Flowers that produce unpleasant odors serve a precise biological function. These scents are a specialized strategy to attract specific pollinators, highlighting diverse ways plants ensure reproductive success.
The Chemistry of Foul Scents
The “bad” smells emitted by these flowers originate from specific volatile organic compounds (VOCs) released into the air. Examples include dimethyl sulfides, like dimethyl disulfide and dimethyl trisulfide, which contribute to a rotting odor. Amines such as putrescine and cadaverine mimic decaying flesh, while isovaleric acid can smell like sweat.
These compounds create scents resembling decaying organic matter, feces, or spoiled food. Indole, for instance, can contribute a fecal note, and methyl thioacetate adds a cheesy or garlic-like aroma. Some plants, like the Titan Arum, enhance scent dispersal through thermogenesis, heating reproductive structures to vaporize compounds more effectively. This chemical mimicry draws in insects that associate these smells with essential resources.
Pollinators Attracted to Bad Smells
The primary pollinators attracted to foul-smelling flowers are scavenging flies and beetles, including carrion flies, blowflies, flesh flies, and various dung and carrion beetles. These insects are drawn to such odors while searching for sites to feed, breed, or lay eggs in decaying organic matter. The flowers deceive these insects into perceiving them as suitable locations.
Notable examples include the Titan Arum (Amorphophallus titanum), also called the Corpse Flower, which attracts carrion beetles and flesh flies with its strong putrid scent. Other examples are species within the Rafflesia and Stapelia genera. Stapelia species, with their hairy, often reddish-brown flowers, lure scavenging flies. Aristolochia species, known for their pipe-shaped flowers, attract phorid flies and other small dipterans, sometimes mimicking invertebrate carrion.
How Pollen Transfer Occurs
Pollen transfer mechanisms in foul-smelling flowers are often elaborate, utilizing trap mechanisms for efficient pollination. The Titan Arum, for example, has a large spathe that unfurls to reveal a central spadix. This spadix heats up, attracting insects that become temporarily trapped within the spathe’s chamber.
Inside, female flowers are receptive first, allowing incoming pollinators to deposit any pollen they carry. After a period, male flowers mature and release pollen, dusting the trapped insects. The spathe then wilts or reorients, allowing pollen-laden insects to escape and potentially pollinate another flower.
Stapelia flowers often feature specialized grooves or structures that guide flies to sticky pollen masses (pollinia) that attach to their legs or heads. Aristolochia flowers typically have a bent floral tube lined with downward-pointing hairs, trapping insects until pollen release, then allowing them to exit as hairs wilt.
The Evolutionary Success of Odorous Blooms
Emitting foul odors for pollination represents a successful evolutionary adaptation. This specialized approach targets a niche of pollinators, flies and beetles, not typically attracted to sweet-smelling blooms. Focusing on these less conventional pollinators may reduce competition with generalist pollinators like bees and butterflies.
This relationship often involves co-evolution, where both plant and insect partners have adapted to each other. The flowers’ mimicry of decaying matter, without offering a nutritional reward, is a deceptive strategy that ensures pollen transfer. The independent evolution of this “carrion mimicry” in multiple plant families, known as convergent evolution, highlights its effectiveness in ensuring reproductive success and contributing to biodiversity.