The Outside Story: Sneaky plants trick gullible ants

I don’t trust flowers.
There are grounds for my suspicion; flowering plants are proven masters of deception. For instance, the sundew uses sparkling droplets of sticky “faux dew” to ensnare and digest curious flies; bee orchids dupe male wasps into wasting their copulatory efforts on floral structures that look and smell like a female wasp. And what about humans? As I labor on behalf of flowers, fertilizing, tilling, watering and sweating, I sometimes wonder if I’m being led down the proverbial garden path. Exactly who’s cultivating whom?
Flowering plants have many strategies for disseminating their progeny. Some dispersal methods are straightforward: maple keys helicopter hundreds of yards, springy jewelweed pods eject their spawn, and tiny willow seeds float miles on watery currents. Many, like the burdock, hitch rides on animals (as my grandfather used to say, “burdock-burdock, he’s your friend, he’ll stick with you ’til the end”). I can even respect the lowly dandelion with its swarm of germinal fuzz and naked ambition for earthly domination — at least it’s honest.
But others, the shiftier of the lot, are not below a trick or two. I recently spoke with Dr. Elizabeth Farnsworth (co-author of “A Field Guide to the Ants of New England”) and it turns out that loads of spring perennials employ an interesting scheme called myrmecochory. They use ants to spread their offspring. Species including the bloodroot (Sanguinaria canadensis), Dutchman’s breeches (Dicentra cucullaria) and various violets grow extra tough seeds capped with fleshy appendages called elaiosomes. These structures, packed with ant-edible lipids, proteins, carbohydrates and vitamins, are also laced with oleic acid — a pheromone that is present in insect corpses and has been shown to attract carnivorous ants.
What usually happens is that ants are drawn to the ripening seeds, then lug them back to (and often into) their burrowed nests. There, in the relative safety of the colony, the ants feed the elaiosomes to their maturing larvae and discard the seeds.
For decades, this relationship has been touted as a fine example of interspecies quid pro quo. In comparison to the plant-insect interaction of the sundew, the relationship a local trillium patch has with a neighboring Aphaenogaster colony seems downright quaint. The ant gets a snack. The plant benefits from seed transport away from the parent (and thereby reduced competition between seedlings) to sites that are protected from predators and often, ideal substrates for germination. 
Seed dispersal by ants is a worldwide phenomenon involving more than 11,000 plant species (4.5 percent of all plants) across multiple ecosystems. In fact, myrmecochory is a key evolutionary driver for flowering plant diversity. Ant-dispersed lineages in 241 genera from all continents except Antarctica contain on average more than twice as many species as do their non-myrmecochorous kin. But scientists are starting to wonder if this ain’t a lopsided deal for the ant.
Recent studies have shown that even though ants can digest elaiosome nutrients, the benefit to the ant colony is questionable — or at least, highly variable. Colony success can be measured in pupae production (number and mass) and sex ratio (queens versus workers). In several experiments, sexual production, allocation and weight did not differ significantly between elaiosome-supplemented and unsupplemented diets. In other experiments, ants preferred oleic acid laden seed that actually reduced reproductive output.
On a recent spring day, after clearing more space for hollyhocks, I was snapped out of my horticultural complacency when our mailman delivered the 10th in a series of unsolicited seed catalogs. There, printed on the pulpy backs of their cousin trees was one glossy, floral display after another— flowering plants shamelessly luring me to buy, ship and sow their offspring. When I finally pulled my nose from the enticing pages I noticed, as if for the first time, that flowering greenery had completely surrounded our home. I dropped the seed rag where I stood, slack jawed. The flowers had played me like an ant.
Howard Krum is a science writer, aquatic animal veterinarian and author of the novel “An Animal Life: The Beginning.” The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlandsmagazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: [email protected].

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