We present experimental evidence supporting the hypothesis that increased spine length in acacia species is a defense induced by herbivory. Acacia drepanolobium is the dominant tree over large areas of East Africa. Each individual tree is occupied by one of four ant species at our study site. Using two types of electric fences, we have effectively controlled herbivory by megaherbivores (elephants and giraffes) and other large mammalian herbivores at a field site in Laikipia, Kenya since 1995. Mean spine lengths of new spines on trees occupied by the most abundant ant species (presumed to be a defensive mutualist) have shown a slow and steady decline over the first five years of the experiment on branches protected from these herbivores. This reduction has been 35–40%, or approximately half of the reduction in spine length that we anticipate will eventually occur, based on trees that have been protected from herbivory for many years. In contrast, trees occupied by a resident ant species that systematically prunes shoots have shown no reduction in spine length associated with herbivore exclusion treatments. Experimental pruning of shoots similar to that carried out by this ant species resulted in longer spines on seedlings in a greenhouse setting. Simulated large mammal browsing in the field rapidly (re-)induced greater spine lengths on trees that had been protected from large mammals for five years. The slow relaxation of spine length in the absence of herbivory, contrasted with its rapid induction after simulated browsing, suggests that there is a difference in the reliability of these two signals. Spine length responses to herbivory were extremely local (limited to individual branches). These branch-specific responses are consistent with the hypothesis that induced defense in this system evolved in the context of within-tree spatial variation in herbivore pressure, in particular variation in branch height.