As the World Turns
All biological clocks are adaptations to life on a rotating planet.
Colin Pittendrigh, undated.
Periodicity in the geophysical world from the rising and setting of the sun to
the waxing and waning of the moon has been observed and recorded since
earliest human times. People also long observed physiological and behavioral
cycles, such as waking and sleeping patterns, menstrual cycles, the seasonal
appearance and disappearance of certain plants, and the migration of birds. What
is the connection between biological and geophysical periodic cycles?
It has long been assumed that circadian clocks capable of being synchronized
or entrained to the environment are an evolutionary adaptation to life on a
rotating planet. Organisms have their own endogenous (internally produced)
biological clocks, which regulate all aspects of their physiology. These
internal clocks are reset by environmental cues known as zeitgebers (German
meaning time-givers), thus keeping them synchronized with the external
environment. Light is the most common zeitgebers. In humans, the brain's master
clock is reset via light receptors in our eyes.
The Periodic Universe.
This late 19th century mechanical planetarium, known as an orrery, illustrates planetary movements.
An Illuminated World.
This modern globe shows the natural cycle of day and night as the Earth rotates on its axis.
What do these actograms show?
The multi-crab "live" recordings of crab activity should theoretically match the
one displayed above, which shows the activity of a single crab in a controlled
laboratory setting. In the laboratory-generated actogram, the crab's locomotor
activity can be seen to follow tidal rather than circadian rhythms. The circles
indicate high tide, which occurs every 12.4 hours, while the bar at the top of
the page indicates sunrise and sunset. To see if these live crabs are following
tidal patterns, you will need to check their activity against the tide chart.
Fiddling while the Tides Rise Uca minax.
These tiny fiddler crabs show tidal and circadian rhythms. Their exterior color
changes according to a 24-hour schedule, whereas burrowing and other locomotor
activities correlate with high and low tides. These tidally timed behaviors
continue even when crabs are placed in a laboratory aquarium that has no tidal
movement. These crabs, which are normally quite "shy," may not behave as
expected in the presence of artificial light and visitors.
The ocean's daily tides are driven by shifting gravitational pulls that are
created as the moon orbits the Earth.
For humans and most other organisms, light receptors are used to reset the clock,
thus keeping the organism synchronized with its environment.
A Single-Celled Clock with a Glowing Reputation Gonyaulax polyedra.
Gonyaulax are single-celled free-swimming algae, that exhibit bioluminescence
according to a circadian schedule. Bioluminescence is the abilty of living
organisms to emit visible light. This circadian expression of bioluminescence is
the result of the daily synthesis and destruction of two specialized proteins.
The color image showing day and night fluorescence is for a single organism and
shows the specific sites within the cell that emit light.
Seasonal Migrants Sandhill Crane, Inca Tern, and Grey Seal.
From building nests and reproducing to migrating, many animals follow circannual
(yearly) rhythms. How do the migrating animals find their way, often across
thousands of miles to precise locations? They rely on internal clocks and their
position relative to the sun to determine their longitudinal (east-west)
positions, and they may rely on their position relative to the stars or to the
Earth's magnetic fields to determine their latitudinal (north-south) positions.
The Seasonal Fashions of Siberian Hamsters.
The Siberian hamster, like many other mammals found in northern regions, has a
light coat color in winter and a dark one in summer. The circadian clock in the
hamster's brain registers progressive changes in the length of the night, as
winter or summer approaches — and hormonally triggers the change in coat color.
This is accomplished through the release of the hormone melatonin. As the length
of the night changes, so too does the amount of melatonin released.
Yearly and Daily Cycles.
In addition to circadian cycles, many animals also follow circannual or yearly
cycles. One such cycle is hibernation, which occurs when daylight hours
decrease. Bears are so-called opportunistic hibernators in that they will
periodically arouse from the hibernating state. Even when animals are
hibernating, however, their circadian systems remain operational.