THE CESTROUS CYCLE IN THE MAMMALIA 47 



she had failed to become pregnant. 1 Heape states that, very 

 exceptionally, mares are momestrous. Blood has been observed in the 

 mare's prorestrous 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 suckling 

 the mammary gland becomes congested and increases in size during 

 the heat. At the same time some mares develop 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 condition, which in a few instances can only be 

 determined by the behaviour of the mare towards the stallion. 2 



The elephant in captivity is said to be polycestrous, but I can find 

 no record of the duration of the dioestrous cycle. Since pregnancy 

 is very prolonged (twenty months), the sexual season cannot occur 

 more than once in two years ; that is, if the animals breed. The 

 elephant in the Zoological Gardens in London is stated to have 

 persistent cestrus probably for three or four days. 



CETACEA 



Little is definitely known about the periodicity of breeding in 

 Cetacea. According to Millais, 3 the right whale brings forth in 

 March in every other year, the young being suckled for about twelve 

 months. The humpbacked whales, blue whales, and sperm whales 



1 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. 



2 Wortley Axe, "The 'Mare and the Foal," Jaw. of the Royal Agri<: $"<'., 

 3rd Series, vol. ix., 1898. Ewart ("Studies on the Development of the Horse," 

 Tram. Rot/. Roc. Edin., vol. li., 1915) says that the period of a-strus in mares 

 tends to be shorter the later in the season, and when the food becomes less 

 plentiful and less nutritious all external signs of ivstrus disappear. Under 

 favourable conditions, however, mares may become pregnant in winter. 

 Ewart gives the following as the periods of gestation in various Equida; : 

 Asses and zebras, 358 to 385 days ; Prjewalsky's horse, 356 to 359 days ; 

 Celtic pony, 334 to 338 days. In coarse-headed tj pes of horse it is about the 

 same as in Prjewalsky's horse, but in the finer breeds the period is the same 

 as in the Celtic pony. In abnormal cases pregnancy may be unduly prolonged 

 in mares as in other animals, a maie occasionally going twelve months in foal 

 instead of eleven. 



3 Millais, The Mammals of Great Britain, vol. iii., London, 1906. 



