COMMUNITY ORGANIZATION: PERIODISM 



553 



Bremer, 1926; Kalmus, 1935, 1940; Lewis 

 and Bletchly, 1943; O. Park, 1940; Scott, 

 1936). We are not concerned at this point 

 with the experimental analysis of such pat- 

 terns, but it should be noted that these 

 insect pupation rhythms normally are in 

 step with the diel cycle, although, experi- 

 mentally, many such rhythms show a re- 

 markable degree of stability under con- 

 stant conditions (Fig. 194). 



In any discussion of diel periodicity it 

 is essential for a full appreciation of the 

 whole problem to remember that a period 

 of relative activity presupposes a period of 

 relative inactivity. The rhythmic alternation 

 of active and inactive states, in diurnal 

 and nocturnal species, is complementary. 

 Feeding and mating are consummated in 

 active states, and physiological recupera- 

 tion (rest, sleep) in a more or less pro- 

 tected habitat niche in inactive states. 



This is generally true for the majority of 

 metazoans under normal conditions. The 

 literature upon sleep, or its physiological 

 equivalent, is large, especially with respect 

 to sleeping attitudes, times of inactivity, 

 and the physiological investigation of the 

 sleep of mammals.** At the community level 

 many animals escape diurnal competition 

 for food, or may gain protection from ene- 

 mies, by a nocturnal period of activity. This 

 may be viewed from two points of view. 

 From the evolutionary standpoint (p. 

 682), many nocturnal species are thought 

 of as more primative than their diurnal rel- 

 atives. Kennedy (1928) discussed this 

 problem for insects and found that the 

 primitive orders were either nocturnal in 

 activity, or occupied cool, darkened habitat 

 niches, or were both nocturnal and occu- 

 pants of such habitats. Barbour (1934, p. 

 8) expressed the same general view that 

 primitive reptiles are often nocturnal, and 

 Clark (1914) found that such typical noc- 

 turnal animals as elephants, hippopotami, 

 tapirs, sloths, various edentates, many le 

 murs, nonaquatic monotremes, geckos, ony- 

 chophores, and diplopods have long pala- 

 eontological records where there is rela- 

 tively little imperfection in geological his- 



** The interested student will find the follow- 

 ing of value in pursuing the subject of sleep, 

 in its broadest sense: Conklin (1927), Fiebrig 

 (1912), Kleitman (1929, 1933, 1939), Pieron 

 (1913), Polimanti (1911), and Rau and Rau 

 (1916). 



tory. Many such animals are found in equa- 

 torial rain forest communities, and probably 

 the more vulnerable escape competition of 

 a rich fauna by their nocturnalism (O. 

 Park, 1940). Such nocturnalism should be 

 distinguished from that of many species 

 living in hot, dry desert communities. The 

 latter may be viewed as the development 

 of a nocturnal fauna, in part from the 

 forces exerted by physical influences of the 

 community. Both types of response, ob- 

 viously, are products of long periods of 

 selection. 



This first aspect, the evolutionary aspect, 

 of the development of nocturnality, is al- 

 lied to the development of diel rhythmicity 

 in general. Certainly the diel activities of 

 individuals, populations, and communities 

 are correlated with the physical environ- 

 mental rhythms. That this is a real, rather 

 than an apparent correlation, seems to be 

 well established. Biological rhythmicity is 

 a result of rhythmicity in the physical en- 

 vironment, as has been suggested often in 

 the past (Reynolds, 1920; Bouvier, 1922; 

 Kleitman, 1933; Welsh, 1938; Park, 1940). 

 If the diel cycle of activity and inactivity 

 does represent an adjustment of organ- 

 isms, populations, and communities for sur- 

 vival, then nocturnality and diumahty be- 

 come integral parts of the problem. 



In addition to this long-range evolution- 

 ary aspect of the development of nocturnal- 

 ity, there is a second aspect, in which 

 a population, or a part thereof, develops 

 nocturnality as an adjustment to an imme- 

 diate pressure of its biological environment. 

 For example, Roosevelt (1910) expressed 

 the view that in regions where game is 

 hunted persistently, many of the persecuted 

 animals tend to become nocturnal, whereas 

 the same species were often diurnal in re- 

 gions not systematically hunted over by 

 man. 



Diel cycles of activity of the community 

 are best known from the data on sound 

 production (sonification) and on vertical 

 migrations. Almost countless animal sounds 

 over the diel period give evidence of activ- 

 ity in the majority of temperate and tropi- 

 cal terrestrial communities. When such 

 sounds are analyzed at the species level, 

 the apparent diel character of this sonifica- 

 tion, both in terms of diurnal and nocturnal 

 species, and in the quantitative ebb and 

 flow of the volume of sound emitted by a 



