46 THE PHYSIOLOGY OF REPRODUCTION 



together, seem, at the distance of a mile or two, like the noise of 

 "flistant 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 

 referi'ed to. The effects of castration upon the growth of the antlers 

 are described in a later chapter (p. 321). 



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 oestrous cycle. Under domestica- 

 tion, 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 duiring 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 polyGestrous,-the normal dioestrous cycle being about 

 three weeks and the oestrous period a week, though its actual length 

 may vary by three or four days.* The sexual season in the absence 

 of the stallion extends throughout the spring and early summer 

 months, and is' generally longest in the more domesticated breeds. 

 Professor Ewart informs me that in a pony imported from Tinior, 

 which is in the Southern Hemisphere, oestrus was experienced in 

 the autumn, or at the same time as the spring in Timeif (c/. camels, 

 p. 45). The period of gestation in the mare is eleven months, and 

 " heat " recurs eleven days after parturition. This is called the " foal 

 heat." Certain mares are irregular in the recunence of the "heat" 

 periods, and, in some, "foal heat" does not occur until seventeen 

 days after parturition instead of the usual eleven days. In 

 exceptional cases a mare, like a cow, may conceive at the "foal 

 heat" and yet take the horse three weeks later, just as though 



' Fleming, Veterinary Obstetrios, London, 1878. . 



2 The times of breeding may be altered by farm practice. See Pearl, f'The 

 Seasonal Distribution of Swine Breeding," Scientific MmitMy, September 

 1918. The dioestrous cycle is usually three weeks. 



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

 Live Stock of'Oreat Britain, 4th Edition, London, 1907. 



* Ewart found that in Eqmis prjewalskii, cestrus lasted a week. 



