128 WHALES 



that Cetaceans are by far the best divers. A dog dies if it is kept under- 

 water for more than 4 minutes, cats and rabbits die after 3 minutes, and 

 normal hinnan beings do not last for more than one minute, though 

 experienced sponge and pearl divers can stay submerged for up to 27^ 

 minutes. Aquatic mammals are, of course, far better adapted to under- 

 water life. Thus, the hippopotamus can remain underwater for up to 

 15 minutes; the beaver for up to 20; the muskrat for up to 1 2 ; the platypus 

 for up to 10; and the sea-cow for up to 16, while the figures for sea-otters 

 and polar bears are only 5 and i^ minutes respectively. Seals and sea- 

 lions can dive for 5-15 minutes, i.e. considerably longer than porpoises 

 or Common Dolphins. Even so, we may say of Cetaceans in general that 

 they excel over terrestrial mammals in their ability to hold their breath, 



AVhat of this respiratory frequency ? Here comparisons are not so straight- 

 forward, since this is largely determined by body size. The number of 

 breaths per minute is roughly 100 in mice and rats, 70 in squirrels, 58 in 

 rabbits, 35 in cats, 20 in dogs, 16 in men, 10 in lions, 7 in bisons, and 6 

 in elephants. On closer investigation it would appear that the number of 

 breaths per minute is a function of the ratio of surface area to lung 

 capacity. This is quite logical since oxygen needs depend on the rate of 

 combustion. Now, since combustion serves, inter alia, for compensating 

 heat losses, and since animals lose heat primarily through their skin, the 

 greater the surface area, the greater the demand for oxygen. Now, with 

 diminishing body size, the surface area decreases proportionally to the 

 square, while the capacity of the lungs (which have to provide the 

 oxygen) decreases proportionally to the cube, of the decrease in total size. 



Do Cetaceans fit into this picture? In other words, is their respiratory 

 frequency regulated by their size to the extent that it is in other mammals ? 

 In considering this question, we must, of course, ignore 'panting' and 

 consider normal breathing alone. Thus the Sperm Whale's rate of six 

 breaths per minute between deep dives is exceptional, and we must 

 consider instead its total respiration, diving and surfacing included. If 

 we do so, we shall find that the animal takes only one breath per minute. 

 Now these figures are precisely what we would expect judging from the 

 animal's size, and so are the figures for Rorquals and Right Whales. 



Things are, however, different with dolphins, whose overall respiratory 

 frequency is about 1-6 breaths per minute, and 3-8 breaths per minute 

 between dives. Comparing these figures with those for terrestrial mammals 

 of corresponding weight (150-400 lb.), e.g. bears, stags, pigs, antelopes, 

 sheep, and men who breathe 14-16 times per minute, we find that 

 dolphins have an exceptionally low respiratory frequency, the more so 

 since the figures for terrestrial mammals refer to normal breathing, while 

 those for dolphins refer to animals in motion and sometimes in very quick 



