3. Horses generally live eighteen or twenty years; some live twenty-five or thirty years; but if they are carefully treated, their life may be extended to fifty years. Thirty years, however, is a very long life for the male, and twenty-five for the female. Some have been known to live forty years. Males live a shorter time than females, on account of the act of sexual intercourse; and those that are brought up separately longer than those which live in herds. Females attain their proper length and height in five years; the males in six. In six more years the fulness of body is acquired, which continues till they are twenty years old. The females attain perfection more rapidly than the males; but in the uterus the males are the more rapidly developed. This is also the case in the human subject. This also takes place in those animals which produce several at a birth. 4. They say that the mule sucks for six months, but the mare will not permit it to come afterwards, because it drags and hurts her. The horse sucks for a longer time. The horse and the mule attain perfection after casting their teeth; and when they have cast them all, it is not easy to know their age. Wherefore they say that, before casting its teeth, the horse has its mark, which it has not afterwards. After the teeth have been changed, the age is usually ascertained by the canine tooth; for that in riding horses is generally worn down, for the bridle rubs against it. In horses which have not been ridden, it is large and not worn. In young horses it is small and sharp. 5. The male copulates at all seasons, and as long as he lives; the female also as long as she lives; and at all seasons, unless they have on a fastening or some other hindrance, no peculiar time is appropriated for copulation in either sex, for there is no period of coition when they cannot also bring up their young. In Opus there was a horse in a herd which engendered when he was forty years old; but it was necessary to lift up his fore legs for him. Mares begin to desire sexual intercourse in the spring; and when the mare has foaled, she does not become pregnant again immediately, but waits for a time, and produces better foals at the end of four or five years. It is quite necessary that she should wait one year, and should pass through a fallow, as it were.