BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.8. Innate Behavior

17.8.5. Biological Rhythms

Many behavioral patterns occur with regular periodicity and serve as one of the manifestations of biological rhythms (biorhythms). Well-known Examples include courtship and nesting periods in birds in spring or the migration of certain species to warmer regions in autumn. The intervals between periods of activity can range from a few minutes to several years, depending on the animal species. For example, the lugworm (Arenicola marina), which lives in U-shaped burrows in muddy or sandy bottoms, sticks its HEAD out of the burrow every 6–7 min and performs feeding-related movements. This rhythmic activity lacks any obvious external or internal physiological motivational stimuli and appears to be regulated solely by a "biological clock"—a mechanism that in this case depends on a pacemaker. Such a pacemaker is located in the supraoesophageal ganglion and periodically sends signals throughout the worm's body along the ventral nerve cord.

Rhythms driven by an internal pacemaker are called endogenous, as opposed to exogenous rhythms, which are regulated by external factors. With the exception of cases like the lugworm's feeding cycle, the vast majority of biological rhythms are mixed, i.e., partly endogenous and partly exogenous.

In many cases, the primary external factor regulating rhythmic activity is the photoperiod, i.e., the day length (and night length). This is the only reliable indicator of changing seasons against which biological clocks can be "set." The exact Nature of the clock is unknown, although it is undoubtedly driven by some physiological mechanism that may involve both neural and endocrine components. The effects of photoperiod have been extensively studied in mammals, birds, and insects. Although it clearly plays a vital role in controlling activities such as preparation for hibernation in mammals, migration in birds, and diapause in insects, it is not the only external factor regulating biological rhythms. The activity of certain species is also influenced by lunar rhythms. For example, the palolo worm, which inhabits the South Pacific, spawns across its entire range on only one day a year—in the third quarter of the moon following mid-October, averaging November 2. METABOLISM/18.html">The Influence of lunar rhythms on tides is well known, and these two exogenous factors are closely linked to The behavior of the midge Clunio maritimus. Its larvae feed on red Algae, which are exposed only during the lowest spring tides twice a lunar month. Under natural conditions, adults emerge from pupae, mate, and lay eggs within a mere 2 hours while this strip of the littoral zone is free of Water, after which they die. In the laboratory, under an artificial photoperiod of 12 h light/12 h dark, mass emergence of adults continued at intervals of about 15 days, indicating the presence of an endogenous clock programmed for approximately half the lunar cycle of spring tides, which is 14.8 days.

The behavior of many strictly terrestrial insects appears to be regulated by endogenous rhythms synchronized with the alternation of day and night. For example, Drosophila fruit flies eclose at dawn, while cockroaches are most active immediately after sunset and right before sunrise. These biological rhythms with a period of approximately 24 hours are called circadian (from Latin circa – about, dies – day). Studies of the activity of cockroaches of the genus Periplaneta under two lighting regimes (12 h light/12 h dark for 10 days, followed by complete darkness for the next 10 days) showed that under the second regime, animal activity was restricted to a time approximately coinciding with the post-darkness activity period of the first regime. The results of these observations, shown in Fig. 17.60, indicate that the circadian rhythm persists even in the absence of any external time cues, although the onset of activity varies slightly from day to day. This Supports the concept that circadian rhythms are controlled by an endogenous mechanism—a biological clock whose pacing is adjusted by exogenous factors.

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Fig. 17.60. Results of cockroach activity recording over 20 days. For the first 10 days, the cockroaches were kept under a 12-hour day and 12-hour night. The insects were then kept in complete darkness. The black areas correspond to the time and duration of their activity "bursts" on different days.

Circadian rhythms are believed to have diverse adaptive significance specific to each species and, in particular, related to orientation. Long-distance migrating fish, marine turtles, birds, and certain insects use the Sun and stars as a compass. Other animals, such as honeybees, ants, and amphipod crustaceans, use The Sun as a landmark for homing and foraging. However, sun and moon orientation is reliable only if the animal is somehow able to tell time in order to account for the diurnal movements of these celestial bodies. An example of a situation where the human innate physiological circadian rhythm deviates from the natural alternation of day and night is "jet lag," which long-haul airline passengers increasingly encounter.