Spines of Acacia tortilis: What do they defend and how?

Spines, prickles and thorns are assumed to act as plant defenses against mammalian herbivores. In this study, I tested experimentally whether increasing densities of spines affect the feeding rate and behavior of goats browsing shoots of Acacia tortilis in Tanzania. Goats may harvest their food by pruning (removal of both twig and leaves) or by picking (only leaves removed). Feeding rate (biomass consumed/minute) was positively correlated with biomass pruned, and negatively related with biomass picked, both for spiny and spineless shoots. Increased spine density negatively affected the proportion of the shoot that could be pruned, but had no effect on leaf biomass removed by picking. Feeding rate was negatively correlated with spine density. Spiny shoots, available simultaneously with spineless shoots, bring associative protection to spineless neighbors against pruning, but not against picking. Protection of spineless shoots is positively correlated with spine density in spiny neighbors. I conclude that increases in spine density of A. tortilis protect the twigs of the plant, but not necessarily its leaves from being browsed. The cost for the plant of producing spines is more than balanced by reductions in losses of shoot biomass by browsing.