Thursday, February 12, 2009

Happy Birthday Darwin!

Darwin's Moth and Columbines: Bee Gardening and Evolution

There's a debate going on in the scientific community about pollination and evolution. In 2007, Justen Whitall and Scott Hodges did a study on different varieties of the columbine genus Aquilegia. There are many varieties of Aquilegia that grow wild in North America, and the interesting thing is the variety in the length of their spurs, which is where their nectaries are located. Those with the shortest spurs are pollinated by bees, the medium spurs are pollinated by long-tongued bees and hummingbirds, and those with the longest spurs are pollinated by hawk moths. Whitall and Hodges observed that the nectar spurs tended to evolve into longer lengths in order to attract the most efficient pollinators. If the bees' tongues weren't long enough to reach the pollen, then the flowers grew longer spurs to entice hummingbirds, who were more efficient at pollinating the plant. There's a good photo of the differing columbines here at

There is another theory for the increasing length of columbine spurs that is based on the work of Charles Darwin. Now some of you may know the story of Darwin's moth. Actually, there are two famous stories about Darwin and moths. The first story is about the peppered moth with had a salt and pepper pigmentation that helped it camouflage itself against speckled tree bark. During the industrial revolution, the trees in the "black country" were covered in factory soot. The peppered moth evolved to become black to blend in with its changed surroundings. Nowadays the black moths are more likely to be picked off by birds because the trees are no longer covered with soot. You can see a photo of the peppered moth on this BBC website.

The other moth story involves a case of cryptozoology. In 1862 Darwin observed an orchid in Madagascar called the Angraecum sesquipedale that had a nectary almost a foot deep. He conjectured that its pollinator was probably a large moth with a proboscus as long as the nectary. The moth did indeed exist. It was discovered 41 years later, in 1902 and named Xanthopan mogani ssp. praedicta, otherwise known as Darwin's moth. There is an excellent article on this story on this web site.

Darwin's theory of a coevolutionary "race" between pollinators and flowers help scientists conjecture that columbine nectaries may be getting longer because the pollinators are evolving with longer tongues in order to become more efficient at gathering nectar and pollen. I'm not a scientist, but I don't see why the theories have to be mutually exclusive. In any case, the point of writing about this in the context of creating a bee garden is that in order to attract bees you have to 1) decide what kinds of bees you'd like to attract, 2) learn how long their tongues are and 3) compare the lengths of the nectaries of the flowers you would like to grow.

In an article on, R. A. Ennos, points out that bees do not coil their tongues as moths do, therefore the evolution of their straw-like tongue is limited to their body size.

"The maximum tongue length of a bee is unlikely to be significantly greater than its body size, since the tongue must fit lengthwise beneath the body in flight. Thus in an ancestral, bee pollinated Aquilegia, co-evolution will lead to a spur length close to the size of the bee's body.

Spurs of enormous length could ultimately evolve under co-evolution with the hawkmoth because the maximum size of a hawkmoth tongue is staggering (up to 220 mm), a fact made possible by their ability to coil the proboscis neatly beneath the head when not in use."

If you fancy doing some scientific experiments of your own, why not try to grow several types of columbines that attract different pollinators? You may be speeding up natural selection by putting plants next to each other that have evolved in geographic isolation. Apparently columbines can create fertile hybrids. If you save some seeds, you could become an amateur plant breeder. Birds love the seeds too, by the way. You can make notes on which colours attract the most bees. Apparently bees see red as black, so they will see the Aquilegia formosa flowers as white centres with black edges. In an abstract for an article on bird-pollinated flowers Quentin Cronk and Isidora Ojeda say that black is a bee-deterrent color. If the Aquliegia formosa depends on both bees and hummingbirds, then it makes sense that the flower is both red and white. I would think that the black would serve as a kind of nectar guide in itself, giving the flower a high contrast edge to read.

"Red coloration appears to play a major role in both bee-deterrence and bird-attraction. Other mechanisms of bird-attraction include the production of abundant dilute nectar and the provision of secondary perches (for non-hovering birds)."

In the book British Wildflowers--In their Natural Haunts Vo 2-4" A. R Horwood has an excellent detailed description of how bees pollinate wild columbine. I wish there were more examples of this kind of writing for bee gardeners.

"The five petals are large and conspicuous, each one hollowed from the claw upwards, to form a hollow spur or horn-shaped cavity, 15-22 mm. long, with a cup-like mouth, admitting a humble bee's head, and the narrow tubular part is curved inwards and downwards above, containing the honey secreted by a fleshy thickening in the spur. Bees with a long proboscis hold on to the flower below, clutching hold of the base of the spur with their fore legs, and with their mid and hind legs they clasp the stamens and pistil, which project obliquely downwards in the middle. They introduce the head into the aperture of the spur where the outer wall touches the end of the proboscis following the curve of the spur. In younger flowers the hind part of the bee's body touches the anthers, closely surrounding the carpels covered outside with pollen, and in older flowers the same parts touch the carpels which have become elongate, and spread the stigmas farther apart. So cross-pollination follows.

The visitors are Bombus hortorum, B. terrestris, B. agrorum, Halictus. B. terrestris cannot reach the honey and bites a hole at the base of the spur in order to obtain it. Holes may frequently be seen and are due to this cause."

Horwood also explains that the flower derives its Latin name from the talons of an eagle, which finally makes sense to me! The columbines that are native to BC (Aquilegia formosa) were used medicinally by First Nations, but be advised the Aquilegia plant has poisonous parts. So go ahead, order some columbine seeds and grow a bee garden that would make Darwin proud!

No comments:

Post a Comment