the organism matures.
For example, chickens become more adept at pecking as they get older, but this does not mean that pecking is a learned behavior; on the contrary, it is innate. The improvement in pecking aim is not the result of learning and correction of errors but rather is due to a natural maturing of muscles and eyes and the coordination between them.
In studying fixed-action patterns of behavior, or FAPs, Lorenz and Tinbergen observed numerous interesting phenomena. Male stickleback fish, for example, recognize potential competition—other breeding stickleback males—by the red stripe on their underside and thus engage in the FAP of attacking anything red on sight. Tinbergen discovered that jealous stickleback males were so attuned to the red stripe that they tried to attack passing British mail trucks, which were red, when they could see them through the glass of their tanks. Tinbergen termed the red stripe a behavioral releaser, or a simple stimulus that brings about a FAP.
Once a FAP is initiated, it continues to completion even if circumstances change. If an egg rolls out of a goose's nest, the goose stretches her neck until the underside of her bill touches the egg. Then she rolls the egg back to the nest. If someone takes the egg away while she is reaching for it, the goose goes through the motions anyway, even without an egg. Not all animal behavior is quite so predictable, however. In contrast to FAPs are complex programmed behavior patterns, which comprise several steps and are much more complicated. Birds making nests or beavers building dams are examples of complex programmed behavior.
As we noted earlier, Lorenz initiated the study of a learning pattern that came to be known as imprinting. Witnessed frequently in birds, imprinting is the learning of a behavior at a critical period early in life, such that the behavior becomes permanent. The very young bird or other organism is like wet concrete, into which any pattern can be etched; once the concrete has dried, the pattern is set.
Newly hatched geese are able to walk. This is something they learn the moment they are hatched, and they do so by following their parents. But how, Lorenz wondered, do young geese distinguish their parents from all other objects in their environments? He discovered that if he
Lorenz concluded that during a critical period following birth, the goslings follow their parents' movement and learn enough about their parents to recognize them. But since he also had determined that young geese follow any moving object, he reasoned that they first identify their parents by their movement, which acts as a releaser for parental imprinting. (Imprinting is discussed further in Instinct and Learning.)
Much of an animal's behavior (this is true of the human animal as well) takes place in interaction with others. This interaction may include rudimentary forms of communication, such as bee dances, studied by Lorenz and Tinbergen's colleague Frisch. As he showed in perhaps the most important research of his career, bees communicate information about food supplies, including their direction and the distance to them, by means of two different varieties of "dance," or rhythmic movement. One is a circling dance, which informs the other bees that food is near (about 250 ft., or 75 m, from the hive), and the other is a wagging dance, which conveys the fact that food is farther away.
There are numerous other forms of communication using one or more sense organs. Birds hear each other sing, a dog sees and hears the spit and hiss of a cornered cat, and ants lay down scent signals, or pheromones, to mark a trail that leads to food. This is only one level of interactive behavior, however. Quite a different variety of interaction is courtship, discussed in Reproduction. Other forms of interactive behavior include the establishment of an animal's territory, a subject we discuss at the conclusion of this essay.
Interactive behavior comes into play when animals live in close proximity to one another. Certainly there are benefits to group life for those species that practice it: the group helps protect individuals from predators and, through cooperation and division of labor, ensures that all are fed and sheltered. In order to be workable, however, a society must have a hierarchy. Thus, in a situation quite removed from the human ideals of freedom and democracy, insect and animal societies are ones in which every creature knows its place and sticks to it.
Bees, ants, and termites live in complex communities in which some individuals are responsible for finding food, others defend the colony, and still others watch over the offspring. In such a highly organized society, a dominance hierarchy or ranking system helps preserve peace and discipline. Chickens, for example, have a pecking order from the most dominant to the most submissive. Each individual knows its place in the order and does not challenge individuals of higher rank. This, again, is quite unlike humans, who at least occasionally step out of line and challenge bullies; by contrast, that never happens with chickens (fittingly enough).
Almost everyone has seen a dog "mark its territory" by urinating on a patch of ground or has watched a cat arch its back in fury at an intruder to what it perceives as its territory. In so doing, these household pets are participating in a form of behavior that cuts across the entire animal kingdom: territoriality, or the behavior by which an animal lays claim to and defends an area against others of its species and occasionally against members of other species as well.
The physical size of the territory defended is extremely varied. It might be only slightly larger than the animal itself or it might be the size of a small United States county. The population of the territory might consist of the animal itself, the animal and its mate, an entire family, or an entire herd or swarm. Time is another variable: some animals maintain a particular territory year-round, using it as an ongoing source of food and shelter. Others establish a territory only at certain times of the year, when they need to do so for the purposes of attracting a mate, breeding, or raising a family.
Territorial behavior offers several advantages to the territorial animal. An animal that has a "home ground" can react quickly to dangerous situations without having to seek hiding places or defensible ground. By placing potential competitors at spaced intervals, territoriality also prevents the depletion of an area's natural resources and may even slow down the spread of disease. Furthermore, territorial behavior exposes weaker animals (which are unable to defend their territory) to attacks by predators and thus assists the process of natural selection in building a stronger, healthier population.
A territory established only for a single night, for the sole purpose of providing the animal or animals with a place to rest, is known as a roost. Even within the roost, there may be a battle for territory, since not all spots are created equal. Because roosting spots near the interior are the safest, they are the most highly prized.
Another type of specialized territory is the lek, used by various bird and mammal species during the breeding season. Leks are the "singles bars" of the animal world: here animals engage in behavior known as lekking, in which they display their breeding ability in the hope of attracting a mate. Not surprisingly, leks are among the most strongly defended of all territories, since holding a good lek increases the chances of attracting a mate. Like the singles-only communities that they mimic, leks are no place for families: generally of little use for feeding or bringing up young, the lek usually is abandoned by the animal once it attracts a mate or mates.
An animal has to be prepared to defend its territory by fighting off invaders, but naturally it is preferable to avoid actual fighting if a mere display of strength will suffice. Fighting, after all, uses up energy and can result in injury or even death. Instead, animals rely on various threats, through vocalizations, smells, or visual displays.
The songs of birds, the drumming of woodpeckers, and the loud calls of monkeys may seem innocuous to humans, but they are all warnings that carry for long distances, advertising to potential intruders that someone else's territory is being approached. As noted earlier, many animals, such as dogs, rely on smells to mark their territories, spraying urine, leaving droppings, or rubbing scent glands around the territories' borders. Thus, an approaching animal will be warned off the territory without ever encountering the territory's defender. Or, if the invader is unfortunate enough to have trespassed on a skunk's territory, it may get a big blast of scent when it is too late to retreat.
Suppose an animal ignores these warnings, or suppose, for one reason or another, that two animals meet nose to nose at the boundaries of their respective territories. Usually there follows a threatening visual display, often involving exaggeration of the animals' sizes by the fluffing up of feathers or fur. The animals may show off their weapons, whether claws or fangs or other devices. Or the two creatures may go through all the motions of fighting without ever actually touching, a behavior known as ritual fighting.
The degree to which a creature engages in these displays of bravado helps define its territory. If the creature perceives that it is at the center of its own territory and is being attacked on home ground, it will go into as threatening a mode as it can muster. If, on the other hand, the animal is at the edge of its territorial boundaries, it will be much more halfhearted in its efforts at intimidation. As with humans, few animals want to fight when there is nothing really at stake. Also like humans, animals many times may seem to be spoiling for a fight without actually fighting, such that when a fight does break out, it is an aberration. This typically happens only in overcrowded conditions, when resources are scarce—again, not unlike the situation with humans.
Late in his career, Lorenz devoted himself to studying human fighting behavior. In Das sogenannte Böse ( On Aggression, 1963), he maintained that fighting and warlike behavior are innate to human beings but that they can be unlearned through a process whereby humans' basic needs are met in less violent ways. Just as fighting in animal communities has its benefits, Lorenz maintained, inasmuch as it helps keep competitors separated and enables the larger group to hold on to territory, so fighting among humans might be directed toward more useful means. As discussed in Biological Communities, it is possible that sports and business competition in the human community provides a more peaceful outlet for warlike instincts.
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