Poor little junkie ants. The first taste is free, then you gotta pay.
The Central American acacia tree and the ant Pseudomyrmex ferrugineus live in a symbiotic relationship. The tree provides sweet nectar for the ants, and the ants protect the tree from weeds and animals. But we now know that the relationship is rather one-sided, as the tree not only causes the ants to become addicted to its nectar, but also damages the ants to make them unable to digest any other food! Martin Heil of Cinvestav Unidad Irapuato in Mexico studied the ants, and found that they are born with the ability to digest a variety of sugars, but then lose their invertase, an enzyme that breaks down sugars.