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It looks like a win-win scenario. An Indian tree helps ants live in its branches in return for the protection the insects offer against grazing animals by patrolling the tree and . The trouble is, ants and worms that provide no protection also move in. But that might not be bad news for the tree after all.

and her team at the Indian Institute of Science in Bangalore fed an amino acid containing a heavy isotope of nitrogen to protective and to another, non-protective, species that lives in the tree. Later, they found traces of the isotope in the tree’s tissue, suggesting that both sets of ants benefit the tree by feeding it.

It is an excellent study, says at the French National Centre for Scientific Research in Montpellier. “Plant benefits from symbiosis with ants were thought to be mostly based on the protection against herbivores – but studies now suggest that plants also get nutrients from the activity of their symbionts,” he says.

Chanam’s team speculates that invertebrates were energy providers for the tree before the relationship developed to include the protection the ants offer.

“This helps us to resolve the classic conundrum of how co-operation can persist in the face of exploitation,” says at the University of Arizona in Tucson. “The apparent exploiters may be subtly benefiting their partners.”

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