Seed Dispersal by Ants: A Primer

A critical and common, but much ignored, plant-animal interaction is primary seed dispersal by ants (myrmecochory). Ants are ubiquitous across many landscapes, so it may not be surprising that many plants have evolved to utilize them to move their seeds. In fact, myrmecochory has independently evolved over 100 times. Myrmecochory carries all the usual benefits of biotic seed dispersal, such as reduction of competition with siblings, as well as a few more (notably, escape from fire). Seeds dispersed by ants generally possess an attached food body (elaiosome), which attracts and rewards ants. Elaiosomes are commonly described as fat bodies comprising lipids. However, their nutritional and chemical composition can vary widely, with some plant species producing elaiosomes with higher concentrations of protein or carbohydrates than of lipids. Elaiosomes of even quite closely related plant species can vary in chemical composition. Once ants encounter a seed with an elaiosome, they generally return to the nest with that seed, remove the elaiosome, and consume it, although which individuals within the nest consume the elaiosome is in many cases unclear. Once the elaiosomes have been removed, seeds are then disposed of either within the nest or outside of the nest, where they can be potentially secondarily dispersed by wind, water, or animals, including other ant species. This primer will focus on primary dispersal, the initial movement of seeds away from a parent plant. This primer defines myrmecochory, provides an overview of elaiosome traits, summarizes how ants choose and interact with elaiosome-bearing seeds, describes the current understanding of how elaiosomes evolved, and touches on how ant seed dispersal may respond to anthropogenic changes, such as the introduction of invasive species and climate change. This primer is guided by five major questions: (i) What is seed dispersal and myrmecochory? (ii) What are the benefits of myrmecochory? (iii) What is an elaiosome? (iv) How do ants choose and interact with seeds? (v) How did elaiosomes and myrmecochory evolve?