In Central America, ants act as bodyguards for acacia trees, defending them from weeds and hungry animals in exchange for room and board, one of the most iconic alliances in nature. (See "Ant Bodyguards Get Exclusive Contract From Trees.")

But Martin Heil of Cinvestav Unidad Irapuato in Mexico has found that the tree's sugary snacks are laced with an enzyme that prevents the ants from eating other sources of sugar.

One sip, and the insects are consigned to a life of indentured servitude.

"It was surprising to me that the immobile, 'passive' plant can manipulate the seemingly much more active partner, the ant," says Heil.

The report illustrates how evolution keeps cooperative relationships among some species going, even when one partner is clearly reaping most of the benefits in the arrangement.