Not all ants are equal: Obligate acacia ants provide different levels of protection against mega-herbivores

In obligate ant-plant mutualisms, the asymmetric engagement of a single plant species with multiple ant species provides the opportunity for partners to vary in their behaviour. Variation in behaviour has implications for the interactions with third-party species such as herbivores. This study assessed the effect of obligate ant-mutualists (Crematogaster mimosae, Crematogaster nigriceps and Tetraponera penzigi) inhabiting the African ant-acacia (Acacia drepanolobium) on three mega-herbivore browsers: the Maasai giraffe (Giraffa camelopardalis tippelskirchi), the reticulated giraffe (Giraffa c. reticulata) and the black rhino (Diceros bicornis). Giraffes are abundant and wide-ranging herbivores of the acacias, whereas black rhinos are localized and perennial herbivores of the acacias. Multiyear field studies comparing the ants' aggressive behaviour and browsing by mega-herbivores suggested differences between the tending abilities of the primary ant species inhabiting A. drepanolobium. Trees occupied by the aggressive ant species C. mimosae had significantly less browsing by giraffes and black rhino than trees occupied by other ant species. The results of this study provide evidence that ant-mutualists on African acacias can serve as deterrents to mega-herbivores and that different ant species vary in their tending abilities.