Chapter 9 

 ADAPTATIONS TO NOCTURNAL ACTIVITY 



(A) NOCTURNALITY AND THE EyE 



Nocturnality and Crude Vision — The support of nocturnality, in 

 animals whose eyes mean much to them, comes wholly from great sensi- 

 tivity to light. This is possible only with a preponderance of rods in the 

 retina, which in turn makes for low visual acuity. However, if a noc- 

 turnal animal emphasizes rhodopsin and the length of his rods rather 

 than their diameters, and keeps summation in optic nerve fibers at a 

 minimum, he may be able to retain good resolving power in bright light 

 — ^provided he has means of reducing greatly the sensitivity of the eye 

 under those conditions. Such means, as we shall see, are exemplified by 

 the common slit-shaped pupil and the rare occlusible version of the 

 'tapetum lucidum'; and the geckoes show what can be done to make an 

 extremely sensitive eye very valuable in the daytime even to an essen- 

 tially nocturnal animal, if that animal insists upon being able to come 

 out by day with safety. 



Nocturnal adaptation of the eye need not, therefore, be as restrictive 

 as bright-light adaptation. No cone-rich or pure-cone eye is useful at 

 night, but a pure-rod eye may be quite useful by day. But it is only 

 among the geckoes, in Sphenodon, and perhaps in the owls that forms 

 having great sensitivity have been able to combine with it a respectable 

 degree of resolving power. By and large, ocular adaptations for sensi- 

 tivity demand such a sacrifice of visual acuity that they make nocturnal 

 animals largely dependent upon senses other than vision. 



The nocturnal animal is primarily an ear- and nose-animal; and this 

 is particularly true of aquatic forms, to which the chemical and auditory 

 senses are especially important because of their enhanced value over 

 distances in water. Both audition and olfaction are promoted under 

 nocturnal conditions, though not because of anything the nocturnal 

 animal has done to modify the receptors of those senses. Odors and 

 sounds are carried better by the night air and are dispelled more slowly 

 because of the absence of rising air-currents. At night, too, sounds have 

 an augmented attention-value since they are of fewer kinds and are out 

 of competition with abundant visual stimuli. 



