490 



Comparative Animal Physiology 



ond. This variation with temperature makes the frequency of the nerve 

 impulses relatively inconstant for the purpose of frequency discrimination. 

 Therefore, as pointed out by these investigators, an adequate system of tem- 

 perature regulation seems to be a prime prerequisite for frequency discrimi- 

 nation. 



The lizards Lacerta and Tachydromus respond to sounds up to 8000 and 

 10,000 cycles, but there is no evidence that they are capable of frequency 

 discrimination.'^' ^^ 



Birds. The ear of birds is very similar in structure to that of reptiles (Fig. 

 161 and Fig. 167). In the basilar membrane of birds there are only 3000 

 fibers (24,000 in man) and there are 20 to 30 hair cells (4 in man) in each 

 row across the cochlea. Therefore, one might expect a lower degree of fre- 



endylymph duct 



anterior canal 

 posterior canal 



lagena' 



Eustachian tut)e 



Fig. 167. Diagrammatic presentation of the ear of a bird (Gallus), as seen from the 

 posterior. From Adams.^ 



quency discrimination and perhaps a lower frequency range than that of 

 man. However, there is good evidence^"* that parrots and crossbills (Loxifl 

 curvirostra') have a frequency range of 40 to 14,000 cycles, and a sense of 

 frequency discrimination approximately equal to that of man. Wever and 

 Bray'*-'^ demonstrated a microphonic effect in the pigeon for the range of 100 

 to lb,000 cycles. 



In general, the hearing of birds is as well developed as that of mammals. 

 The ability of birds to discriminate frequencies is apparent to anyone who has 

 watched birds respond to the call of their mates or to signals of alarm from 

 birds of the same or of different species. A walk through any bird nesting 

 ground during the breeding season provides ample evidence of both hearing 

 and frequency discrimination. This is also obvious in the mimicry of cer- 

 tain birds (e.g., mockingbirds and catbirds) which copy the sounds of other 

 birds, and in the mimicry of parrots which copy human speech. Birds are 

 also capable of accurate localization of sound, provided both ears are normal 

 and uncovered. 



