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  • Tick-Tock Biological Clock
    How Animals Tell Time

    Among the most important skills of a migratory animal is its ability to tell time. As you track migrations this spring you'll find countless cases where a creature's very survival depends on being at the right place at the right time. Thus, it's no surprise that animals have internal timing mechanisms, or "biological clocks", with which they tell time. While the underlying science of biological clocks is complicated and not fully understood, we do know that animals set their master clocks according to the world's most dependable time keeper, the sun. Photoperiod--the length of daylight hours--governs the annual rhythm of life throughout the plant and animal kingdoms. Its cyclical changes during the course of a year provide a dependable measure of time.

    Students enjoy discovering examples of their own biological clocks. These clocks moderate sleep and energy cycles in interesting ways. In this lesson students will explore the concept of time and their abilitiy to keep time. Then they'll consider the importance of an accurate biological clock to a migratory species.

    1. Cover the classroom clock and ask students who wear watches to take them off for the day. Have students write the numbers 1-10 on a piece of paper. At random intervals during the day, ask students to guess the time and record the guess on the paper. Have them list the internal clues (such as hunger, fatigue, restlessness) and external clues (school bells, movement in the hallway, smells from the lunchroom) they used to judge the time. At the end of the day see how accurately students were able to estimate the time and compare the clues they used.

    2. On the same day, test how well students can remember how daylight changes during the course of a year. Ask them to record the following dates on a piece of paper. For each date, record the time they think the sun rises and sets: February 1, April 1, June 1, August 1, October 1, December 1. Using a calendar with sunrise and sunset data, compare these guesses to the actual amount of daylight on those days. Ask students to consider why its important for a migratory species to notice changes in photoperiod.

    3. Ask all students to stand. Divide your class into groups acording to the following characteristics: Have those that usually wear a watch move to one side of the room and those that don't to the other. While staying on the same side of the room, have those that wake to an alarm move to the back. Have those that wake naturally move towards the front. If these four groups are too large for a discussion, have them break into triads or foursomes. Ask students to talk about time: What interesting experiences have they had because of oversleeping, or with jet lag or when changing to daylight savings time? Do they have pets that seem to be able to tell time? Does time seem to pass more slowly at the dentist's office or when eating dessert? What does this mean? How would life be different without clocks? Is there any correlation between these students' performance on the time-tests above and the groups they are in?

    4. Have students try an experiment. During the week, have them record the number of hours of they sleep each night. On Friday, calculate the average for the week. On the weekend, have each student record the time he/she goes to sleep on Friday night and the exact time he/she wakes up on Saturday morning. Students should wake-up on their own--no alarm, no wakening by another person. How close was their wake-up time to the average number of hours of sleep they had during the past week?

    5. Chose one Journey North migratory animal. What would happen if this animal were to lose track of time? Write a story about the consequences that would result. Be sure to consider all the activities the animal does according to a daily and annual cycle.


    1. How closely were you able to guess the time without using a clock? What clues do you use to tell the time when there is no clock available?
    2. How do you use the sun in telling time?
    3. What happened in the sleep experiment? What did you learn from that?
    4. How do the different groups in activity one respond to having all the clocks stopped for a day? Does one group find that more difficult than another?
    5. Why is it important for animals to be able to tell time? What about plants?

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