THE ALLIES OF THE SQUIRREL 



419 



Horns for defense have been de- 

 veloped in many species of ungulates. 

 They are of various sorts. The rhi- 

 noceroses (Fig. 220) have one or more 

 median horns, which are composed 

 of a thickened and hardened portion 

 of the skin and hair, covering a short 

 protuberance of the skull. In the 

 giraffes there are one or two pairs of 

 horns consisting of a layer of skin 

 over bony processes of the skull. 

 Neither in the rhinoceros nor the 

 giraffe are the horns ever shed. In 

 the North American pronghorn an- 

 telope {Antiloca'pra america'na) the 

 horns are branched and consist of 

 a hardened and thickened skin on a 

 bony core. The thickened skin is shed 

 periodically but the core is retained. 

 The horns in true antelopes and in 

 oxen, sheep, and goats, are of similar 

 structure but are never shed. They 

 are usually found in both sexes. In 

 the deer family (Fig. 221) the horns, 

 called antlers, consist of outgrowths 

 of bone covered each year during the 

 period of growth with a sensitive skin 

 called "the velvet." When the an- 

 nual growth is completed the supply 

 of blood to the antlers ceases and the 

 velvet peels off, leaving the bone 

 bare. After a time the antlers sepa- 

 rate from the skull and are shed. In 

 most deer this takes place annually. 



Fig. 221. Series of antlers 



American Museum of Natu- 

 ral History 



