This phenomenon is known as heteropaternal superfecundation. The pair are thus an example of heteropaternal superfecundation. Much more rarely, mixed twins can arise as a result of heteropaternal superfecundation. Only Jana is Achim's child, making Nico and Jana to a special phenomenon called Superfecundation. The theme occurs in other mythologies as well, and is called "superfecundation". The term superfecundation is derived from fecund, meaning the ability to produce offspring. Heteropaternal superfecundation is common in animals such as cats and dogs.