Ants are cool. It’s true. An example? Seed dispersal.
Plant dispersal is pretty awesome in and of itself, with the flora of the world containing a variety of specialized morphologies that are clearly adapted for specific means of dispersal, but I find ant-assisted seed dispersal to be particularly intriguing. One interesting component is the use of an elaiosome as a reward for the ant to disperse the seed. An elaiosome is a fatty, protein-rich body that is attached to a seed. Below is a picture of Aphaenogaster ants dispersing a seed with an elaiosome on top.
(Photograph by Alex Wild)
Tasty. As can be imagined, offering such a costly (and delicious!) reward for dispersal must result in an advantage for the plant species. However, this benefit can sometimes be hard to glean. For example, a study by Rowles and O’Dowd (2009) found that on average, Acacia sophorae seeds contain an elaiosome that is 20 percent of the total mass of the seed. What makes this confusing is the seemingly ridiculously short distance at which seeds are dispersed. In this experiment, the authors considered dispersal 5cm away from the seed source as significant. 5cm. The farthest away that a seed was dispersed was in the nest of the disperser under study (the invasive Linepethema humile, the Argentine ant), at a whopping 1.15m away from the source.
How, then, is such seemingly pathetic dispersal worth all of the energy required to produce an elaiosome? I don’t think the answer is entirely known, but several components undoubtedly contribute to benefits for plants. First, and probably foremost, is avoidance of intraspecific competition – if a daughter feeds off the same local soil as her mother, the fitness of both suffer. Another related possibility is nutrient availability. If nutrients diminish locally for a plant population, the plants want their offspring to explore the soil that is richer on the other side of the ecosystem. An additional benefit for using ants as a means for dispersal, and perhaps the coolest, is due to the fact that ants are clean critters. Dispersal into an ant’s nest means, in reality, being thrown onto the refuse pile, which is a rich mine of carbon and other resources that offer a huge advantage to a seed. There are even some elaiosomes that may contain oleic acid, the same chemical that stimulates ants to throw their dead onto the refuse pile! Possums got nothing on elaiosomes.
Source: Rowles, A &. D.J. O’Dowd. 2009. New mutualism for old: indirect disruption and direct facilitation of seed dispersal following Argentine ant invasion. Oecologia. 158:709-716.