Do snakes exhibit shifts in feeding ecology associated with the presence or absence of potential competitors? A case study from tropical Africa

Abstract

Two species of rain-forest snakes, the colubrids Natriciteres fuliginoides and Natriciteres variegata, are potential competitors, because they attain, on average, the same body size (20–30 cm long), have similar habits and ecological requirements (they are both diurnal and nocturnal, often semi-aquatic), and have similar dietary spectra ( including both small vertebrates and invertebrates). I studied prey type and predator size – prey size relationships of these snakes in four swamp-forest areas of southeastern Nigeria (West Africa): only N. fuliginoides was found in two of these areas, whereas both species were common in the other two areas. The feeding ecology of N. fuliginoides shifted greatly in the presence of its potential competitor, N. variegata, in terms of prey-type preferences and predatory behaviour as well as in terms of predator size – prey size relationships. In particular, the effects of the potential competitor on the feeding ecology of N. fuliginoides were (i) a shift toward eating many invertebrates from a diet based on many small vertebrates, (ii) a shift toward eating many terrestrial organisms from a diet based on many aquatic organisms, and (iii) a shift in mean prey size for females toward relatively larger prey from a diet based on smaller prey. The resulting effect was a clear partitioning of food resources between the two predator species in both the areas where they are sympatric, whereas N. fuliginoides had a wider dietary spectrum (covering that of the two species together) in the areas where its competitor is not found. The role of intraspecific competition seems less strong, as no apparent resource partitioning in prey type or prey size was observed between males and females of either species or in any study area. However, mean prey size was significantly greater for females than for males of both species and in all study areas, but depended on the significant reverse sexual size dimorphism observed in both species.