Superfecundation in the context of "Castor and Pollux"

Twins in mythology are in many cultures around the world. In some cultures they are seen as ominous, and in others they are seen as auspicious. Twins in mythology are often cast as two halves of the same whole, sharing a bond deeper than that of ordinary siblings, or seen as fierce rivals. They can be seen as representations of a dualistic worldview. They can represent another aspect of the self, a doppelgänger, or a shadow.

Twins are often depicted with special powers. This applies to both mortal and immortal sets of twins, and often is related to power over the weather. Twins in mythology also often share deep bonds. In Greek mythology, Castor and Pollux share a bond so strong that when mortal Castor dies, Pollux gives up half of his immortality to be with his brother. Castor and Pollux are the Dioscuri twin brothers. Their mother is Leda, a being who was seduced by Zeus who had taken the form of a swan. Even though the brothers are twins, they have two different fathers. This phenomenon is a very common interpretation of twin births across different mythological cultures. Castor's father is Tyndareus, the king of Sparta (hence the mortal form). Pollux is the son of Zeus (demigod). This brothers were said to be born from an egg along with either sister Helen and Clytemnestra. This teleologically explains why their constellation, the Dioskouroi or Gemini, is only seen during one half of the year, as the twins split their time between the underworld and Mount Olympus. In an aboriginal tale, the same constellation represents the twin lizards who created the plants and animals and saved women from evil spirits. Another example of this strong bond shared between twins is the Ibeji twins from African mythology. Ibeji twins are viewed as one soul shared between two bodies. If one of the twins dies, the parents then create a doll that portrays the body of the deceased child, so the soul of the deceased can remain intact for the living twin. Without the creation of the doll, the living twin is almost destined for death because it is believed to be missing half of its soul. Twins in mythology are often associated with healing. They are also often gifted with the ability of divination or insight into the future.

Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In very rare cases, fraternal or (semi-) identical twins can have the same mother and different fathers (heteropaternal superfecundation).

In contrast, a fetus that develops alone in the womb (the much more common case in humans) is a singleton; one of a multiple birth is a multiple. Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgänger.

In Greek mythology, Iphicles (/ˈɪfɪˌkliːz/ or /ˈaɪfɪˌkliːz/; Ancient Greek: Ἰφικλῆς Iphiklēs), also called Iphiclus, was the maternal half-twin brother of Heracles and one of the Calydonian boar hunters.