Chapter 8 

 ADAPTATIONS TO DIURNAL ACTIVITY 



(A) DiURNALITY AND THE EyE 



D'turnality and Sharp Vision — The adoption of diurnality entails a 

 sacrifice of sensitivity. This is hardly possible without a marked increase 

 of visual acuity, for if the cones are multiplied at the expense of the rods, 

 resolving power inevitably rises. While it is theoretically possible for an 

 animal with a pure-rod retina and crude vision to be strictly diurnal, 

 given the right type of pupil, in actual fact it never happens. 



Adaptation to diurnality is thus, at the same time, adaptation for sharp 

 vision. Diurnal animals are relatively keen-sighted, and their other habits 

 are such as to demand keen sight; but it is of course impossible in most 

 cases to say whether they are diurnal and cone-rich in order to have sharp 

 vision (which is probably true of the birds) or have only cones simply in 

 order to be diurnal, without making the most of the opportunity to gain 

 sharp vision (which may hold for the snakes) . The relationship between 

 visual acuity and diurnality, in so far as it expresses needs and the pro- 

 duction of adaptations to fill those needs, is perhaps most easily seen in 

 a rough analysis of feeding habits : 



Diurnality, Acuity, and Food — Animals which feed upon small ob- 

 jects such as seeds and insects must be able to resolve them, which is pos- 

 sible only for an eye rich in cones and hence diurnal in capacity. Most 

 lizards, birds, and primates are in this category; as are also the tree- 

 shrews, at least, among the insectivores. It is important to remember that 

 insects themselves are poikilothermous, hence most species are most active 

 and available under diurnal conditions. Nocturnal insect-feeders can 

 place no reliance upon vision, but must either rely upon hearing and 

 touch for securing individual insects (bats) or else 'trawl' blindly through 

 the air for flying insects with wide-open mouth (goatsuckers, frog- 

 mouths) . The dependence of most birds upon sunlight is proverbial. So 

 is their visual acuity. In this respect, man acknowledged even the small 

 birds to be his superior, centuries ago — it was the habit of the medieval 

 falconer to carry a caged shrike on his saddle, to keep track of the falcon. 

 As long as the shrike acted fearful and excited, the hawker knew that his 

 proud tiercel was in sight — though not to him\ 



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