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 south-eastern Nigeria

Abstract

Aspects of coexistence between the dwarf crocodile Osteolaemus tetraspis and the Nile monitor lizard Varanus niloticus were studied in swamp rain forests of south-eastern Nigeria, central Africa. Crocodiles were significantly smaller than monitor lizards. There was no significant sexual size dimorphism 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 breadth than O. tetraspis. Crabs constituted the main prey type category for both species, whereas vertebrates were more rarely preyed upon. The diet composition of males and females was similar in V. niloticus, but it was different 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 were found were similar. These interspecific similarities suggest that O. tetraspis and V. niloticus could be potential competitors in the freshwater ecosystems of the Nigerian rainforest. However, it is suggested that the main prey type for these species (crabs) is not limited in the environment, which may minimize interspecific competition. Interference competition can also occur between these species, as suggested by one case of direct predation of the one species (V. niloticus) towards the other species (O. tetraspis). A case of cannibalism was observed in V. niloticus. This could indicate that intraspecific competition can be important for regulating the dynamics of monitor populations in the rainforests of south-eastern Nigeria.