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

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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.

Deux espèces de cobras habitent la zone forestière du sud du Nigéria (Afrique de l'Ouest). Alors que l'une des espèces (Naja nigricollis) se retrouve souvent dans les habitats modifiés (y compris les banlieues), l'autre (Naja melanoleuca) est une espèce typique des forêts dont les populations subissent un déclin rapide à cause des changements importants de l'habitat forestier et des modifications du paysage dans cette partie de l'Afrique. Nous avons tenté de déterminer si la taille du corps, le régime alimentaire et les stratégies de recherche de nourriture des deux espèces ont changé en fonction du type d'habitat et si le succès écologique de l'une des deux espèces dans les habitats transformés peut être attribuable à une flexibilité alimentaire plus grande quant au type de proies et à leur taille. Nous avons réparti nos données selon trois catégories d'habitat : (1) les banlieues, (2) la mosaïque plantations–forêts et (3) les forêts en pleine maturité. Le dimorphisme sexuel de la taille est faible chez les deux espèces et dans tous les types d'habitat; les différences entre mâles et femelles quant au choix et à la taille de leurs proies sont peu importantes. Malgré cela, nous avons observé un changement ontogénique remarquable de la composition taxonomique du régime alimentaire chez l'une des espèces (les juvéniles de N. nigricollis consomment presque exclusivement des lézards, alors que les adultes consomment des petits mammifères, des oiseaux et des lézards), mais pas chez l'autre. Curieusement, chez l'espèce la moins bien adaptée aux banlieues, la taille moyenne du corps diminue des forêts aux banlieues, ce qui peut représenter une adaptation sub-optimale aux habitats fortement modifiés. La taille des proies, ainsi que la relation entre la taille des proies et celle des prédateurs, sont semblables chez les deux espèces et dans les trois types d'habitat. Il semble donc peu probable que la flexibilité dans le choix des proies puisse expl

Document Type: Research Article

Publication date: January 1, 2002

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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