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The Behavior of Animals

Grade 8 Science Worksheets

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Response to Stimulus

All living organisms respond to the environment in which they live. This behavior can vary from actions such as –

  • Movement of a muscle
  • Secretion from a gland
  • Emotions and thoughts in higher animals.

The behavior is almost always in response to a stimulus received from the external environment. A bright light makes a human being blink. The same bright light makes a running deer freeze in their path. The sound of a predator elicits fear in a prey. We sweat or shiver if exposed to hot or cold temperatures. Such basic and spontaneous responses to external stimuli are native in every organism and are termed reflex actions. They take place without any thought process triggering them. They are the very basis of survival.

All animals have to cope with external disease and infection or any such threat to their existence and survival. Much of their behavior is induced by bacteria, viruses, parasites which infect their bodies. Animals are born with immune system passed on through their genes known as inborn immunity. This mechanism creates antibodies inside the organism to fight off disease and infection. Acquired immunity is the second kind that develops over time. The immune mechanism is yet another example of a response to external stimuli.

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The Nervous System

Almost all multicellular organisms have some form of a nervous system, which varies from simple to very complex. It is a collection of special cells, known as neurons, which transmit signals between various parts of the body of the organism.

The simplest of nervous systems control responses to external stimuli such as light and touch, and in locomotion and other simple body functions.

Complex organisms have a brain which directs and controls these neurons. Neurons play different roles such as conveying information from sensory organs, managing motor activity, controlling internal organs and glands and enabling the learning process through past experiences.

The Endocrine System

All animals possess a system of glands that secrete hormones that control various body functions. Hormones are chemical fluids carried in the bloodstream to help perform these various functions. The endocrine glands are much smaller than other organs but they perform very important functions such as growth, reproduction, sleep and wake times, hunger, thirst, digestion, moods, and emotions.

Natural Defenses

Animals exhibit many behaviors as a form of natural defense from predators. Some common ones are –

  1. Hiding
  2. Fleeing
  3. Showing aggression and fighting when isolated
  4. Vocalizing or communicating with others of their species
  5. Playing dead to fool their enemies
  6. Using protective colors to disappear in their environment
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Social Defense Mechanisms

Animals also exhibit social or group behavior such as flocking or moving in shoals or packs to ensure survival.

Some typical examples are –

  1. Birds stay together in flocks and fly in formation to reduce fatigue, enabling them to fly farther. They also tend to sick birds in their flock.
  2. Fish stay in shoals and the shoal swims together, known as a school. This protects them from predators as no member can be singled out for any attack.
  3. Wolves and lions hunt in packs to tackle a larger prey


Animal behavior is made up of a complex set of interacting mechanisms. They range from instant reflex actions, to “control and command” systems such as the nervous and endocrine systems, to social and group behaviors. When studied together, a complex and wondrous picture of the evolution of organisms emerges.

Check Point

  1. A spontaneous response to any external stimuli is known as a ______.
  2. Animals cope with disease and infection with ______ which produces ______.
  3. The nervous system has special cells called _____ to transmit signals between various parts of the body.
  4. Hormones are secreted by _____ in the endocrine system.
  5. Natural defenses are a form of protection from _____.

Answer Key

  1. Reflex action
  2. Immune system; antibodies
  3. Neurons
  4. Glands
  5. Predators

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