tliough tlu're may he a secondary prc-dusk period of 

 activity (Calluniii 1^)44-46). 



A ciassiticatiqn of the diel activities of aiiinials in 

 respect to controlling influences can he made (O. 

 Park in Allee ct al. 1949: 558): 



I. Periodic activity. Regularly most active for 

 a specific ])eriod in the diel cycle. 



1. Exogenous type. .Activity rhythm directly 

 induced and controlled hy periodically re- 

 current environmental conditions. 



2. Endogenous type. .Activity rhythm more 

 or less inde]H"ndent of obvious factors 

 in the enviroinncnt : persists even under 

 controlled, apparently uniform environ- 

 mental conditions. It is possible, however, 

 that obscure environmental factors may 

 still be regulative (Brown 1959). 



a. Habitual activity. An endogenous 

 rhythm that has become established as 

 tlie result of previous experience of the 

 individual. 



b. Inherent activity. .An endogenous 

 rhythm that is inherited. 



3. Composite type. .A rhythm pattern that 

 is partly endogenous but is accentuated 

 when the animal is exposed to environ- 

 mental conditions periodically recurrent. 



II. Aperiodic (arhythmic) activity. Xo con- 

 sistency between individuals of a species in 

 exhibiting an activity pattern relating to a 

 specific time of day or night. 



Endogenous diel rhythms have been demonstrated 

 in Coelenterata. Platyhelminthes, Echinodermata, 

 Crustacea, Insecta, Cyclostomata. Pisces, Amphibia, 

 Reptilia, Aves, and Mammalia, although they are not 

 often inherent. Probably the great majority of species 

 have rhythms of the composite type. Periods of rest 

 or sleep alternating with activity seems to be a funda- 

 mental protoplasmic requirement (Park 1940). 



Carnivorous, herbivorous, and omnivorous noc- 

 turnal animals occur throughout the metazoans. It 

 is of interest that nocturnal animals are less fre- 

 quently gregarious and social than are diurnal forms. 

 Adjustments for night activity involve development 

 of luminescent organs such as fireflies (Lampyridae) 

 possess : infrared-sensitive vision, suggested for some 

 insects and birds but not proven for owls (Dice 

 1945) : increase in visual acuity by modification of 

 eye structures (Walls 1942) ; keenness of smell dis- 

 played by some mammals ; and increased sensitivity 

 to sound, remarkably developed in bats. Bats have 

 evolved a radar system, called echolocation, whereby 

 the animals emit ultra high frequency sound waves, 

 which are reflected from objects back to die ears 

 (Griffin 1953). Color vision, well developed in some 



diurnal insects, lisli, rejjtiles, amphibians, birds, and 

 mammals, is largely lost in those nocturnal species 

 active at such low intensities of light that colors 

 would be indistinguishable anyway. Correlated with 

 loss of color vision is restriction of body coloration to 

 blacks and whites or intermediate shades. Nocturnal 

 .inimals escape such diurnal predators as reptiles, 

 liirds, and hymenoi)terous insects. There are noc- 

 turnal ]5redators, to be sure, but jjredation pressure 

 at night ajjpears to be less intense than during the 

 day. 



There is decreased competition for food and 

 shelter when some species are active by night, others 

 l)y day, over the same range. Butterflies are pre- 

 dominantly diurnal; moths, nocturnal. .Animals with 

 moist skin, like snails and amphibians, suiTer less 

 evaporation of water from their bodies at night, when 

 the relative humidity is higher and the temperature 

 is lower. There is some belief (Clark 1914) that noc- 

 turnal forms are derived from originally diurnal 

 forms; an adjustment, perhaps, to avoid competition 

 from aggressive diurnal sjiecics occupying the same 

 niciies. 



Seasonal variations (aspection) 



In tropical rain forests there is very little sea- 

 sonal variation in the number of species active and 

 size of populations attributable to length of day, tem- 

 perature, and humidity, for these factors are nearly 

 uniform throughout the year. In other parts of the 

 tropics, however, there are definite wet and dry sea- 

 sons, and a considerable change in numbers and ac- 

 tivities of animals correlates with the seasonal varia- 

 tions in vegetation and food supply. In temperate 



|JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCTjNOV|0EC| 



FIG. 8-3 Monthly variation in daily photoperiods at 

 latitudes of the Northern Hemisphere (after Boggs 1931 ). 



Rock, sand, and clay 101 



