230 ADAPTATIONS TO NOCTURNAL ACTIVITY 



required by some vertebrates because of an important difference between 

 a camera and an eye: for the eye, exposure-time cannot take the place 

 of intensity — the eye can only take 'snapshots'. 



Under nocturnal conditions, a visual object may be brighter than its 

 surroundings, or it may form a shadowy silhouette against a background 

 brighter than itself. There is a perennial argument as to whether a tape- 

 turn enhances visibility by sometimes promoting the perception of the 

 object, or by sometimes increasing the apparent brightness of the back- 

 ground. The argument is quite pointless; for, no matter which has the 

 greater brightness — object, or ground — the reflections from the tapetum 

 will increase the absolute and relative differential between the two, and 

 thus increase their discriminability. 



Not all animals which have eyeshine possess any definite tapetum, 

 as an examination of the pertinent Table VII (pp. 240-1) will show. In 

 the ostrich, at least, the light reflex has been attributed to the lamina 

 vitrea between pigment epithelium and chorioid, as the lamina is extra- 

 ordinarily thick in this bird. A number of other birds, both nocturnal and 

 diurnal, also show eyeshine, but with no known structural basis for it. 

 There are also many fishes, anurans, and snakes (but not lizards) in 

 which there is eyeshine and in which the reflecting material has not 

 been identified, though it is certainly nothing especially differentiated 

 for the purpose. 



An anomalous eyeshine even occurs in a few humans. It is normally 

 lacking in all diurnal monkeys and apes, and Ernest Walker found only 

 a "faint suggestion of a shine" in the diurnal Lemur catta. Among the 

 other mammals, the rodents and lagomorphs are conspicuous for having 

 a dull eyeshine (whose basis is yet to be found) in nearly all species, 

 including even the strongly diurnal squirrels. Only one rodent, Cunt- 

 cuius paca, is known to have a tapetum; and even here the reflex is said 

 to be of only moderate brilliance. The Hystricidae may prove to have a 

 tapetum of some sort, for in these exotic porcupines the silvery eyeshine 

 is described as being particularly brilliant, and visible through a wide 

 angle. 



In snakes, the eyeshine varies from faint to brilliant in both diurnal 

 and nocturnal groups. Klauber states that it can be seen through only a 

 narrow angle, which suggests that it may come wholly from the myelin- 

 ated optic-nerve head and means nothing to the scotopic vision of the 

 animal. 



