THE TAIL 



when the animal is nuzzling about in the mud for its food. The latter 

 function would seem to be the critical one. 



Externally the tail of the beaver is phenomenally broadened and the 

 integument is scaly in appearance, although cornification is not marked. 

 Subcutaneously the tissue is fatty, for the most part, but the lateral 

 muscles at the base of the member are very broad and powerful. The 

 caudal vertebrae are unusual in that they exhibit marked broadening, 

 especially those proximad, and this character is shared by the sacral 

 series. In the caudal compliment there are about 23 to 25 vertebrae, 

 which -are short, wide and depressed, with very wide transverse processes 

 which become double at the middle. 



The reason for the sort of flattening shown by the tail of the beaver 

 is puzzling. The function, once popularly believed, as a vehicle for 

 carrying mud, was long since proved to be erroneous. Many people 

 still consider that the flat tail was developed so the animal might slap 

 it on the surface of the water and thus more quickly submerge, but 

 this hardly seems to be sound logic, for the muskrat can disappear with 

 an abruptness that is equally startling. Its present function as an organ 

 for giving warning signals (by surface slappings) is undoubtedly in- 

 cidental and secondary, as is any use to which the tail may occasionally 

 be put for tamping mud while building dams. It is obvious, however, 

 that without a tail flattened in some manner the beaver would progress 

 in circles while towing logs and sticks. This, then, must be listed as 

 a primary need for a very broad tail, but one flattened in the horizontal 

 plane should be much more effective for this purpose, hence, while the 

 rudder function may have very materially assisted in the expansion 

 process, it could hardly have initiated the present direction of flattening. 

 Similarly with the fact that the tail is occasionally used as a scull. I 

 have watched a beaver swimming slowly by the tail alone, pulling the 

 appendage latero-ventrad first to one side and then the other. In this 

 also a laterally flattened tail would be more effective. As the beaver 

 swims by alternate strokes of the hind feet it seems that this almost 

 certainly introduced some original stimulus for flattening the tail in 

 the transverse plane. Hence there appear to be at least three factors 

 for which a narrowed tail would be more favorable than a broadened 

 one, and it accordingly appears likely that the broadening stimulus 

 would have to be an unusually strong one to overcome them. 



The beaver often walks erect with an armful of mud, and also uses 

 the tail as a prop while cutting trees (Bailey, 1923), or it may make 

 more use of this member for keeping near the stream bottoms than is 



[193] 



