Mutually beneficial relationships are common, but what happens when one partner stops enforcing the other’s good behavior? An exception to the usual relationship between figs and their pollinator-wasps, may hold the answer.

The finely-tuned relationship between many different species of fig trees and their wasps took shape between 70 and 90 million years ago: a female wasp squeezes through a hole in the end of a fig losing her wings in the process. Once inside this sphere full of flowers, she places pollen and eggs on some of the flowers, and as she does, she may also deposit a drop of fluid that causes the developing flower to form gall tissue to feed wasp larvae. Wasps mate inside the fig, males chew exit holes and then females crawl out, carrying pollen as they fly off to repeat this drama in the next fig.

«The currency is unambiguous,» said Allen Herre, staff scientist at the Smithsonian Tropical Research Institute in Panama. «One flower can either become one seed, which is good for the future of the tree species or one wasp, which is good for the future of the wasp species, and also is good for the tree — if the wasp carries its pollen to the next flower.»

But what would happen if figs were not pollinated? No fig seeds would develop, and eventually, there would be no more figs trees. That would be a disaster for tropical forests where a huge number of animals, from birds, monkeys and bats in the treetops to wild pigs and even fish, depend on fig fruits to survive.

Botanists in China’s Yunnan province discovered a fig species, Ficus microcarpa, is visited by two different, related wasp species. The first, Eupristina verticillata, is an active pollinator, has combs on its legs to harvest pollen, and pollen pockets. The second, another Eupristina species that has not yet been named, lacks combs and pockets. It lays its eggs in fig fruits and its larvae eat gall tissue, but it doesn’t pollinate the fig.

«Once you have a mutualism established, because everybody benefits, you might not expect to lose it,» Herre said. «We know of relatively few cases where this has clearly happened.»

In previous studies of 16 fig species, fig trees appear to reward wasps that actually pollinate them and provide severe disincentives to wasps that do not. Trees drop figs containing large numbers of unpollinated flowers on the ground where they rot before the young wasps can develop and leave the fig. This punishment, or sanction, for non-pollinators should get rid of wasp species that don’t pollinate.

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Materials provided by Smithsonian Tropical Research Institute. Note: Content may be edited for style and length.