70 THE SENSES OF ANIMALS 



their requirements, to receive these vibrations. Some may 

 be able to receive and use those sound waves which are of 

 a very high frequency — bats for instance ; while others, like 

 some of the whales, respond to low frequencies. Song birds, 

 which on the whole live in a world of high frequency sounds, 

 will not be adapted to deal with those of very low frequen- 

 cies ; and our human ears are not sufficiently sensitive for us 

 to hear the very high-pitched squeaks which enable bats to 

 navigate and even catch food. 



The sense of hearing, however, may at times be as vital, 

 or even more vital, to some groups of animals as is sight ; 

 and general observations on certain creatures when asleep 

 will satisfy anyone on this point. It is maintained that in 

 normal life very few birds or mammals sleep really deeply — 

 their hearing mechanisms are always subconsciously on the 

 alert. Were this not so, many would fall a prey to animal 

 or human enemies simply because their sleep might be too 

 deep for them to be awakened in time to take defensive or 

 avoiding action. 



All the same it must not be forgotten that certain special 

 circumstances will show that human beings share this semi- 

 alertness during sleep with some wild animals. A mother 

 with an infant or small child may be well and truly asleep ; 

 but the slightest cry from the child will waken the mother 

 when a thunderstorm might not. The fortunate father is far 

 less likely to be awakened by his offspring's night cries, and 

 this seems to show that the female at such times either sleeps 

 less heavily — though she may be unconscious of this — or 

 else, deep down within her she is, at least temporarily, 

 extremely sensitive to high-frequency sounds. Of course, the 

 wild animal mother will also be equally super-sensitive re- 

 garding noises made by her young; but basically the wild 

 creature is at all times acutely alert, having more dangers 

 and threats about her than a human mother. In this con- 

 nection it is interesting to remember that on the whole 

 women are heavier sleepers than men, and it would be 

 instructive to know the differences in depth of sleep among 

 wild animals as between the two sexes. 



In this book we are endeavouring to refer to as many 

 different kinds of animals in relation to their senses as is pos- 



