The age at puberty depends essentially on the adult size of the breed. It can range from six months in miniature dogs to eighteen months in the giant breeds. In the male, it corresponds to production of the first fertile spermatozoa. Since fertility declines with age, and large breeds age earlier than small breeds (a phenomenon probably linked to the aging of the thyroid gland), large dogs have a correspondingly shorter fertile period. In giant breeds, the fertility of the sperm sometimes begins to decline as early as the age of seven years.
As in males, puberty is later in large-breed females than in small-breed females (also between six and eighteen months). The first heat periods are relatively unobtrusive and may pass unnoticed. In bitches, there is a difference between puberty (the ability to ovulate) and nubility (the ability to carry a pregnancy to term and to whelp), which is why it is not recommended to breed from a bitch at her first heat, when her birth canal is not yet fully developed.
Beginning at puberty, the female genital tract adopts a cyclical rhythm that is generally expressed as two heat periods per year.
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