170 ADAPTATIONS TO DIURNAL ACTIVITY 



Poikilothermous vertebrates, generally, may be diurnal for the sake of 

 the activating effect of sunlight upon metabolism and locomotor activity, 

 unless they happen to be particularly defenseless or especially dependent 

 upon prey which in itself is nocturnal. Predaceous fishes, most reptiles, 

 and some frogs fall here. 



Predaceous vertebrates, generally, require fairly sharp vision at rela- 

 tively close range in order to pursue and capture prey and obtain a grasp 

 upon it which will be advantageous to them in any ensuing combat. Be- 

 ing ordinarily swifter than the prey — at least for short bursts — there is 

 added need for acuity of vision, which must keep pace with speed if 

 'crashes' are to be avoided. These factors are especially operative in 

 fishes, lizards, and birds; and they are largely responsible for the acuity- 

 adaptations tenaciously retained by those carnivorous mammals which 

 attempt to compromise between sensitivity and acuity by having large 

 eyes. Small, small-eyed carnivores on the other hand are nocturnal, 

 largely because the small prey animals which they are able to master 

 have taken refuge from them in nocturnality. 



Defenseless, herbivorous prey animals which rely upon speed for escape 

 must recognize enemies at a distance. This in itself demands high visual 

 acuity; and the factor of distance, besides reducing the retinal-image size 

 of the potentially dangerous object seen afar, greatly reduces its bright- 

 ness. Vision at a distance is therefore altogether impossible in dim light. 

 The ungulates, and the more strictly diurnal mammals, must have high 

 visual acuity for safety, and their acuity-devices will work only under 

 bright-light conditions. Lastly, predators which specialize on diurnal prey 

 must ordinarily be permanently or temporarily diurnal themselves — the 

 hawks, for example, as also the bear during his annual gorge on salmon. 



However, we must not suppose that in every act of predation both par- 

 ties are under optimal conditions and fighting to best advantage. On the 

 contrary, many a predator is nocturnal in order to seek out prey which, 

 being itself diurnal, is asleep at night and hence at a disadvantage. Con- 

 versely, the diurnal predator may depend not upon diurnally active food 

 animals, but upon nocturnal ones which, sleeping in their burrows by 

 day, are then easily surprised and subdued. The diurnal snake exploring 

 the nests of slumbering rodents, the nocturnal marten investigating a 

 squirrel's dray, are examples of these advantageous employments of the 

 sleep-time of the victim. 



We can easily imagine that diurnality and nocturnality have come and 

 gone, sometimes repeatedly, in particular lines of descent. Prey animals 



