2l6 THE SENSES OF ANIMALS 



can thus locate tiieir prey in the dark, for most of them hunt 

 by night, though they are often seen out in dayhght basking 

 in the warmth of the sun. (See Plate 19.) 



It has been suggested that owls similarly find their prey 

 in the dark by seeing the infra-red radiation given off from 

 the bodies of small animals. There are however theoretical 

 grounds for rejecting this idea, and experiment has con- 

 firmed that the eye of owls does not respond to infra-red 

 radiation. Infra-red radiation, "black light" as it has been 

 called, has been used for illuminating small mammals in the 

 dark so that they can be photographed by films sensitive to 

 those wave-lengths. Most small mammals can be equally 

 easily observed by red light visible to our eyes, which pro- 

 duces little or no response in theirs. 



There is another sense, or response to a stimulus, that 

 remains to be mentioned. It is found in animals that live 

 in water, and is now well known in the fishes. It is a reaction 

 to electric currents passing through the water, and is 

 probably quite different from the sense that responds to the 

 electric field set up by the electric fishes in their echo- 

 location. This reaction is exploited in the recently developed 

 techniques of fishing by electric currents. If an electric 

 current is passed through water in which fish are living they 

 are attracted to the cathode, provided the current is of suit- 

 able strength. Thus the anode can be made a metal plate 

 fixed under a boat and the cathode the ring of a hand net ; 

 when the net is dipped in the water the fish swim to it and 

 can be lifted out. This works well in fresh-water but the 

 method is not so easy to use in sea-water which is about 

 500 times a better conductor than fresh-water, and so short- 

 circuits the electrodes. In sea-water the current must be in 

 the form of pulses of short duration but high current density. 

 Methods have also been invented for attracting fish elec- 

 trically to the mouth of a pipe lowered over the ship's side 

 through which they are pumped on board. By adjusting the 

 strength of the current fish can be attracted, repelled, stunned 

 or killed. Electrified fences can be used for keeping fish away 

 from such places as the intakes of turbines, or for leading 

 them in a desired direction. There is a point about the 

 middle of a fish's body that is most sensitive to electric 



