BEHAVIOUR 189 



one of these animals for a whole month in a Japanese tank, state that it 

 did not sleep at all but merely increased its respirational and other activities 

 from nightfall until about midnight, thereafter to become more restful again. 



The fact that aquatic animals sleep at all may seem strange, since we 

 usually associate the idea of sleep with a supine position, but if we recall 

 that horses and elephants have no difficulty in sleeping on their feet, there 

 is really nothing that need occasion us any surprise. Some fish sleep 

 during the night, while others which are most active during that period 

 (e.g. some sharks) spend their days dozing at the bottom of the sea, or 

 near the surface (e.g. the Basking Shark and the Moonfish). Although 

 seals and sea-lions generally sleep ashore, they, too, can occasionally doze 

 in tlie water, sometimes with their noses just above the water, but gener- 

 ally floating below the siu-face and coming up for air at regular intervals. 

 They often sleep with their eyes shut, and give the impression of being very 

 fast asleep indeed, though it is still debatable whether their 'sleeping' is 

 comparable with ours. When we sleep, our blood pressure and respiratory 

 frequency drops, our muscles become relaxed, and there is a decrease in 

 activity of certain nerve centres. It is, of coui'se, extremely difficult to 

 establish the existence of similar phenomena in animals, and particularly 

 in fish and dolphins, though we do know that most terrestrial mammals 

 (some monkeys excluded) sleep much less deeply than we do. Cows, 

 horses and donkeys never seem to sleep properly at all, but to doze 

 gently instead. 



The Sperm Whale seems to be the deepest sleeper of all Cetaceans. A 

 number of observers have stated repeatedly that this animal can stay near 

 the surface for hours on end, apparently very fast asleep. This is also 

 borne out by the many stories of ships colliding with sleeping Sperm 

 Whales. Thus, one dark night during the Second World War, the crew 

 of an American destroyer felt a heavy jolt as their ship rapidly lost speed. 

 Thinking that they had been torpedoed, they took to the boats only to 

 discover that there was no apparent damage. Next morning the body of a 

 large Sperm Wliale was found right across the bows. A similar experience 

 was had by Capt. A. P. Disselkoen on 22nd March, 1955, aboard the S.S. 

 Amerskerk. The ship was making 17 knots west of Cape Guardafui 

 when she suddenly had a mysterious collision. It was found that, just 

 below the water-line, she had struck the head and body of a thirty-two- 

 foot Sperm Whale. The engines had to be reversed to shake ofif the 

 animal which had been killed by the impact. It seems likely that Sperm 

 Whales were also responsible for the reported collisions of the Russian 

 whaler Aleut near the Panama Canal, the 24,000- ton American liner 

 Constitution ofif Genoa, and the Willem Ruys (Royal Rotterdam Lloyd) 

 between Cape Town and Colombo (all three in 1956). 



