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.