Strange Sex Lives of Orchids

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Forget about birds and bees—if you want to learn about the varieties of sexual practices in the wild, study orchids. They're the most rich and varied family of flowers by far, with about 24,000 species (another estimate is 30,000 species). And many of those species have evolved elaborate tricks to get hapless birds and bees and other pollinators to lovingly embrace their flowers.

Some orchid flowers look just like their pollinators and thereby lure the real thing. In a special issue on orchids in the Annals of Botany this month, an introduction points out that Carl Linnaeus appreciated one superb mimic:

Its flowers bear such a resemblance to flies, that an uneducated person who sees them might well believe that two or three flies were sitting on a stalk. Nature has made a better imitation than any art could ever perform.

(See for yourself here.)  Linnaeus didn't figure out what the orchid was up to, but Darwin did. The National Museum of Natural History had a gorgeous exhibit of live orchids this spring called Orchids Through Darwin's Eyes, which Sarah photographed.

Botanists recognized orchids' visual mimicry first, but lately they've uncovered even more interesting scent-based mimicry. Basically, the orchids emit chemicals that smell, to a male insect, just like the sex pheromones emitted by the female of his species. In an interesting twist last year, researchers found that a bee-pollinated orchid produced chemicals that are similar but not identical to a female bee's scent. It's not that the orchid is a bad mimic, they researchers conclude, but that male bees are most attracted to a scent that's not too familiar.

Aside from feeling used, do pollinators suffer from being tricked by orchids? Maybe so. As a paper in the American Naturalist last year pointed out:

While some sexually deceptive orchid species require only pollinator gripping or brief entrapment for effective pollination, other orchid species coerce their pollinators into energetic copulation. Although these copulations are often described as “pseudocopulations,” the vigorous response of pollinators suggests that true matings with ejaculation and costly sperm wastage may indeed occur.

Sure enough, they found that male wasps pollinating Australian tongue orchids do indeed ejaculate, which is a waste of time and energy for the wasps.

For the orchid, the relationship with pollinators is all about sex; but for the pollinators, sometimes it's about food. A study that comes out in Current Biology later this month shows that a Chinese orchid mimics the scent of a honeybee's distress signal—a scent that attracts honeybee-eating hornets. Wicked!

But orchids don't always need pollinators. Sometimes they have sex with themselves. A study a few years ago showed that another Chinese orchid, if no wind or pollinators are around, will twist its pollinia into its own stigma:

Here we describe a new type of self-pollination mechanism in the tree-living orchid Holcoglossum amesianum, in which the bisexual flower turns its anther against gravity through 360° in order to insert pollen into its own stigma cavity — without the aid of any pollinating agent or medium.

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