50 THE PHYSIOLOGY OF REPRODUCTION 



when they congregate into such masses in some places as 

 literally to blacken the prairies for miles together. It is no 

 uncommon thing at this season, at these gatherings, to see 

 several thousands in a mass, eddying and wheehng about under 

 a cloud of dust, which is raised by the bulls as they are pawing 

 in the dirt or engaged in desperate combats, as they constantly 

 are, plunging and butting at each other in the most furious 

 manner. In these scenes, the males are continually following 

 the females, and the whole mass are in. a constant motion ; 

 and all bellowing (or ' roaring ') in deep and hollow sounds 

 which, mingled together, seem, at the distance of a mile or two, 

 like the noise of distant thunder." 



That the antlers are the fighting weapons in stags, and that 

 their growth is associated with the advent of the sexual season, 

 after which time they are cast off, are facts which have been 

 already referred to. The effects of castration upon the growth 

 of the antlers are described in a later chapter (p. 305). 



Passing to the non-ruminating Ungulata, we find that the 

 wild sow has only one annual sexual season. It is not certain 

 whether this consists of more than a single cestrous cycle. Under 

 domestication, however, the sow is polyoestrous, and may take 

 the boar five weeks after parturition. The duration of the 

 dioestrous cycle is from two to four weeks, according to Fleming.^ 

 The period of gestation is about four months. Litters are 

 usually produced only in spring and autumn ; but by weaning 

 the young early (or partially weaning them), and feeding the 

 mother liberally, it is possible to get five litters in two years. 

 A sanguineo-mucous flow has been observed issuing from the 

 genital aperture during the prooestrum. At the same time 

 the vulva is distinctly swollen. 



Wiltshire ^ states that in the hippopotamus in captivity a 

 condition of oestrus may be experienced at regular monthly 

 intervals. This animal has been known to breed in Zoological 

 Gardens. 



The mare is polyoestrous, the normal dioestrous cycle being 

 about three weeks and the cestrous period a week, though its 



^ Fleming, Veterinary Obstetrics, London, 1878. 



2 Wiltshire, loc. cit. See also Ellenberger, loc. cit.\ and Wallace (R.), 

 Farm Live-Stock of Great Britain. 4th Edition London. 1907. 



