According to a 2015 study, crocodiles engage in all three main types of play behaviour recorded in animals: locomotor play, play with objects and social play. Play with objects is reported most often, but locomotor play such as repeatedly sliding down slopes, and social play such as riding on the backs of other crocodiles is also reported. This behaviour was exhibited with conspecifics and mammals and is apparently not uncommon, though has been difficult to observe and interpret in the past due to obvious dangers of interacting with large carnivores. Taxonomy and phylogeny Crocodylidae is cladistically defined as a crown group composed of the last common ancestor of the Nile crocodile (Crocodylus niloticus), the Dwarf crocodile (Osteolaemus tetraspis), and all of its descendants. It contains two subfamilies: Crocodylinae and Osteolaeminae. Crocodylinae contains 13-14 living species, as well as 6 extinct species. Osteolaeminae was named by Christopher Brochu in 2003 as a subfamily of Crocodylidae separate from Crocodylinae, and contains the two extant genera Osteolaemus and Mecistops, along with several extinct genera. The number of extant species within Osteolaeminae is currently in question. Subfamily Crocodylinae Genus Crocodylus Crocodylus acutus, American crocodile Crocodylus halli, Hall's New Guinea crocodile found South of the New Guinea Highlands Crocodylus intermedius, Orinoco crocodile Crocodylus johnsoni, freshwater crocodile, or Johnstone's crocodile Crocodylus mindorensis, Philippine crocodile Crocodylus moreletii, Morelet's crocodile or Mexican crocodile Crocodylus niloticus, Nile crocodile or African crocodile (the subspecies found in Madagascar is sometimes called the black crocodile) Crocodylus novaeguineae, New Guinea crocodile found North of the New Guinea Highlands Crocodylus palustris, mugger, marsh or Indian crocodile Crocodylus porosus, saltwater crocodile or estuarine crocodile Crocodylus raninus, the Borneo crocodile, is currently considered to be a synonym of Crocodylus porosus; whether or not it is a distinct species remains unclear. Crocodylus rhombifer, Cuban crocodile Crocodylus siamensis, Siamese crocodile (may be extinct in the wild) Crocodylus suchus, West African crocodile, desert or sacred crocodile Crocodylus anthropophagus Crocodylus checchiai Crocodylus falconensis Crocodylus palaeindicus Crocodylus thorbjarnarsoni Genus Voay Voay robustus (formerly Crocodylus robustus) Subfamily Osteolaeminae Genus Osteolaemus Osteolaemus tetraspis, dwarf crocodile (There has been controversy as to whether or not this is actually two species; recent (2010) DNA analysis indicate three distinct species: O. tetraspis, O. osborni and a third, currently unnamed.) Genus Mecistops Mecistops cataphractus West African slender-snouted crocodile Mecistops leptorhynchus Central African slender-snouted crocodile Genus Brochuchus Brochuchus pigotti (formerly Crocodylus pigotti) Brochuchus parvidens Genus Euthecodon Euthecodon nitriae