6 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



leading to temporary sterility. Microscopical study showed a marked 

 decline in the motility of the sperm cells. Nevertheless, such off- 

 spring as were obtained from the late services were in every way as 

 vigorous as those from early services. 



Artificial insemination may be mentioned at this point as a practice 

 which is useful in extending the service of a valuable male and in 

 overcoming certain forms of sterility. This practice is especially 

 useful in the case of horses, but has also been used to some extent with 

 cattle and dogs. 



THE BREEDING SEASON AND (ESTROUS CYCLE. 



Wild animals generally have a fairly definite breeding season, 

 which in most cases occurs at such a time that the young are born 

 in the spring or summer. The smaller animals, in which the gesta- 

 tion period is very short and which develop rapidly, such as mice, 

 rats, rabbits, and moles, usually have an extended breeding season 

 from early spring through summer; wolves and foxes, with a gesta- 

 tion period of 2 months, breed in winter. Where the gestation 

 period approaches half a year, the breeding season comes in the fall, 

 as in the wild sheep and goats and the Texas armadillo. The bison 

 and most deer, with a longer gestation period, breed late in the 

 summer or early in the fall. There are some curious exceptions, 

 such as bats, whose breeding season is in the fall; the ova remain 

 unfertilized all winter and go through a development lasting 2 

 months in the spring. 



The breeding season has become much obscured or wholly lost in 

 most of the domestic animals. Most breeds of sheep, however, 

 retain the definite fall breeding season and consequent spring lamb- 

 ing season of their wild forbears. The Dorset breed is exceptional 

 in that the ewes will breed in the spring soon after the birth of lambs, 

 conceived in the preceding fall. The sheep of Australia have come 

 to breed at all seasons of the year. Mares come in heat most regu- 

 larly in the spring and summer and the great majority of the foals 

 are born in the months of April, May, and June after a gestation 

 period of 11 months. Some are born, however, in all months of the 

 year. About 50 per cent of all the calves and pigs are born in the 

 months of March, April, and May. There is a secondary rise in the 

 number of births early in the fall in both cases. Under favorable 

 conditions sows are often bred to produce two litters a year with 

 profit. 



During the fall breeding season ewes have a number of periods of 

 heat, each lasting 2 or 3 days and at intervals of 2 to 3 weeks. Mares 

 come in heat normally about 9 days after foaling. The heat period 

 lasts several days and recurs at intervals of 3 or 4 weeks. Cows 



