Biological clocks provide important time cues for many behaviors.

Like us, other animals live in space and time. So far, we have focused on their behavior in space. For many species, time is a life-or-death matter. When to migrate or mate is a critical decision in a seasonal environment. Researchers are beginning to unravel the neural and genetic underpinnings of the clocks in some species. In model organisms like Drosophila, for example, researchers have identified a number of genes that, when mutated, cause the clock to run slow or fast.

A biological clock is produced by a set of interacting proteins that cycles on its own to provide a regular rhythm. Different clocks work on different timescales. Daily cycles are governed by a circadian clock. Circadian clocks regulate many daily rhythms in animals, such as those of feeding, sleeping, hormone production, and core body temperature. Some species, especially seacoast species living in habitats where the tides are important, time activities by a lunar (moon-based) clock. There are also annual (yearly) clocks. For example, periodical cicadas are insects in the genus Magicicada that have a generation time of either 13 or 17 years: In a given location, there will be a cicada outbreak every 13 or 17 years.

Circadian clocks are observed in many organisms, including plants, fungi, and animals. Humans have circadian clocks, as jet lag never fails to remind us. The circadian clock is based on a set of “clock genes” whose protein products oscillate through a series of feedback loops in a roughly 24-hour cycle. Thus, when animals that, like us, are active during the day and inactive at night are placed in artificial conditions that are always lit, they continue to follow a basic day–night, active–inactive cycle. The clocks are not perfect, though: As the period spent in constant light is prolonged, the circadian clock drifts slowly until the animal is eventually no longer synchronized with the true day–night cycle.

Biological clocks remain synchronized with the day–night cycle because they are often reset by external inputs. For the circadian clock, light is the primary input, so the natural light–dark cycle keeps the clock from drifting. For clocks related to the seasons, day length (known as photoperiod) is the critical input because it is a good indicator of the time of the year (Chapter 30). For example, many mammals produce their offspring in the spring so that they can grow over the summer, when resources are most abundant. These mammals have been selected over many generations to synchronize their reproduction at a time when the young have the greatest chance of survival. Photoperiod determines the timing of many kinds of behaviors, including migration, development, and reproduction.

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The importance of the sun compass coupled with the biological clock in homing pigeons can be demonstrated by experimentally disturbing the birds’ clock, as shown in Fig. 45.12.

HOW DO WE KNOW?

FIG. 45.12

Does a biological clock play a role in birds’ ability to orient?

BACKGROUND One suggestion for how pigeons home is that they use a sun compass. If you are in the Northern Hemisphere and you know the time is 12 noon, then the sun is due south of you. Orienting yourself by this method is possible only if you know the time, so the question arises whether homing birds have the ability to tell the time. One way to answer this question is to “clockshift” the birds. Researchers raise birds in an artificial day–night cycle that is out of sync with the actual one. When released into a sunlit environment, these birds’ sense of time is shifted by a set number of hours.

event dawn dusk
actual time 6 am 6 pm
clockshifted time 12 noon 12 midnight

HYPOTHESIS If a bird’s ability to home is dependent on an internal clock, clockshifting should affect the bird’s homing ability in a predictable way. Given that the sun travels 360 degrees in 24 hours, a 6-hour clockshift will result in a 90-degree error in homing direction because 360/(24/6) = 90.

EXPERIMENT Birds were clockshifted by raising them in a chamber under an artificial light. Birds from a home loft in Ithaca, New York, were released on a sunny day at Marathon, New York, about 30 km east of Ithaca. Release on a sunny day made it possible for the birds to use the sun’s position to navigate.

RESULTS As expected, the control birds (those that were not clockshifted) were usually good at picking the direction of their home loft, heading approximately westward toward Ithaca. The results for the clockshifted birds were very different: They miscalculated the appropriate direction. These birds headed approximately northward, as shown by the positions of the red triangles on the compass in the figure.

image
FIG. 45.12

INTERPRETATION Assume the birds are released at 12 noon, when the sun is due south. The control birds know to fly in a direction 90 degrees clockwise from the direction of the sun, but the clockshifted birds “think” it is 6 p.m., so they expect the sun to be in the west. Their 90-degree clockwise correction, then, has them flying due north.

CONCLUSION The clear difference between control and clockshifted birds in the experiment shows that an internal time-based sun compass is an important component of the birds’ homing abilities. However, the scatter of points (for both experiment and control) suggests that other factors are also important. This conclusion is reinforced by the observation that birds home well on cloudy days, when they cannot use a sun compass, suggesting that birds use multiple cues and navigational systems when they are homing.

SOURCE Keeton, W. T. 1969. “Orientation by Pigeons: Is the Sun Necessary?” Science 165:922–928.

Quick Check 3 Why is photoperiod such a widely used time cue?

Quick Check 3 Answer

Photoperiod is a good marker for time of year, and therefore for season. Many species have season-specific behaviors—hibernation in bears, migration in birds—that must be timed appropriately.