The twin species, called cryptic, are different species but so similar to each other that they are indistinguishable. They are, however, isolated from a reproductive point of view, so they are not genetically equal, and have different ecology and behaviors. Usually, in these organisms the recognition between the sexes takes place with other mechanisms (with respect to sight), such as the emission of sounds or odorous molecules.

RELATIVES. Among the invertebrates there are many cryptic species and, in the last decade, thanks to genetic analysis, twin species have also been found among vertebrates. Bats, monitor lizards, pygmy lemurs, dolphins and crocodiles have been added to the list. The Nile crocodile would not be a single species, but a complex of species that live in nearby areas, without crossing.