Sources : Dolphin

Pliny the Elder [1st century CE] (Natural History, Book 8, 38): Dolphins enter the mouth of the Nile river, but are driven away by crocodiles that claim the river as their own. Crocodiles are much stronger than dolphins, so the dolphins must use strategy rather than strength. To defeat the crocodiles, dolphins dive below them and cut open their soft bellies with the sharp fin the dolphin has on its back. (Book 9, 7): Dolphins are the swiftest of all animals found in the sea or on land. Because their mouths are much below their snouts, they must turn on their back in order to seize fish. They breathe air from their backs. While pursuing fish to great depths, and so having held their breath too long, dolphins shoot up to the surface with such force that they fly into the air, sometimes flying over a ship's sails. Dolphins usually travel in mated pairs. Their voice is like a human moan. Their snouts are turned up, so they all answer to the name "Snubnose" (simonis) and prefer that name to any other. Dolphins love music, and can be charmed by songs sung in harmony or by the sound of the water-organ. Dolphins are friendly to mankind; they often play around ships and race with them.

Aelianus [170-230 CE] (On the Characteristics of Animals, Book 1, chapter 18): ...though one confront [the mother dolphin] with terrors never so great, she is still undismayed, and will not endure to desert her young one which has come to a bloody end; indeed, it is even possible to strike her with the hand, so close does she come to the hunters, as though she would beat them off. And so it comes about that she is caught along with her offspring, though she could save herself and escape. But if both her offspring are by her, and if she realises that one has been wounded and is being hauled in, as I said above, she pursues the one that is unscathed and drives it away, lashing her tail and biting her little one with her mouth; and she makes a blowing sound as best she can, indistinct, but giving the signal to flee, which saves it. So the young dolphin escapes, while the mother remains until she is caught and dies along with the captive. - [Scholfield translation]

Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 12.3): This area harbours very many dolphins. These creatures provide multifarious causes for wonderment. First of all, the seas have nothing more speedy; many of them leap up and fly over the sails of ships. Wherever they go, they go in couples. [4] They beget young; the unborn offspring mature in nine months. The babies are born in summertime, and the mothers nourish them with their teats. When the babies are very young, the mothers take them into their mouths. The mothers follow their babies for some time while they are still weak. Dolphins live for thirty years, as has been investigated in an experiment, wherein some specimens’ tails were chopped off. Dolphins do not have their faces in the same place as other marine creatures do -- they are almost in their stomachs. Unlike other aquatic animals, they can move their tongues. [5] They have sharp dorsal fins, which bristle up when they become angry, and are concealed, as in a sheath, when their minds are in repose. They say that dolphins do not breathe underwater, and they do not take in the breath of life except in the air above. For a voice, they have a moan similar to that of a human being. They are appropriately named “Snubnose”. This name pleases them, and they follow those who call them by it. [6] Dolphins hear men’s voices more easily when in the breezes of the north wind; contrariwise, their hearing becomes blocked up when the wind is blowing from the south. They are delighted by music: they rejoice in the songs of the tibia. Wherever there is harmony, they arrive in throngs. - [Arwen Apps translation, 2011]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:11): Dolphins (delphini) have their name because they follow human voices or because they join together to sing. They are the fastest beasts in the sea; they can jump over most ships that attack them. When they play in strong waves they appear to forecast a storm. The dolphins of the Nile have a saw-shaped back; with this they kill crocodiles by cutting the soft parts of their bellies.