Cracking the AP Biology Exam

13

Animal Behavior and Ecology

INSTINCTS

Instinct is an inborn, unlearned behavior. Sometimes the instinctive behavior is triggered by environmental signals called releasers. The releaser is usually a small part of the environment that is perceived. For example, when a male European robin sees another male robin, the sight of a tuft of red feathers on the male is a releaser that triggers fighting behavior. In fact, because instinct underlies all other behavior, it can be thought of as the circuitry that guides behavior.

For example, hive insects such as bees and termites never learn their roles; they are born knowing them. On the basis of this inborn knowledge, or instinct, they carry out all their other behaviors: A worker carries out “worker tasks,” a drone “drone tasks,” and a queen “queen tasks.” Another example is the “dance” of the honey bee, which is used to communicate the location of food to other members of the beehive.

There are other types of instinct that last for only a part of an animal’s life and are gradually replaced by “learned” behavior. For example, human infants are born with an ability to suck from a nipple. If it were not for this instinctual behavior, the infant would starve. Ultimately, however, the infant will move beyond this instinct and learn to feed itself. What exactly, then, is instinct?

For our purposes:

Instinct is the inherited “circuitry” that directs and guides behavior.

A particular type of innate behavior is a fixed action pattern. These behaviors are not simple reflexes and yet they are not conscious decisions. An example is the egg-rolling behavior exhibited by a graylag goose. If the egg is removed from the goose, it will continue to make the same movements. That is, the innate movements are independent of the environment.

LEARNING

Another form of behavior is learning. Learning refers to a change in a behavior brought about by an experience (which is what you’re doing this very moment). Animals learn in a number of ways. For the AP Biology Exam, we’ll take a look at the key types of learning.

Imprinting

Have you ever seen a group of goslings waddling along after their mother? How is it that they recognize her? Well, the mother arrives and gives out a call that the goslings “recognize.” The goslings, hearing the call, know that this is their mother, and follow her around until they are big enough to head out on their own.

Now imagine the same goslings, newly hatched. If the mother is absent, they will accept the first moving object they see as their mother. This process is known as imprinting.

Animals undergo imprinting within a few days after birth in order to recognize members of their own species. While there are different types of imprinting—including parent, sexual, and song imprinting—they all occur during a critical period—a window of time when the animal is sensitive to certain aspects of the environment.

Remember that:

Imprinting is a form of learning that occurs during a brief period of time, usually early in an organism’s life.

Classical Conditioning

If you have a dog or a cat, you know that every time you hit the electric can opener, your cat or dog comes running. This is a form of classical conditioning. To feed your pet, you need to open its can of food. For your pet, the sound of the opener has come to be associated with eating: Every time it hears the opener, it thinks that it is about to be fed. We can say that your pet has been “conditioned” to link the buzz of the can opener and its evening meal.

The classic experiment demonstrating conditioning was done by a Russian scientist named Ivan Pavlov, who made his dogs salivate by ringing a bell. He did this the same way you make your dog come running with the can opener. Each time he fed his dogs, Pavlov rang a bell. Eventually, the dogs came to “associate” the bell with the food. By the end of the experiment, Pavlov had merely to ring the bell to start the dogs salivating. This type of learning is now known as associative learning.

Pavlov’s experiments demonstrated what many of us know already: We can learn through conditioning, or repeated instances of an event.

Operant Conditioning

Another type of associative learning is operant conditioning (or trial-and-error learning).

In operant conditioning, an animal learns to perform an act in order to receive a reward. This type of behavior was extensively studied by psychologist B. F. Skinner. He put a rat in an experimental cage and watched to see if it would randomly press different levers. Through trial and error, the animal figured out that one lever in particular would always produce food from a dispenser. Over time, the animal made an “association” between pressing the lever and getting food (the reward). Skinner even detected that some rats were so “conditioned” that they started to “hang out” near the lever. These same rats were also subjected to a negative form of operant conditioning, where touching the bar led to an electric shock. As you could imagine, rats quite quickly learned not to touch the bar that gave them the shock.

If the behavior is not reinforced, the conditioned response will be lost. This is called extinction.

Here’s one thing to remember:

In operant conditioning, the animal’s behavior determines whether it gets the reward or the punishment.

Habituation is another form of learning. It occurs when an animal learns not to respond to a stimulus. For example, if an animal encounters a stimulus over and over again without any consequences, the response to it will gradually lessen and may altogether disappear. One example is how a marine worm, Nerels, withdraws into its protective tube if a shadow passes over. With repeated exposure to the stimulus, however, the response decreases.

Insight

This is the highest form of learning and is exercised only by higher animals. Insight means the ability to figure out a behavior that generates a desired outcome. It is sometimes referred to as the “aha experience.” As far as the animal world is concerned, human beings tend to be pretty good at using insight, or reasoning, to solve problems. However, we are not the only ones who reason. Chimpanzees, for example, have been known to use rudimentary tools, such as twigs and stones, to get their food.

To recap, there are four basic types of learning:

  • Imprinting occurs early in life and helps organisms recognize members of their own species.
  • Classical conditioning involves learning through association.
  • Operant conditioning occurs when a response is associated with new stimuli (also a form of associative learning).
  • Insight involves “reasoning” or problem solving.

INTERNAL CLOCKS: THE CIRCADIAN RHYTHM

There are other instinctual behaviors that occur in both animals and plants. One such behavior deals with time. Have you ever wondered how roosters always know when to start crowing? The first thought that comes to mind is that they’ve caught a glimpse of the sun. Yet many crow even before the sun has risen.

Roosters do have internal alarm clocks. Plants have them as well. These internal clocks, or cycles, are known as circadian rhythms.

If you’ve ever flown overseas, you know all about these. They’re the basis of jet lag. Our bodies tell us it’s one time while our watches tell us it’s another. The sun may be up, but our body’s internal clock is crying “Sleep!” This sense of time is purely instinctual: You don’t need to know how to tell time in order to feel jet lag.

Circadian rhythms are yet another example of instinct. When an organism does something on a daily basis, we say it acts according to its circadian rhythm. But how do we know for certain that it’s instinctual?

In a famous experiment, an American scientist took a bunch of plants and animals to the South Pole and put them on a turntable set to rotate at exactly the same speed as the earth but in the opposite direction of the earth’s rotation. As a result, the organisms had absolutely no indication of day or night. Yet all of them continued to carry out their regular 24-hour cycles. This proved that the cycles have nothing to do with sunlight and everything to do with the internal clock.

Watch out though: Seasonal changes, like the loss of leaves by deciduous trees or the hibernation of mammals, are not examples of circadian rhythms. Circadian refers only to daily rhythms. Need a mnemonic? Just think how bad your jet lag would be after a trip around the world. In other words:

Circling the globe screws up your circadian rhythm.

HOW ANIMALS COMMUNICATE

Some animals use signals as a way of communicating with members of their species. These signals, which can be chemical, visual, electrical or tactile, are often used to influence mating and social behavior.

Chemical signals are one of the most common forms of communication among animals. Pheromones, for instance, are chemical signals between members of the same species that stimulate olfactory receptors and ultimately affect behavior. For example, when female insects give off their pheromones they attract males from great distances.

Visual signals also play an important role in the behavior observed among members of a species. For example, fireflies produce pulsed flashes that can be seen by other fireflies far away. The flashes are sexual displays that help male and female fireflies identify and locate each other in the dark.

Other animals use electrical channels to communicate. For example, some fish generate and receive weak electrical fields. Finally, tactile signals are found in animals that have mechanoreceptors in their skin to detect prey. For instance, cave-dwelling fishes use mechanoreceptors in their skin for communicating with other members and detecting prey.

SOCIAL BEHAVIOR

Many animals are highly social species, and they interact with each other in complex ways. Social behaviors can help members of the species survive and reproduce more successfully. Several behavioral patterns for animal societies are summarized below:

  • Agonistic behavior is aggressive behavior that occurs as a result of competition for food or other resources. Animals will show aggression toward other members that tend to use the same resources. A typical form of aggression is fighting between competitors.
  • Dominance hierarchies (or pecking orders) occur when members in a group have established which members are the most dominant. The more dominant male will often become the leader of the group and will usually have the best pickings of the food and females in the group. Once the dominance hierarchy is established, competition and tension within the group is reduced.
  • Territoriality is a common behavior when food and nesting sites are in short supply. Usually the male of the species will establish and defend his territory (called a home range) within a group in order to protect important resources. This behavior is typically found among birds.
  • Altruistic behavior is defined as unselfish behavior that benefits another organism in the group at the individual’s expense because it advances the genes of the group. For example, when ground squirrels give warning calls to alert other squirrels of the presence of a predator, the calling squirrel puts itself at risk of being found by the predator.