Is there any interspecific competition between dwarf crocodiles (Osteolaemus tetraspis) and Nile monitors (Varanus niloticus ornatus) in the swamps of central Africa? A study from southeastern Nigeria

Aspects of coexistence between the dwarf crocodile Osteolaemus tetraspis an d the Nile monitor lizard Varanus niloticus were studied in swamp rain fore sts of south-eastern Nigeria, central Africa. Crocodiles were si,significan tly smaller than monitor lizards. There was no significant sexual size dimo rphism in the examined sample of Varanus, whereas the female size exceeded the male size in Osteolaemus. The food niche breadth was narrower in young than in adults of both species, and V. niloticus showed a narrower niche br eadth than O. tetraspis. Crabs constituted the main prey type category for both species, whereas vertebrates were more rarely preyed upon. The diet co mposition of males and females was similar in V. niloticus, but it was diff erent in O. tetraspis. The general diet composition of the two species was similar (78.2% of overlap), and even the habitats where these two species w ere found were similar. These interspecific similarities suggest that O. te traspis and V. niloticus could be potential competitors in the freshwater e cosystems of the Nigerian rainforest. However, it is suggested that the mai n prey type for these species (crabs) is not limited in the environment, wh ich may minimize interspecific competition. Interference competition can al so occur between these species, as suggested by one case of direct predatio n of the one species (V. niloticus) towards the other species (O. tetraspis ). A case of cannibalism was observed in V niloticus. This could indicate t hat intraspecific competition can be important for regulating the dynamics of monitor populations in the rainforests of south-eastern Nigeria.