The Clock Inside You

You probably wear a wristwatch to tell the time of day, but did you know that you have a built-in clock, too?

In fact, the bodies of all living things have ways of keeping track of time. These inner, or biological, clocks control how long to sleep and be awake, when to grow into an adult, and how long to live. They tell birds when to fly south and bears when to hibernate. They let tadpoles know when to change into frogs. They even tell flowers when to blossom. In all forms of life on Earth, biological clocks are marvelous mechanisms that keep life running on time.

See how some inner clocks tick!

there is a picture of a butterfly

A butterfly's inner clock lets it know when to change from a caterpillar into a butterfly.

there is a picture of a parrot

Pet parrots can live to be 100 years old—which is nothing to squawk about!

there is a picture of a hamster

Pet hamsters have 3 or 4 years to run on that wheel! And they do keep busy: females can have 18 babies a year!

there is a picture of a dog

Dogs live from 7 to 12 years or more. Big dogs develop more slowly than small ones, but small dogs tend to live longer.

there is a picture of a duck

From fuzzy duckling to old age, ducks live 7 to 12 years. Within 36 hours of birth, they can run, swim—and quack!

there is a picture of a tarantula

Some female tarantulas live 30 years, though a male's life span is shorter.

there is a picture of a goldfish

Goldfish can live as long as 10 years!

there is a picture of a horse

Horses live from 20 to 30 years.
All racehorses have their official birthdays on January 1, regardless of their real birthdates.

there is a picture of a girl in a yellow sweater

On average, people live into their 70s. The oldest person on record lived to be 122 years old!

there is a picture of a tortoise

The tortoise may be the longest-lived animal; some live well past 150. Let's hear it for life in the slow lane!


  1. In what ways are the life cycles of the animals in this article alike? In what ways are they different?
    [anno: All of the animals go through different stages of growth. The butterfly, duck, tarantula, parrot, goldfish, and tortoise are all born from eggs. The hamster, dog, person, and horse are all live births.]
  2. Which animal changes form during its life cycle? How does it change?
    [anno: The butterfly changes form. It starts life as an egg. When egg hatches it is a larva or caterpillar. Then the caterpillar becomes a pupa and forms a chrysalis. An adult butterfly comes out of the chrysalis.]