Reproductive Biology and Food Habits of Horned Adders, Bitis caudalis (Viperidae), from Southern Africa

Abstract

Horned adders (Bitis caudalis) are small heavy-bodied viperid snakes widely distributed across a range of habitat types in southern Africa. Measurement and dissection of 580 preserved specimens in museum collections provided information on morphology, food habits, and reproductive biology of this species. In particular, it enabled us to assess the effects of sex and habitat type (arid to mesic) on adult body sizes and shapes, dietary composition, and reproductive output. Female horned adders mature at larger sizes than do males and grow much larger. At the same snout-vent length, females have larger heads and shorter tails than do males. Arid-zone snakes are longer and thinner than conspecifics from more mesic areas and have longer tails and larger heads. Horned adders feed primarily on lizards (especially lacertids, skinks, and geckos) but also take other small vertebrates. Dietary composition varies according to the snake's body size, sex, and geographic location: endothermic prey are taken mostly by larger snakes; by females rather than males; and by arid-zone rather than mesic-habitat snakes. Most prey are small relative to predator size, especially in large snakes. Litter sizes (3-19 offspring) increase with maternal body size, with no significant geographic differences in this relationship. However, reproductive frequency (as inferred from the proportion of adult females that were reproductive when collected) was significantly higher in mesichabitat snakes (> 50%) than in their arid-zone relatives (15%). Habitat-associated differences in resource availability may have affected traits such as body size and shape, degree of dietary specialization, growth rates and female reproductive frequencies.