Some plants produce on the surface substances such as nectar, not only in flowers. With this species of nectar are fed the ants that protect the plant from herbivorous insects. It is an example of mutualism.
But now there are researchers who have seen that the issue is even more complicated, it seems that this species of nectar puts ants in the craving of protein.
To demonstrate this, he has worked with cactus wislizenii. This cactus produces a substance similar to nectar and attracts four species of ants. A few cactus removed the nectar and others left them as they were. Then he put pieces of sugar and meat in the cactus and saw that the ants that ate nectar also ate meat eager. Instead, those who were in the cactus without nectars had not so much interest.