The number of penguin species is still disputed to be between 16-19 species. Penguin species range from the southern tropics to Antarctica. Larger penguin species can generate and retain heat better and can therefore live further south. The emperor penguin is the epitome of this trend, living the furthest south out of all the penguin species.

Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Aves
Order: Sphenisciformes
Family: Spheniscidae
Genus: Aptenodytes
Species: Aptenodytes forsteri

The chick-kidnapping behavior has not been identified in other species of penguins. Scientists have also been unable to determine when the kidnapping behavior evolved in emperor penguins. In addition, prolactin is present in fish, mammals and birds during reproductive periods, which adds to the complications experienced when trying to trace the origin of this behavior. However, based on the fact that only the emperor penguin engages in this behavior, it is most likely a derived trait.