Removing Environmental Factors

The effect of removing an environmental factor

Experiments have been done where environmental influences from the light/dark cycle are entirely removed, to observe the response of the organism.  Some plants, if kept in constant light and constant tem
Figure 3.5
perature, lose their leaf-motion cycle completely and immediately. In these plants the motion cycle seems to be controlled entirely by the environment. In other plants, the cycle will continue for a week or more in either constant light or constant darkness.  For example, Figure 3.5 shows a seven-day record of leaf angles for a silk tree plant kept on a repeating schedule of 16 hours of light and eight hours of darkness. (The dark sections on the time scale indicate periods of darkness.)
Figure 3.6
Figure 3.6 shows a record for another silk tree plant kept on the same schedule at first, but then kept in constant darkness after the third day. The leaf movement is not as large without the influence of the light, but it does continue. The leaf motion cycle of this plant seems to be an innate biological cycle that is amplified by environmental influence. This persistence of cycles without any known external influence from the environment may surprise you, but it has been known for over 250 years. In 1729 the French scientist Jean Jacques de Mairan published a description of how he had kept plants in the dark and observed the continuation of their "sleep movements." He concluded that the plant could "sense the sun without seeing it in any manner."

Figure 3.7
In animals, too, biological variables continue to rise and fall in cycles even when environmental conditions are kept constant. For example, Figure 3.7 is a 17-day record of the internal temperature of a rat. For the first nine days, the rat was kept on a schedule of 12 hours of light and 12 hours of darkness. From day ten on, it was kept in constant light (the light is never turned off). The temperature in the cage was kept constant throughout. (Measurements were made automatically: A tiny temperature-sensing radio transmitter was surgically implanted in the rat's abdomen, and the radio signals were detected by an antenna under the floor of the rat's cage.) There was only a slight reduction in the range of the cycle when the rat's lighting clues were taken away. Just as for the plants above, the cycle here was originating inside the organism itself. There also seems to be a four or five day temperature cycle, (associated with the rat's estrus cycle) which is confirmed by statistical analysis. Less obvious, but statistically demonstrable in longer series of data, is an about-seven-day cycle.

Figure 3.8
An organism does not have to be as complicated as a silk tree plant or a rat to show the persistence of circadian cycles in the absence of environmental cues (under constant conditions). Cycles persist even in one-celled plants and animals, even in simpler microorganisms such as the bacteria. For instance, there is a species of one-celled algae (species Gonyaulax polyedra) that has a circadian cycle of glowing with a pale green light. Kept under constant conditions in a laboratory, this species of algae continues its glowing cycle indefinitely, as the results of the experiment in Figure 3.8 show. In the figure, the amount of light given off by a test tube of algae is graphed over a span of six days. You can see that the period of the cycle is close to 24 hours.

About-Yearly Cycles
There are many examples of biological cycles with periods longer than a single day. These longer cycles are most often approximately a week, a month, or a year in duration. One dramatic exhibition is an experiment on four ground squirrels, who were raised in the lab from birth, in constant darkness and with no contact with the outside world.  They hibernated each winter and awoke each spring, even without environmental cues. Each year of the four-year study, the squirrels' hibernation schedule varied by only a few weeks from those of squirrels in the wild.  Such internal cycles are also found in migratory birds that spend their winters in the tropics. Experiments have shown that when these birds are kept under constant laboratory conditions, they show persisting about-yearly cycles in body weight, in molting, in size of male sex glands, and in“"migration restlessness" (repeatedly hopping in the direction in which they would normally be migrating). Other experiments under constant conditions have shown that sheep maintain an about-yearly cycle in wool growth as do deer in antler growth.

About-Monthly Cycles
Crustaceans collected on sea coasts continue to show cycles of activity that match the tides where they were found, even when they are kept under constant laboratory conditions, having no exposure to tides. An interesting example of the tidal cycle is found in a tiny crustacean called the sand hopper (species Talitrus saltator). This animal stays buried in the sand of the beach except when the edge of the high tide reaches it, allowing the sand hopper to swim for a while. In one experiment, sand hoppers were collected from the shore, placed in beakers, and kept under constant conditions in the laboratory for four days.

Figure 3.9
The lower graph in Figure 3.9 shows how many sand hoppers were swimming, recorded every three minutes from July 18 to July 21. The upper graph shows how the height of the tide varied over those four days on the shore where the sand hoppers had been collected. (Because of the position of the sun and moon on these dates, the two tides each day were unequal.) Though there was no tide in the beakers, the swimming cycles of the hoppers still appeared, just as if they were still periodically getting wet on the shore.

Are these cycles really caused within the living organism or tissue? Or is the environment still exerting some influence that the experimenters have not been able to keep constant? Can it be that organisms can somehow "sense the sun" as de Mairan thought over two centuries ago? How else could a biological system keep nearly perfect time?

Variation in Free-Running cycles

Figure 3.10
When environmental cues are removed, the resulting cycles are called free-running.  The truth is that persisting cycles (cycles that persist when environmental cues are removed) rarely match environmental cycles exactly.  We have said that organisms under constant conditions show persistence of about-daily, about-weekly, about-monthly, or about-yearly cycles. In fact, the free-running cycles of organisms under constant conditions are seldom exactly the same as the environmental cycles they follow under normal circumstances. Figure 3.10 shows the activity records of two different species of fruit bats kept under constant conditions. The graphs show measures of their activity during each half hour of a 10-day experiment. The graphs for the 10 days are stacked on top of one  another, with the first day at the top. Bat 1 started its activity about a half hour earlier each day, which means that !ts free-running period was shorter than 24 hours about 23 1/2 hours.
Figure 3.11
Bat 2 started its activity about a half hour later each day, so its free-running period was about 24 1/2 hours. Such differences in free-running periods are found even among members of the same species. Figure 3.11 shows records of wheel-running activity for two flying squirrels kept separately under constant conditions in a 26-day experiment. Each squirrel had a steady drift in the time of day when it started to run continuously on its wheel. Squirrel l's period was only two minutes short of exactly 24 hours, while squirrel #2's period was 21 minutes longer than 24 hours. In an experiment with 50 flying squirrels kept separately in constant darkness, all but five had free-running periods between 23 and 24 hours. This kind of variety in periods is typical of free-running cycles. People who have been isolated for long spans of time in caves or in isolation rooms also show such drifts.  one human subject kept under constant conditions in a bunker settled into a sieep/waking cycle of about 33 hours. For four weeks his temperature was measured continually, and his urine was analyzed to find the amount of calcium and potassium excreted. Calcium excretion showed the same 33-hour period as sleep, but the temperature and potassium cycles showed a more typical period of 24.7 hours.