Genetic differentiation of the African dwarf crocodile Osteolaemus tetraspis Cope, 1861 (Crocodylia: Crocodylidae) and consequences for European zoos

The endangered African dwarf crocodile Osteolaemus tetraspis is distributed in Central and Western Africa. Conventionally, two subspecies were distinguished: Osteolaemus tetraspis tetraspis and Osteolaemus tetraspis osborni. The taxonomic significance of diagnostic morphological characters is still being discussed and the existence of additional species in the Osteolaemus group remains unclear. Recent molecular studies suggest the existence of three allopatric species in the genus Osteolaemus. These results supported a division of the dwarf crocodile into a Congo Basin form (O. osborni), an Ogooué Basin form (O. tetraspis), and a third separate evolutionary lineage from Western Africa. Several European zoos host African dwarf crocodiles. For reasons of conservation and possible reintroduction, it is important to clarify provenance of these zoo animals. Therefore, we conducted molecular and phylogenetic analyses of three mitochondrial and two nuclear gene sequences with all available samples from European zoos and museums. We also estimated the origin of the zoo animals by comparing sequences of wild animals and museum samples of known provenance. Our study strongly supports three distinct lineages of Osteolaemus as recently postulated, but also reveals a fourth evolutionary lineage. We demonstrate that, of the European zoo animals sampled, only one dwarf crocodile corresponds to the Congo Basin form (O. osborni) whereas the majority of individuals correspond to the three other forms. Four zoo animals belong to the new fourth group; but their provenance is still unresolved. The origin of these animals is probably located in an African region from which no wild animal samples are currently available. Further investigations and sampling of other regions should be completed to clarify the identity of this fourth lineage. We found potential hybrids from European zoological gardens using nuclear DNA sequences. The European Studbook will use these results for further breeding programmes to keep genetically suitable ex-situ populations as reassurance colonies for prospective reintroduction into African countries.