A Japanese flower lures in its pollinators with a morbid perfume — the scent of injured ants.
The unusual scent belongs to Vincetoxicum nakaianum, a recently named species of Japanese dogsbane. Botanist Ko Mochizuki of the University of Tokyo discovered the plant’s grisly allure after noticing clouds of scavenging grass flies hovering around its unassuming flowers. Experiments revealed that the plant’s odor is a near-perfect chemical match to the distress signals released by injured ants, Mochizuki reports in the Oct. 20 Current Biology. The trick dupes the flies into visiting and inadvertently pollinating the blooms.
Mochizuki first noticed the grass flies while studying pollination at the Koishikawa Botanical Gardens in Tokyo and realized that some of the fly species could be kleptoparasitic. Such flies don’t hunt their own prey but instead pilfer the food of other predators, like injured honeybees or plant bugs.
Grass flies swarming around a flower is “really weird and rare scenery in nature,” Mochizuki says. These flies “are known to feed on damaged insects,” not nectar.
But the flies also serve as useful pollinators. So, Mochizuki hypothesized that the flowers must be tricking these insects into pollinating them by releasing scent molecules or volatiles akin to those released by the injured insects the flies feed on. A few plants have been documented doing this, including the parachute plant (Ceropegia sandersonii), whose flowers smell like wounded honeybees, and smearwort (Aristolochia rotunda), which mimics the odor of injured plant bugs.
After confirming that pollen-carrying grass flies also visited natural populations of V. nakaianum, Mochizuki cataloged the volatile compounds released by the flowers. He discovered that those aromatics perfectly matched the mix of chemical distress signals released by injured ants. The grass flies also showed interest in a synthetic re-creation of this volatile mix, suggesting that they could be after ants.
In a final experiment, flies in a maze were able to home in on ants killed by spiders, showing that they’re able to hunt by scent alone — and suggesting that V. nakaianum flowers are drawing in the hungry flies.
Mochizuki says that he now wants to study related plant species to understand how plants “get such weird mimicry systems during the course of evolution.” Other plants that lure in kleptoparasites have trap-shaped flowers designed to easily capture (and later release) the pollinating flies. In contrast, the flowers of V. nakaianum look rather ordinary.
“Strange mimicry [is] not restricted to the flowers with very strange morphology,” Mochizuki says.
Cornell University biologist Robert Raguso is reminded of the German word umwelt, which describes the unique sensory world experienced by an organism. “We can barely imagine the sensory realities perceived by other organisms with whom we share the biosphere,” he says. “A small, inconspicuous flower like this is nevertheless capable of conjuring the chemical essence of wounded ants.… It almost seems like a magic trick.”
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