How Fig Trees Kill Cheating Wasps

Figs and fig wasps have lived in symbioses for centuries — wasps lay their eggs in figs, and pollinate the trees as recompense. But it's not all altruism. The trees will fight back if the wasps renege on the deal.

The symbiotic relationship has been in existence for around 80 million years, with the wasps stashing their eggs in the fig fruits where they can develop safely, and pollinating the plants in return for the service. This relationship is so functional that there are over 700 different figs and wasp species pairs that engage in it.

So what's to stop the wasp from reneging on their side of the deal, and leaving the fig to its own devices — effectively shifting from a symbiotic to a parasitic relationship? It turns out the fig trees have a surprisingly effective trick up their branches.

If the wasps don't do their duty, the trees respond by enacting a sanction — aborting their fruit, killing off the teeming mass of baby wasps. A new study of this killer tree phenomenon, published in Proceedings of the Royal Society B comes from Cornell University and The Smithsonian Tropical Research Institute, shows that negative reinforcement may be an important part of symbiotic relationships.

Pollination by wasp comes in two varieties: passive and active. With passive pollination the wasps carry pollen that happens to stick to their bodies; where with active pollination they collect pollen in special pouches to deliver to the flowers.

With the passive pairings, the fig trees abort their fruit far less often than with active pairs. In the actively pollinating groups, the tree species that tend to enforce sanctions less often have a higher occurrence of freeloader wasps, who take advantage of the figs without doing any of the work. Inversely, by using the sanction option more frequently, some fig species have a lower incidence of non-pollinating insects.

Lead author of the study Charlotte Jandér said:

Sanctions seem to be a necessary force in keeping this and other mutually beneficial relationships on track when being part of a mutualism is costly. In our study, we saw less cheating when sanctions were stronger. Similar results have been found among human societies and social insects. It is very appealing to think that the same general principles could help maintain cooperation both within and among species.

via Proceedings of the Royal Society B [full scientific paper] and Cornell University [release]

Image Copyright Christian Ziegler, via Cornell University