If you were to combine ants’ dispersal of seeds and plant protection interactions, and maybe squint a little, you might see something like epiphitic ant gardens. Ant gardens form when tree-nesting ants collect the seeds of some epiphytes—plants evolved to grow in the branches of trees—and the collected seeds sprout. The nests provide congenial conditions for the plants, since gardening ants frequently use dung as a building material. The roots running through the nest help stabilize its structure and suck out moisture to control interior conditions.
This adds up to a mutually beneficial relationship between ant and epiphyte [$a]. A number of tropical epiphytes grow almost exclusively in ant gardens, and the inclusion of plants in the structure of their nests apparently helps gardening ant species to establish nests wherever food is most abundant.
Association with ant gardens has evolved independently in a number of epiphytic species, from arums like Anthurium gracile (pictured to the right) to orchids and philodendrons. When distantly-related species begin to perform the same ecological role, they often evolve convergent traits that facilitate the common role. Almost all ant-dispersed plants attach fatty bodies called elaisomes to their seeds to reward the ants that pick them up. Almost all ant-protected plants grow domatia in which the ants can nest, and nectaries to reward them with sugary sap. But plants that grow in ant gardens don’t seem to have a common trait that prompts ants to collect their seeds. Can it be that every ant-garden plant species has a unique way to be an ant-garden plant?
That’s what studies of ant-garden plants, including a new one just published in PLoS ONE, suggest. Plants associated with ant-gardens don’t have elaisomes on their seeds. Many produce fleshy fruit, but ants will collect their seeds even if no shred of fruit flesh clings to them. In some cases, ants will even collect seeds from the dung of fruit-eating birds and mammals.
This leaves the possibility that ant-garden plants produce some ant-attracting chemical in their seeds. In the new paper, Elsa Youngsteadt and her coauthors set out to identify chemical compounds that might be the common attractant used by nine different ant-garden plants from seven different plant families. Youngsteadt et al. isolated seven different compounds found in the seeds of ant-garden plants but not in closely related species that do not grow in ant gardens. (The absence of the seven compounds from the non-ant-garden relatives is established, rather amusingly, with a blank data table.)
The authors then painted crude extractions of all soluble organic compounds from two ant-garden plants onto seeds from species that gardening ants do not cultivate, and found that the ants were indeed more likely to collect them. (As a control, the ants were also offered seeds coated in the pure solvents used to extract attractive compounds. They didn’t like those.) However, analysis of the extracts failed to find a compound or set of compounds present in all three species.
It’s possible that Youngstead et al. simply failed to isolate the compound or compounds that all three ant-garden plants use to prompt ants to collect their seeds. But it’s not that far-fetched to think that these distantly-related plants might each use different attractive compounds to interact with ants in the same way. Natural selection may often arrive at different solutions when shaping different species for the same ecological role. It might also be that ant-garden relationships were established not by plants evolving a way to prompt ants to pick up their seeds, but by ants evolving to recognize seeds of plants that work well in gardens.
Davidson, D. (1988). Ecological studies of neotropical ant gardens. Ecology, 69 (4), 1138-52 DOI: 10.2307/1941268
Youngsteadt, E., Guerra Bustios, P., & Schal, C. (2010). Divergent chemical cues elicit seed collecting by ants in an obligate multi-species mutualism in lowland Amazonia. PLoS ONE, 5 (12) DOI: 10.1371/journal.pone.0015822