A closely related to the bullhorn acacia is the whistling-thorn acacia growing on African grassland. Jacob Goheen of the University of British Columbia in Canada and Todd Palmer of the University of Florida in Gainesville were studying satellite images of the Lewa Wildlife Conservancy in North central Kenya when they noticed something odd about the acacia trees. The ever-present thorny trees were noticeably thinning in the northern part of the 62,000-acre property but not in the southern sections. That was odd, because, with ongoing conservation, elephant population had soared to three times their normal numbers throughout the region. What was making the big difference?
A trip to the park revealed that the most obvious difference between the northern and southern acacias was the presence of ants living in the hollowed-out thorns of trees in the south. Surely ants couldn’t prevent elephants browsing, could they? The ants were known to discourage foregoing of smaller animals but it was assumed that acacias protected themselves from the big animals with their thorns. After all, ants are a tiny fraction of the size of an elephants, the largest land animals and one that has no natural predators. And Elephants have such thick skin. The elephant’s nickname of pachyderm, means “thick skin”. How would an elephant even know that ants were on its skin? How would an ant bite through such thick skin? For scientist, this was a time to do some experiments. In the controlled environment of an elephants orphanage, Goheen and Palmer tested various trees with and without ants. Sure enough, ants on the foliage discourage browsing. Then out to the field for further tests. When some southern trees had their ant population removed, the elephants moved in and browsed their foliage. Their surprising conclusion: Crematogaster ants, known to be guardians of the acacias from giraffes and other grazer, also protected the acacias from even the largest plants eater on our planet. There is power in numbers. Apparently elephants do not like having a trunk full of stinging ants.
Since both the bullhorn and whistling-thorn acacias lack the chemical defenses of their other acacia cousins, they are fortunate to have the ant colonies to defend them from herbivores.