Ants have a close association with red and white dead nettles (Lamium purpureum and L. album) and carry off the seeds to their nests, where they feed the smooth folded, nutritious attachment (elaiosome) to their developing larvae. The seed itself is not consumed, and is either left in the nest or dispersed a short distance, e.g. to a pile or midden outside the nest.

This relationship is a form of mutualism, a symbiotic association where both species benefit from their interaction, although not necessarily to the same extent. There are probably millions of such associations in the living world, where two or more species have evolved a relationship where they effectively barter – exchange goods or services – or just work together, for their mutual benefit. In effect, the plant rewards the ants for their transportation services, by attaching a nutritious appendage to each seed.

N.B. Commensalism is the name given to a relationship where only one species benefits.

Myrmecochory is the name given to this form of seed dispersal, where ants are recruited by plants to disperse their seeds. It is advantageous for plants to disperse their seeds as widely as possible, although even a few centimetres is advantageous, for a number of reasons. The seeds generally fare better away from the parent plant, by avoiding parent-sibling competition and potentially falling in more suitable soil. Studies have shown that there are often higher soil nutrients near ant nests. When ants bury myrmecochorous seeds within their nests, they also provide protection from predators, or seed scavengers (granivorous ants: see below), and in some circumstances, from lethal soil surface temperatures that occur during fires.

Plants reward the ants by by providing a lipid rich appendage called an elaiosome. This type of seed dispersal is remarkably common, occurring in more than 11,000 plant species (in 334 genera in 77 families). Myrmecochory is thought to have evolved independently at least 100 times in the past 30 Myr or so, which suggests that there are strong pressures to develop these sorts of mutual relationships. Such widespread myrmecochory is a good example of convergent evolution and is an important driver of plant diversity around the world. In other words, without the contributions of ants, and other insects of course, there would be far fewer plants for us to admire; arriving as we humans have, long after such relationships evolved.

The specialised fleshy and edible seed appendages have been derived from a variety of different seed structures (see below) but are all dispersed by ants of one sort or another.

Foraging ant workers pick up diaspores, which is the name given to the seeds with attached elaiosomes, and carry them back to their nests. Ants generally remove complete diaspores much more rapidly than they do seeds stripped of elaiosomes, often using the elaiosome as a handle. Once in the nest, ants generally remove and feed elaiosomes to larvae before depositing seeds in a midden pile inside or outside their nest. Some harvester ants collect seeds without elaiosomes, take then to their nests and consume the entire seed. However, this may nevertheless still benefit the plant, because many seeds get dropped or discarded by the ants, and thus germinate away ftom the parent plant. This is called non-specialised myrmecochory.

“Plants commonly trick insects into self-serving behaviours” (Warren & Giladi, 2014).

Although the benefits of ant-mediated seed dispersal by plants are fairly clear, ants are not totally dependent on myrmecochorous plants and sometimes prefer arthropod prey over seeds. Finally, some plants deceive ants by providing false elaiosomes, which only contain low concentrations of nutrients, which suggests that ants are susceptible to being manipulated by some species plants (e.g. Prokop et al., 2022).

So it seems that some of these interactions between plants and ants are more one-sided than was originally thought, with the plants tricking the ants into self-serving behaviour, and in some cases creating a dependency on the nutrients provided! As with all such symbiotic relationships, the state of play remains in flux, with the balance of advantage shifting one way or the other as circumstances change.

But who would have thought that nettles were such tricky old things!

Links

https://www.science.org/content/article/don-t-crush-ant-it-could-plant-wildflower

https://www.sciencephoto.com/media/1180384/view/seed-dispersal-by-ants-sem

Spring Ephemerals and Elaiosomes

https://roadsendnaturalist.com/tag/elaiosome/

https://ozarkbill.com/2020/12/07/myrmecochory-seed-dispersing-ants/

https://www.science.org/content/article/don-t-crush-ant-it-could-plant-wildflower

References

Gorb, E., & Gorb, S. (2003). Seed dispersal by ants in a deciduous forest ecosystem: mechanisms, strategies, adaptations. Springer Science & Business Media.

Lengyel, S., Gove, A. D., Latimer, A. M., Majer, J. D., & Dunn, R. R. (2009). Ants sow the seeds of global diversification in flowering plants. PloS one, 4(5), e5480.

Lengyel, S., Gove, A. D., Latimer, A. M., Majer, J. D., & Dunn, R. R. (2010). Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey. Perspectives in Plant Ecology, Evolution and Systematics, 12(1), 43-55.

Prokop, P., Fančovičová, J., & Hlúšková, Z. (2022). Seed Dispersal by Ants in Three Early-Flowering Plants. Insects, 13(4), 386.

Turner, K. M., & Frederickson, M. E. (2013). Signals can trump rewards in attracting seed-dispersing ants. PloS one, 8(8), e71871.

Warren, R. J., & Giladi, I. (2014). Ant-mediated seed dispersal: a few ant species (Hymenoptera: Formicidae) benefit many plants. Myrmecological News, 20, 129-140.

Zinger, E., Gueijman, A., Obolski, U., Ram, Y., Ruby, E., Binder, M., … & Hadany, L. (2019). Less fit Lamium amplexicaule plants produce more dispersible seeds. Scientific Reports, 9(1), 6299.