6 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
frequent service they found a marked decline in the percentage of 
pregnancies induced and ultimately a decline in the size of litters, 
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 
