Plant-ant relationships: plants on top?
Ants disperse the seeds of several ‘ancient woodland species’ in the UK, such as dog’s mercury. These are woodland plants that take a long time to arrive when a new wood forms, so you tend to only find them in old woods. In the tropics, ‘ant plants’ take it even further: they house and sometimes feed a colony of particular ant species, which in turn protect them from herbivores, and in one case even kill competing plants.
But this co-operation is a trading relationship, not a romance. Both partners give something and get something, and they’re both looking for the best deal. It might seem that ants, being able to literally run rings around the plant, could also do so figuratively, but a couple of pieces of research suggest that plants can use their chemical wizardry to get the upper hand.
First off, seed dispersal. ‘Honest’ plants wanting their seeds to be carried by ants typically form seeds with a fatty lump, called an elaiosome. That’s what the ants are interested in; they take the seed back to the nest and eat the elaiosome. But the plant has to put energy in that it could be spending on more seeds, or larger ones. So some plants cheat, giving their seeds the smell of elaiosomes with oleic acid. Ants pick them up and take them away, even though there’s nothing for them to eat.
How did they work that out? It’s easy enough to see that plants do cheat: watch the ants long enough, and you’ll notice them picking up some seeds that don’t seem to offer any food. But to find how, you’ve got to experiment. The scientists made seed dummies—little seed shaped lumps of perlite which could be soaked in different mixtures. When they soaked them in extracts from the real seeds, extracts from ant-dispersed seeds got the most attention from ants, showing that scent was important (also, ants weren’t very interested in cleaned, scentless real seeds of any shape). Then they added particular chemicals to the dummies one at a time, to find that oleic acid, one building block of fats, was the most important.
The other piece of research began with some of those ant plants. Ants might be useful for many things, but for pollination, their knees aren’t the bees’ knees—bees (and moths, birds, bats, etc.) can fly to other plants. So it’s in the plant’s interest to keep ants out of its flowers. It could use a ‘carrot’, like providing nectar from the stems to attract ants away, or, as in this study, it could use a stick. Specifically, it seems that something in the pollen deters the ants. By touching young, ant-repelling flowers to old, non repelling ones, the old flowers would deter ants again, as if some sort of scent was being transferred.
More recently, the researchers found that some plants in the UK, even without a close partnership with ants, seem to repel them from their flowers in the same way. If the anthers (the bits that make pollen) were cut off, the ants no longer reacted. This seems to be one of two or more options: plants keep ants out of their flowers with barriers to stop them walking in, or with a deterrent smell, but none of the plants they studied invest in both.
So, in these two cases, it looks like plants are in control, either tricking ants into doing their work, or controlling where they go, for the plant’s benefit. But no doubt there are other examples where it’s the ants which have the upper hand.
Pfeiffer, M., Huttenlocher, H., & Ayasse, M. (2009). Myrmecochorous plants use chemical mimicry to cheat seed-dispersing ants Functional Ecology DOI: 10.1111/j.1365-2435.2009.01661.x
Willmer, P., Nuttman, C., Raine, N., Stone, G., Pattrick, J., Henson, K., Stillman, P., McIlroy, L., Potts, S., & Knudsen, J. (2009). Floral volatiles controlling ant behaviour Functional Ecology, 23 (5), 888-900 DOI: 10.1111/j.1365-2435.2009.01632.x