208 ADAPTATIONS TO NOCTURNAL ACTIVITY 



Advantages and Limitations — It may be stated categorically that 

 nocturnality, wherever it is characteristic of a large taxomic group, has 

 always been adopted secondarily by the ancestral form of the group. 

 Even more certainly, any nocturnal member of an otherwise diurnal 

 group has become nocturnal independently. We can be sure that all 

 vertebrate species would be diurnal if they could 'get away with it'. 



The original chordates were bright-light animals. The early fishes 

 invented rods in order to extend their day and to be able to venture 

 from the surface to depths where they were safer, but where the lessened 

 illumination made necessary greater ocular sensitivity. The first land 

 animals were quite without predaceous enemies and were able to enjoy 

 the benefits of sunshine by becoming diurnal and heliothermic. But 

 increasing competition on land drove some forms into the cavern of 

 nocturnality to escape their enemies and to be able to feed in compara- 

 tive peace. These nocturnal amphibians and reptiles were the better off, 

 the smaller their bodies and the less they were dependent upon the sun 

 for the maintenance of rapid metabolism. The advent of small-bodied 

 descendants of the massive stegocephalians made nocturnality desirable 

 for the reduction of water-loss; and the small animal, being able to be 

 more active at a given environmental temperature, suffered no disad- 

 vantage from the change in habits. 



Upon the invention of 'warm-bloodedness', independence of the sun 

 became greater. The mammals for the most part proceeded to become 

 crepuscular and nocturnal. The defenseless plant-eaters then found 

 greater safety in feeding, which is in them an almost continuous and 

 decidedly noisy process which places the animal at a real auditory dis- 

 advantage. Predators were forced into nocturnality by the paucity of 

 diurnal prey. The birds, however, were mostly prevented from aban- 

 doning diurnality by the high requirements imposed upon visual acuity 

 by the habit of flight. The ability to fly, in itself, served as a compensa- 

 tory defense against most predators, for birds are most vulnerable in the 

 form of eggs and young, as easily captured at one time of day as another. 

 The most conspicuously nocturnal birds, the owls, trace their ancestry 

 from diurnal birds through the crepuscular goat-suckers and frog-mouths. 

 They had no trouble in becoming nocturnal, for with their size and 

 roundheadedness, there was abundance of room in their heads for eyes 

 large enough to combine fair resolution with super-sensitivity. 



Though nocturnality is something of a sanctuary from predators and 

 carries with it a coincidental improvement of audition and olfaction 



