Structural defense of Acacias: hooks, spines and architecture

African Acacias form attractive browse for many large mammals, necessitating their development of various structural defenses. Hooked thorns, straight spines and various branching architectures are used to defend against herbivory. The defenses of four Acacia species are quantified using a stripping index, biting index and branching index, which are combined in an overall defensive index. Three feeding simulations were also performed in an effort to quantify the effect of Acacia defenses on browsers in a more holistic manner. Results showed the biting index and clamp tests to be a good measure for the effectiveness of spines. The sock test, stripping index and branching index appear to be useful tools for assessing the defensive functioning of hooks, and the associated importance of architecture. This study provides a useful starting point for the development of a measure of the overall defense of Acacias, which would greatly enhance the understanding of the life history and distributional patterns observed in this genus.