Age The effect of age is closely linked with nutrition, as puberty is determined by body weight not by the age of the animal. The physical size of animals may affect their ability to mate, carry a foetus or give normal birth to offspring. After puberty, fertility generally increases for some time then decreases when the animal gets too old. In contrast, goats have a greater chance of twin kids as they age. The decreasing daylight hours stimulate ewes and does to cycle while the increasing daylight hours stimulate hens and mares to cycle.
Dogs have only one cycle which is unrelated to the season and so are monoestrous.
The period of the year at which animals breed has evolved by natural selection to ensure the young are born at a time giving them the best chance of survivial. In sheep, the mechanism by which the breeding season is brought about is triggered by hours of decreasing daylight. Experiments have shown that, by the use of electric light, sheep in the Northern Hemisphere can be made to breed at the same time of the year as sheep in the Southern Hemisphere. When ewes are taken across the equator from north to south, they change their breeding season to correspond with the hours of daylight. The daylength is also a factor with the production of sperm in the ram but does not seem to be in other animals.