3-way mutualism appears to have evolved between an ant, a butterfly caterpillar,
and an acacia in the American southwest. The caterpillars have nectar organs
which the ants drink from, and the acacia tolerates the feeding caterpillars.
The ants appear to provide some protection for both plant and caterpillar.
Research of Diane Wagner, American Museum of Natural History
Southwestern Research Station
Gregory G. and Mary Beth Dimijian