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
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.
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.
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
Halberg Chronobiology Center > Outreach & Education > Cycles of Nature > Cycles of Nature: The Role of Environmental Influences >