1. Animals also differ in the age at which sexual intercourse commences. For in the first place the period at which the spermatic fluid begins to be secreted, and the age of puberty is not the same, but different; for the young of all animals are barren, or if they do possess the power of reproduction, their offspring are weak and small. This is very conspicuous in mankind, and in viviparous quadrupeds and birds, for in the one the offspring, in the other the eggs, are small. The age of puberty is nearly the same in the individuals of each kind, unless any alteration takes place, either as ominous, or from an injury done to their nature. 2. In men this period of life is shown by the change of voice, and not only by the size but by the form of the pudendum and of the breasts in women, but especially by the growth of hair on the pubes. The secretion of the spermatic fluid commences about the age of fourteen, the power of reproduction at twenty-one. Other animals have no hair on the pubes, for some have no hair at all, and others have none upon their under side, or less than on their upper side, but the change of the voice is conspicuous in some of them. And in others different parts of the body signify the period of the formation of the semen, and of the power of reproduction. 3. In almost all animals the voice of the female and of the young is more acute than that of the male and the older animals, for even the stags have a deeper voice than their females. The males utter their cry at the season of copulation, the females when they are alarmed. The voice of the female is short, that of the male longer. And the barking of old dogs is also deeper than of young ones, and the voice of the horse also varies. The females utter a little small cry as soon as they are born, and the males do the same, but their voice is deeper than that of the female, and as they grow older, it still increases. When they are two years old, and reach puberty, the male utters a great deep voice, that of the female is greater and clearer than it was at first; this continues till they are twenty years old at the outside, and after that the voice, both of the male and female, becomes weaker. 4. For the most part, then, as we observed, the voice of the male differs from that of the female in depth, in those animals which utter a lengthened sound. There are, however, some exceptions, as oxen; for in these animals the voice of the female is deeper than that of the male, and the voice of the calf than that of the full-grown animal; wherefore also in the castrated animals, the voice changes the other way, for it becomes more like that of the female.