Forget mice, elephants intimidated by ants

Swarms on acacia trees keep the mighty beasts at bay

LARAMIE, Wyoming — To get an elephant to mind its manners, call an ant.

IN-HOUSE SECURITY Ants that live in and among the thorns of acacia trees discourage elephants from foraging on the plants, a study in Kenya has found. Pharaoh han

Ants living in whistling-thorn acacia trees on the African savanna may weigh only 3 milligrams, but they can protect their trees from being demolished by elephants weighing a billion times more, biologist Jacob Goheen reported June 12 at the annual meeting of the American Society of Mammalogists.

The elephants still eat a bit of acacia, but overall the ants have such a strong deterrent effect that they have kept a whole acacia landscape in Kenya intact despite rising numbers of elephants, said Goheen, of the University of British Columbia in Canada.

Like many acacias, the whistling-thorn species goes out of its way to recruit live-in guards. The tree sprouts special nubbins called nectaries that make sweet ant treats, and grows little swellings perfect for ant shelters. Offering room and board pays off for the trees, as ants get territorial about their homes.

Acacia ants already had a reputation as mercenaries in tree-defense forces, but not at the elephantine scale, Goheen said. They typically swarm to attack intruders, including insects that chew on trees. But to ward off big mammal browsers, the trees were thought to rely on their long thorns.

Some satellite images of Kenya, however, led Goheen and coauthor Todd Palmer of the University of Florida in Gainesville to ask whether ants might be effective against much larger foes than previously believed. The team noticed an oddity in tree cover in satellite images of the Lewa Wildlife Conservancy between 2003 and 2008.

Elephant numbers had tripled in the reserve, but satellite images showed the expected thinning of tree cover only in the northern section of the conservancy. That zone doesn’t have trees with their own ant militias, but the southern section, which apparently withstood the onslaught of elephants, is about 95 percent ant-defended whistling-thorn acacias.

For a ground-level view of how elephants feel about ants in their food, Goheen and Palmer set up a cafeteria selection of branches at a facility that cares for young orphaned elephants. Elephants readily ate almost all of the branches without ants, including whistling-thorn branches that had been cleared of ants. But the elephants virtually ignored branches with ants, including an otherwise tasty tree to which the researchers added ants.

Watching elephants in the wild, researchers found the same results. Elephants chowed down on whistling-thorn acacias only when a person removed the ants. Elephants didn’t appear to like having an ant horde storming up their trunks, Goheen said. 

The results drew an appreciative grin from mammalian ecologist Rob Swihart of Purdue University in West Lafayette, Ind. The new study raises intriguing questions about the evolution of plant defenses, he says, but foremost he likes the idea that “a tree depends on ants to maintain its population against damage from the largest land herbivore. To me, that’s just sticking it to the man.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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