When folks hear the word "spur," many of them invariably think of cowpokes in the Old West and the jangling metal boot contraptions they wore to urge on their horses. For us plant lovers, however, the word can conjure up something a little more serene. We can think, for instance, of the architecture of a columbine flower, with its distinctive spurs curling outward like the necks of graceful birds in mid-flight.
In the botanical world, a spur is a hollow, elongated tube extending from the petals or sepals of certain flowers. It is usually a reservoir for nectar, which is secreted by special glands inside the tube and collects in a knobby bulge at the base. The floral nectar spur wouldn't be much use during a cattle drive, but it's aptly named, nonetheless. In recent years, scientists have determined that it plays a major role in spurring the increase of species of flowering plants.
Columbines, such as Aquilegia caerulea, are prized for their five elegant flower spurs.
Much of the new research has been conducted on columbines. These spring-flowering wonders are not the only plants that produce nectar inside of spurs. Members of 15 out of the more than 300 plant families do it, too. However, while familiar plants like impatiens, violets, and larkspurs are limited to just one spur per flower, columbines are set apart by having five of them.
Family Ties
It's hardly original to use avian imagery to describe the columbine. The common name comes from the Latin word columba, meaning "dove," a reference to how the spurred flower of some species resembles a small flock of these birds. Moreover, the scientific name for the genus, Aquilegia, is derived from the Latin word aquila, or "eagle," because the columbine's quintet of spurs often assumes the shape of a raptor's talons.
The columbines are a diverse group of flowering plants, totaling about 70 species in the wild. They are widely distributed in the Northern Hemisphere, inhabiting climates that range from warm-temperate to arctic-alpine. Individually, many of these species exist in narrowly restricted geographical ranges. One long-spurred columbine, Aquilegia hinckleyana, grows only on one side of a single mountain in west Texas.
Geographic factors have no doubt played a role in the proliferation of columbine species. Indeed, the erection of physical barriers between populations of organisms—through such events as continental drift and climate change—has been the dominant mechanism behind all speciation (scientific parlance for the evolution of new species) on the planet. But geography only forms a backdrop to the story of how all the different columbines came to be.
Over the last decade, scientists—noting the high number of columbine species (and also struck by their array of fancy floral spurs)—began to ask whether there is a connection between spurs and high rates of speciation in plants. Dr. Scott A. Hodges, an evolutionary biologist at the University of California-Santa Barbara, has been at the forefront of this research. He started by posing a simple question: do groups of plants that have spurs display higher rates of speciation compared to their nearest relatives that don't have any?
Hodges identified eight lineages of flowering plants that possess floral nectar spurs. He found that seven of these have undergone a rate of speciation that was higher than that of their closest unspurred relatives. The lineage including larkspur and monkshood, for example, contains 350 species, while its nearest unspurred relation, a lineage including love-in-a-mist, has only 37 species. As for the columbines, their closest unspurred kin is a group consisting of just a single plant, Semiaquilegia adoxoides (a ratio of 70 species to one).
Pollinator Preferences
Once Hodges established that spurs are associated with higher species diversity, the next question he asked was "why?" The answer lay, naturally, in the spurs' function: the attraction of pollinators like insects and birds. As the nectar source for these plants, spurs play a vital role in their reproductive success. But it turns out that the spurs don't accept advances from just any suitor: their unique structure and orientation determine which animals can obtain nectar and receive pollen from the flower, and which can't.
In the columbine family, for example, the western U.S. species Aquilegia formosa is mostly visited and pollinated by hummingbirds. Its nodding, bright, scarlet-red and yellow flowers have relatively short spurs (1/2 to 3/4 inches long) that stick up in the air like caps, but with their openings facing down. As a hummingbird reaches into the spur openings with its tongue, its head brushes up against the protruding stamens (or male reproductive organs of the flower) and is smudged with pollen, which it carries to the receiving stigma (female organ) of another plant.
Then there's A. pubescens, which grows in the rocky soils of the Sierra Nevadas and isn't so friendly to hummingbirds. Its flowers range in color from white to pink and have very long spurs, up to 1 1/2 inches. They are erect, so that the spurs extend downward, but with their openings facing up. Their pollinators-of-choice are the hawkmoths, nocturnal flying insects with very long tongues that can reach the nectar residing at the base of the spur. The tongues of hummingbirds are too short for them to successfully feed on this plant.
In a series of experiments on A. pubescens, Hodges and another biologist, Michelle Fulton, examined in detail the intimate relationships between columbines and their pollinators. First, they changed the orientation of the flowers from upright to nodding, so that the spur openings were reversed from up to down. As a result, hawkmoths visited the flowers much less often, presumably because they prefer to feed while hovering above them. Then, the scientists shortened the length of the spurs by tying them off with thread. They found that hawkmoths still visited the flowers to feed on nectar, but that they didn't transfer pollen effectively since their bodies never got close enough to the anthers.
Spurred or Spurned
The biologists concluded from their research that, over the course of evolution, floral nectar spurs have enabled columbines (and by extension, many other flowering plants) to specialize on different pollinators. The ability of the columbine flower to "pick a pollinator" has led to an increase in the number of columbine species.
When the first floral spur evolved on a columbine, it had a dramatic effect on pollinator behavior by ensuring that only certain animals were equipped—anatomically and behaviorally—to feed on the plant. Moreover, by creating a niche for certain pollinators, the spur also ensured that when plant pollen was transferred, it went to another spurred columbine. This controlled transfer of genetic material eventually led to a breakaway from the main columbine group and the creation of a new species. The process must have happened over and over, helping to produce the great variety of columbine flowers we see today.
In fact, it's even come full-circle. There is a spurless columbine—Aquilegia ecalcarata. Through DNA testing, scientists have determined that this plant is a true columbine that evolved after the first columbine species got its spurs. It produces nectar in small pouches—not quite spurs—at the base of its petals. In the course of shooting it out with other columbines for pollinator attention, A. ecalcarata found it advantageous to dump the spur and be less exclusive.
So there, in a nutshell, you have it—floral nectar spurs have played an innovative role in the evolution of flowering plants. Something to think about this spring, while you are out in the garden admiring your columbines. Maybe you'll be inspired to come up with new metaphors for describing these spurred beauties. Who knows, you might even start calling them after your favorite Western heroes, like Wild Bill, Doc Holiday, or Calamity Jane. Okay, maybe not.
