Biological rhythms are self –sustaining natural cycles of animal life history which maintain themselves regardless of the environmental factors. All animals possess innate biological clocks which are driven by the biochemical mechanisms. Erwin Bunning (1936) was the first biologist to carry out extensive work on biological rhythms.
They show one-year periodicity, e.g. a large number of animals reproduce once in a year. Flowering in plants also takes place once a year. Insects and amphibians follow a cycle of hibernation and activity. Hummingbirds in South America move to the caves and become inactive in winter in Andes. Famous migration of Monarch butterflies from North America to Mexico and back follows annual cycle. Millions of these butterflies cover a distance of 3200 km to hibernate on trees in San Francisco. Many beetle species hibernate under the snow in Himalaya. Arctic and Antarctic animals generally follow annual cycles of activity.
These rhythms synchronise with the 28 day phases of moon and tidal rhythms. Palolo worm lives in deep sea but swims to surface on the first day of the first quarter of moon in November in Fiji. The sea hare (Aplysia) shows periodicity which is exactly half of the lunar cycle.
They are synchronised with the periodic rise and fall in sea level due to gravitational pull of sun and moon and centrifugal force of the earth. There are daily tides due to earth’s rotation on its axis. Spring tides cause maximum rise and fall in sea level because moon and sun are on the same side of earth. Neap tides occur when sun and moon are on opposite sides of earth at full moon stage.
They follow fortnightly cycle of 14.7 days of high tide after new moon or full moon. Molluscs exhibit egg laying behaviour according to this periodicity. Periwinkle also comes out of burrows on sea shores during high tides.
These follow 12.4 or 24.8 hour cycle that is synchronised with low and high tides twice a day. Animals living in burrows, such as polychaetes, planarians, crab etc. are submerged and exposed alternately and in the process get food brought by water currents. Bivalves such as Mytilus showed shell opening rhythm according to circatidal rhythms even when kept in the lab. Grunion fish spawns precisely at high tides.
These rhythms follow 24 hour cycle of activity and sleeping synchronised with light and darkness. So, the animals can be classified as nocturnal, diurnal and crepuscular, the last ones are active at sunrise and sunset. Birds are mostly diurnal and bats nocturnal which find their way by echolocation. Body metabolism and release of hormones are synchronised with 24 hour cycle.
Honey bees are known to have time memory. In experiments, honeybees after 5 hours of freezing came to food 5 hours late. Human beings experience jet lag when their circadian rhythm is disturbed while travelling in aeroplanes.
Larvae of Wuchereria bancrofti move to peripheral blood in the night but go to deeper blood vessels in daytime, which is synchronised with the blood-sucking habit of Culex mosquito.
Brady (1969) thought that optic lobes play an important role in controlling circadian rhythms in cockroach. Corpora allata and corpora cardiaca also release hormones that control day-night cycle. Cyclic AMP and serotonin are involved in biochemical events that control circadian oscillations.
In vertebrates, neural connections exist between retina and hypothalamus and pacemaker may be located in ventromedian nucleus of hypothalamus. In amniotes, the pineal and parietal bodies regulate photoperiodism. Melatonin secreted by pineal gland has anti-gonadotropic effect. Turtles synthesize serotonin during day and melatonin at night. However, this cycle disappears during hibernation.