3 : 4/ Special Uses of Hearing and Vision 59 



with larger objects (and therefore does not need short wavelengths). 



Certain swiftlets {Collecalic brevirostris unicolor) also live in caves. 

 Although studied in less detail than the oilbird, it has been found that 

 the cave-dwelling swiftlets emit sharp clicks when flying in the dark. 

 The ability of the swiftlet to fly in the light is only slightly impaired if 

 either its eyes or ears are covered. It becomes quite helpless when both 

 are masked. No studies have been made of the frequencies of its clicks. 



Marine mammals, such as porpoises and small whales, have hearing 

 ranges which extend well above 100 kc. Their hearing has been shown 

 by conditioning experiments to be extremely sensitive. All of this 

 group of animals emit short sound pulses. Only the pulses from por- 

 poises have been studied in detail. They emit more and shorter pulses 

 when hunting for fish. Experiments have shown that porpoises use 

 these pulses for echo-location both in navigating and in locating food. 



Other animals are thought to use echo-location, although the evidence 

 is less certain. For example, deep-sea fish emit light flashes and certain 

 electrical fish send out weak electrical impulses. It is quite possible 

 that both of these are also used for echo-location of some type. 1 



There is also some evidence that blind humans use the echoes of their 

 footsteps to sense their closeness to objects. Attempts have been made 

 to extend this sense electronically to give details of size, shape, and 

 hardness. These have all failed because the added information gained 

 was less than that lost by wearing earphones or interfering with the 

 normal hearing of sound. 



4. Sense of Direction in Bees and Ants 



Besides echo-location, other sensory information is used in ways which 

 are unique to limited groups of animals. In this section, the sense of 

 direction in bees and ants is briefly considered. Humans possess a sense 

 of direction and use many different types of clues as guides such as 

 knowledge of the terrain, the stars, the compass, road signs, and mile 

 posts. Most of these are unavailable to bees and ants; nonetheless, they 

 proceed straight from their homes to a food source and back again. 

 This is the biological source of the colloquial expression "made a bee- 

 line." 



There is no doubt that, when ants follow the trails of other ants, they 

 use olfactory senses as a guide to direction. Likewise, they also can use 



1 Many of these electrical fish can detect electrical impulses with sensory 

 receptors, known as the lateral line organs. To some degree, these electrical 

 receptors represent a type of sensory system not found in humans. Although all 

 sensory receptors respond to electrical stimulation, humans have none that are 

 specialized for this type of stimulus. 



