THE OESTROUS CYCLE IN THE MAMMALIA 51 



actual length may vary by three or four days.^ The sexual 

 season in the absenpe of the stalHon 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 Timor, which is in the Southern 

 Hemisphere, oestrus was experienced in the autumn, or at the 

 same time as the spring in Timor (c/. camels, p. 49). 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 recurrence 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, Hke a cow, may conceive 

 at the " foal heat " and yet take the horse three weeks later, 

 just as though she had failed to become pregnant.^ Heape 

 states that, very exceptionally, mares are monoestrous. Blood 

 has been observed in the mare's prooestrous discharge, but it is 

 not generally present. The genitalia, however, are always 

 swollen and congested, and a glutinous secretion is generally 

 emitted from them. The clitoris and vulva often undergo a 

 succession of spasmodic movements, preceded by the discharge 

 of small quantities of urine. Suckling mares tend to fail in 

 their milk supply, and the quality of the milk appears to undergo 

 some kind of change, as it is frequently the case that foals 

 during the heat periods of their dams suffer from relaxation of 

 the bowels or even acute diarrhoea. In mares which are not 

 suckhng the mammary gland becomes congested and increases 

 in size during the heat. At the same time some mares de- 

 velop great excitability, and kick and squeal, becoming dangerous 

 to approach and impossible to drive. There is, however, great 

 variation, for other animals may pass through the " heat " 

 period without exhibiting any well-marked signs of their con- 

 dition, which in a few instances can only be determined by the 

 behaviour of the mare towards the stallion.^ 



' Ewart found that in Equus prjeioalsky, oestrus lasted a week. 



^ Wallace, loc. cit. Professor Ewart informs me that pregnant mares do 

 not necessarily abort as a result of taking the horse at the third, sixth, or 

 even ninth week of gestation. ' 



' Wortley Axe, "The Mare and the Foal," Jour, of the Royal Agric. Soc., 

 3rd Series vol. ix., 1898. Ewart {" The Development of the Horse," to be 



