DAILY RHYTHMS AS COUPLED OSCILLATOR 



SYSTEMS AND THEIR RELATION 



TO THERMOPERIODISM AND PHOTOPERIODISM^ 



COLIN S. PITTENDRIGH AND VICTOR G. BRUCE 



Biology Department, Princeton University, Princeton, New Jersey 



CHRONOMETRY, RHYTHMS, AND PHOTOPERIODISM 



For the last thirty years the fact that organisms can measure time 

 has confronted biologists studying very diverse phenomena. The 

 first clear and explicit demonstration of functional chronometry in 

 organisms was that of Beling (1929) and Wahl (1932), who showed 

 that the honeybee returns to a favorable feeding place for several 

 days at precisely the same sun-hour at which it was initially dis- 

 covered. A very definite instance of chronometry is implied — as later 

 work has shown — in the photoperiodic effect initially discovered by 

 Garner and Allard (1923); many plants and animals identify season 

 by what amounts to a time measurement of night length. A third 

 category of chronometry includes all the remarkable cases of celestial 

 navigation recently uncovered by Kramer (1952), von Frisch (1950), 

 Hoffman (1954), Pardi and Papi (1953), Lindauer (1957), Sauer 

 (1957), and Birukow (1957). In these cases an endogenous timing 

 system is utilized by vertebrates and arthropods in correcting for the 

 shifting position of celestial direction-givers. 



A fourth class of phenomena implying functional time measure- 

 ment by organisms is that of persistent rhythms with a daily, tidal, 

 or lunar period. Observations on persistent daily rhythms are, to be 

 sure, much older than those in the other three categories. Bouvier 

 (1922) cites observations in the nineteenth century, and the begin- 

 nings of essentially modern work on persistent rhythms go as far back 

 as the classical researches of Pfeffer commenced in 1875 (see BUnning, 



1 Previously unpublished experimental data presented in this paper were ob- 

 tained in studies supported by grants from the National Science Foundation and 

 the Eugene Higgins Trust. 



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