Comparative feeding strategies and dietary plasticity of the sympatric cobras Naja melanoleuca and Naja nigricollis in three diverging Afrotropical habitats

Abstract

Two cobra species are found in the forest block of southern Nigeria (West Africa). However, whereas the one species, the spitting cobra (Naja nigricollis), is often found in strongly altered habitats (including suburban areas), the other, the black forest cobra (Naja melanoleuca), is a typical forest species that is currently subject to a rapid decrease in population abundance because of intensive forest alteration and landscape modification in this part of Africa. We studied whether the body sizes, diets, and feeding strategies of these two species changed in relation to habitat type, and whether the ecological success of the one species versus the other in altered habitats depends upon greater dietary flexibility in prey type or prey size. Therefore, we divided our cobra records into three habitat categories: (1) suburbia, (2) plantation–forest mosaic, and (3) mature forest. We observed that sexual size dimorphism was minor in both species and in all habitat types, and that intersexual differences in prey composition and prey size were also minor. Nevertheless, there was a remarkable ontogenetic change in taxonomic composition of the diet for one species (N. nigricollis, with juveniles taking almost exclusively lizards and adults taking small mammals, birds, and lizards) but not the other. Remarkably, the species that is less adapted to life in suburban areas showed a reduction in mean body size from the forest to suburbia, which may also indicate suboptimal adaptation to strongly altered habitats. Prey size was similar for the two species and in the three habitat types, and the relationships between prey size and predator size were similar. Thus, it seems unlikely that flexibility in prey-size patterns explains the greater colonizing success of N. nigricollis. Nevertheless, and although both species exhibited remarkable dietary flexibility, leading them to prey upon homeotherms as well as heterotherms and upon terrestrial as well as arboreal and even aquatic prey, there were important interspecific differences in prey composition that may explain the ecological success of N. nigricollis. The success of N. nigricollis likely lies not in dietary flexibility but in the consistency with which its juveniles prey upon a single prey type (lizards, mainly Agama agama) that is so abundant in nearly every altered habitat in Nigeria and is a virtually unlimited food resource for young N. nigricollis. However, adults of this species also forage frequently upon commensal rodents and poultry, which may also help it to colonize man-made habitats.