Ecological relationships in two Afrotropical cobra species (Naja melanoleuca and Naja nigricollis)

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Abstract
Aspects of comparative ecology were studied in the spitting cobra, Naja nigricollis, and the black forest cobra, Naja melanoleuca, from southeastern Nigeria. Both species were common and widely distributed in the study region. Naja melanoleuca inhabited mainly primary and secondary forest patches, but also plantations and suburbia. The two species differed significantly in terms of habitat types frequented. A logistic regression model showed that the presence of N. melanoleuca was significantly influenced by the presence of a unique macrohabitat category (primary swamp-forest), whereas the presence or absence of N. nigricollis did not correlate with any macrohabitat categories. Aboveground activity of both species was lowest during the hottest months of the dry season (December to February) and peaked during the wettest months of the wet season (June and July). Sexual size dimorphism was minor in both species, but N. melanoleuca attained a significantly greater body size than N. nigricollis. The adult sex ratio was approximately 1:1 in both species. Both species preyed on a wide variety of small to medium-sized vertebrates; adult N. melanoleuca fed with similar frequency upon mammals, frogs, and fish, whereas the young fed mainly upon fish. Adult N. nigricollis preyed mainly on lizards and to a lesser degree on mammals and frogs, whereas the young preyed with similar frequency upon lizards, frogs, and fish. Many of the prey of both these cobra species were terrestrial (rodents and shrews), but semiaquatic (frogs) and aquatic (fish) prey were also frequently taken. In both N. melanoleuca and N. nigricollis, egg laying probably occurred over a broad time-span, there was a positive relationship between maternal size and litter size, and reproduction first occurred when individuals attained a low proportion of their maximum size.

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