234 WHALES 



and ungulates which use their eyes day and night, and in lemurs which 

 only spring to life after sunset. Man shares his lack of a tapetum with 

 diurnal apes, pigs and a number of other mammals. In all Cetaceans, 

 however, the tapetum, which forms a blue or green iridescent layer, is 

 extremely well developed, covering almost the entire surface of the 

 choroid, particularly in Mysticetes where, as in seals, it runs as far as the 

 ciliary body. In Odontocetes, though not so large, it is still considerably 

 bigger than in terrestrial mammals. 



Another adjustment to the paucity of light under the sea is found in 

 the microscopic structure of the retina, whose receptors of sight consist 

 mainly of modified nerve cells called rods and cones, the ability to see in 

 dim light depending largely upon the rods. Now, many investigators have 

 been able to show that Cetaceans not only have a greater number of rods, 

 but that their rods are bigger than those of terrestrial mammals. Mysticetes 

 appear to have the longest rods of all, possibly because of their deep dives. 

 VVe might then expect Sperm Whales and Bottlenose Whales to have very 

 long rods also, but unfortunately not enough data are available to test 

 this hypothesis. 



It was long believed that Cetaceans lacked the cones found in the retina 

 of terrestrial mammals, but in 1946 Fischer managed to isolate such cones 

 in the retina of a Sperm Whale and a Fin Whale. This discovery was 

 highly significant because many authorities believe that the presence of 

 cones is associated with visual acuity and with colour perception. 



A question of great importance for the correct evaluation of Cetacean 

 behaviour is the size of the Cetacean visual field. When we look straight 

 ahead we can take in a field of 160 without having to move our head. 

 A dog covers a field of roughly 250°, and some rodents (e.g. haies and 

 rabbits) can cover 360°, i.e. they can observe everything that goes on 

 around them. Their large visual field, so important in animals threatened 

 from all sides, is, however, offset by loss of stereoscopic vision, which is 

 indispensable for estimating distances correctly and is therefore par- 

 ticularly sharp in hunting animals, or in arboreal animals which jump 

 from branch to branch. Stereoscopic vision occurs when the visual field 

 of the left eye partially overlaps that of the right eye. In man, the stereo- 

 scopic field is 120"" in a total visual field of 160°, in the dog about go°, 

 in the horse 60", and in the rabbit 30° in a forward direction and another 

 9° in a caudo-dorsal direction. 



In other words, hunted animals usually have their eyes placed laterally, 

 whereas hunters, which have to judge their distance from their prey very 

 accurately, benefit from eyes placed more frontally. Thus we might 

 expect Cetaceans, and Odontocetes particularly, to have close-set eyes, 

 were it not that this would vitiate their streamlining. Moreover, frontally 



