Biological Rhythms in Nature

An article by Mohan Pai


“God does not play dice with the Universe”- Albert Einstein


Biological Rhythms refers to periodic biological fluctuation in an organism that corresponds to, and is in response to, periodic environmental change. Examples of such change include cyclical variations in the relative position of the Earth to the Sun and to the Moon and in the immediate effects of such variations, e.g., day alternating with night, high tide alternating with low tide..The internal mechanism by which such a rhythmic phenomenon occurs and is maintained even in the absence of the apparent environmental stimulus is termed a biological clock. When an animal that functions according to such a clock is rapidly translocated to a geographic point where the environmental cycle is no longer synchronous with the animal’s cycle, the clock continues for a time to function synchronously with the original environmental cycle.

Humans similarly transported over great distances often experience fatigue and lowered efficiency for several days, a phenomenon known as “jet lag,” or jet syndrome.A rhythm with a 24-hour cycle is called a circadian (from Latin circa, “about”; di, “day”—i.e., “about a day”), solar day, diel, daily, diurnal, or nychthemeral rhythm. A lunar tidal rhythm—the regular ebb and flow of oceans and very large inland bodies of water—subjects seashore plants and animals to a rhythmic change; typically two high and two low tides occur each day (about 24.8 hours). Many species of shorebirds exhibit this rhythm by seeking food only when beaches are exposed at low tide.

Monthly rhythms, averaging approximately 29.5 days, are reflected in reproductive cycles of many marine plants and in those of many animals. Annual rhythms are reflected in the reproduction and growth of most terrestrial plants and animals in the temperate zones.Animal behaviourPeriodic change with the time of day, month and year is a most spectacular feature of the environment of the earth. The three major periods thus prevalent are those of the solar day (24 hours), lunar month (29 days) and the calendar year (365 days). Most living creatures have adapted themselves in many ways to this temporal order of their environment, giving rise to a variety of biological rhythms. Theses rhythms enable the plants and animals to carry out their various bodily functions at the most advantageous time of the day, month or year.The cold-blooded lizards must raise their body temperature that necessarily falls during the night by sunning themselves in the morning. The most appropriate time for hunting for prey for them is therefore late morning, when they can be warm and active. Crabs on the seashore must adapt their feeding times in accordance with tides which depend on the rotation of moon. Insectivorous birds must adjust their breeding seasons to correspond with the yearly period of maximum abundance of insects to satisfy the requirements of their fast-growing chicks, and so on.

Animals fall into two broad categories of day-active and night-active, depending on the time during which they actively seek food. On land insects have large membership in both these categories. Cold-blooded reptiles are constrained to be day active, and the primarily visual birds are also day-active. Amongst the ground dwelling mammals, both habits are quite common. While the flying mammals bats, are all active at night, taking advantage of the paucity of night active birds.Even within these categories, however, not all animals are equally active throughout the day or night. Their activity patterns vary, depending both on changes in the physical environment, as well as in response to the activity pattern of other animals in their habitat.

Dawn and dusk are periods of the most rapid change in light intensities, and most animal use these as cues to initiate or terminate their period of rest. Thus birds become active and bats go to rest at dawn, and the reverse occurs at dusk. If we look further, the Jungle Crow becomes active at lower light intensities than the Indian Myna, and the pipistrelle bats at higher light intensities than the Flying Foxes. Dawn and dusk are also the times at which the air is least turbulent, permitting sound to be carried farthest. That is why birds indulge in their most intense vocalization at these times, giving rise to dawn and dusk choruses; as do monkeys like the Hanuman Langur.Animals also adjust their periods of activity to minimize competition with other species. Thus various species of bees have peaks of flower-visiting activity at different times of the day, and different species of mosquitoes have peak blood-sucking at different times at night.

Such rhythms have greatly fascinated physiologists who have attempted to study them under experimental conditions. It has been shown that the rhythms are not merely imposed from outside, but persist even under totally unchanging conditions. Under these conditions, however, the period of rhythm is not precisely 24-hours, but nearly so, hence these rhythms are known as circadians (circa, about, dies, day). They are adjusted to the diurnal rhythm of light, temperature, etc. Through the external cues. It is now known that animals use social cues provided by other animals as well in adjusting their rhythms. Thus bats confined to deep part of the cave with no environmental cues of light or temperature can still synchronize their activity with the day-night regime by picking up their cues from the vocalization of the other bats in the cave.
Marine animals too exhibit a number of biological rhythms. The zooplankters migrate towards the surface at night and move down deeper during daytime. The animals on the shore adjust their periods of activity in relation to the tides. Furthermore, the tides change not only once or twice a day, but vary in their magnitude with the phase of the moon and the time of the year. Certain marine animals such as the famous Palolo worm of Fiji seem to synchronize their breeding with these tides. Thus the palolo worm swarms to reproduce every year 7 to 9 days after the full moon in November.At the other end from the palolo worm, the entire population of which breeds on just one day in the whole year, is an animal such as our Asiatic elephant which seems to breed, and also to come to musth, at any time of the year. The Chital has an extended breeding season, its rutting coinciding with the monsoon and the season of the birth of calves peaking from January to March, although some calves are born in every month of the year. This coincidence of birth of calves with the most difficult season of the year in terms of food availability is truly puzzling. In Karnataka, the major predator of Chital, the Wild Dog, breeds from January to March, presumably because its food is most plentiful at the time of fawning by chital. Among our birds, the small insectivores such as Warblers breed during the monsoon, apparently because this is the time of maximum abundance of insects to feed their chicks. The birds of prey, on the other hand, breed mostly during December-March, again apparently because this is the time of maximum abundance of their rodent prey which multiplies following the seeding of grasses and cereal crops towards the end of the monsoon.The breeding of herons, storks and other colonially breeding water-birds coincides with the monsoon. Thus at Bharatpur in Rajasthan or Ranganathittu at Shrirangapatna they breed from July to October, the southwest monsoon bringing most of the rains in these parts. On the other hand, at Vedanthangal near Chennai they breed from January to March, this part receiving most of its rainfall during the northeast monsoon. However, this rule is not without exception. Night Herons breed at Ranganathittu from April to August, but near Bangalore, hardly 120 km away, they breed from January to March; similarly Little Cormorants breed at Ranganthittu from July to October, but hardly 80 km away they breed from January to March.

The migratory birds show a remarkable annual rhythm of long distance movements. Many of our ducks, teals and waders breed in Siberia in summer, from April to September. In autumn they migrate south to India, staying here from October to March, moving north again in the spring. The migratory impulses of these birds is known to be controlled by changing day-length. They migrate southwards after breeding in response to decreasing day-length and north after wintering in response to increasing day-length.

Finally, a most spectacular example of biological rhythm is furnished by some species of tropical bamboos. Our commonest species Bambusa arundinacea, flowers and seeds only once in its lifetime at an age of 45-48 years, after which it invariably dies. Moreover, the flowering is synchronized for a whole population so that all the bamboo species flower and die over a region of several thousands of hectares within the space of three to four years. The significance of this seems to lie in the fact that when seeds are very occasionally produced in such large quantities, predators on the seeds such as rodents can only devour a small fraction of them. If on the other hand a much smaller seed crop was produced every year, a much greater fraction of the seed could be destroyed by the predators. Hence, it is likely that massive seeding in a few years has been favoured by natural selection.Animals not only respond to natural cues, but as the persistence of their rhythms under constant conditions shows, they also have endogenous rhythms – Circadian as well as circannual, and perhaps of much longer duration as well as in the case of bamboos. The precise nature of these biological clocks is yet unknown. Animals also use these clocks for purposes other than adjusting behavioural rhythms. Thus honey-bees, as also some fish and birds, are known to use the sun for navigation. However the position of the sun varies with the time of the day. These animals make fine adjustments for such movements of the sun by using biological clock.

Reference: Encyclopedia of Indian Natural History – Edited by R. E. Hawkins for Bombay Natural History Society. 

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