78 THE SENSES OF ANIMALS 



hear the croaking and will approach. This will stimulate the 

 first male into even louder croaking which is answered by 

 the second male. Should this intruder come near the holder 

 of that territory, it will be attacked in a very determined 

 manner. The attacker will jump at the other male and try 

 to bite at its vocal sacs, thes i. being inflated during croaking 

 so that they stand out on each side of the head, looking like 

 small grapes. 



This very definite attack surely shows that these male frogs 

 instinctively try to damage the organs on which the rival 

 depends for his mating success. Similar attacks on vital 

 organs are known in other animals. 



Reptiles 



The hearing of snakes has already been referred to ; but it is 

 necessary for the student to remember that snakes have no 

 true ears; and the specialized bones, which allow them to 

 detect vibrations from the ground, are not "hearing" 

 mechanisms in the accepted sense of that word. I have tried 

 time and time again to produce response from snakes of 

 many species by playing to them on whistles and a clarinet, 

 and I have never succeeded in doing so. I have, however, 

 caused snakes to protrude their tongues by gently resting a 

 clarinet against their tanks — a sign that the vibrations are 

 detected by the snakes, because they are conducted via the 

 tank itself The action of the tongue means that at times 

 snakes may be aware of possible prey by reason of vibrations 

 made by some animal, in addition to the scent by which 

 snakes normally detect their victims. 



Of course, different frequencies and degrees of vibration 

 mean diflferent things to snakes. A heavy footfall may spell 

 danger and will evoke either retreat or attack; but lighter 

 vibrations may well herald the presence of a meal. 



The hearing of lizards can be easily tested in those species 

 which are often kept as pets. The British Sand Lizard is a 

 good one to use for this purpose since it becomes very tame 

 and soon learns not to react in alarm to slow and quiet move- 

 ments on the part of its owner. There are two ways of con- 

 ducting suitable experiments. 



