December 11, 2025
3 min read
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Before Flowers Existed, These Plants Lured Insects with Heat
New research on strange cycad plants offers a glimpse into the prehistoric origins of pollination
A thermal image of two male cones of the cycad Zamia furfuracea. The cones heat up during pollen release. Some areas of the cones can heat differentially, and these patterns serve as pollination guides.
The words “pollination” and “flower” may seem inseparable, but plants began courting insects millions of years before they evolved flashy petals. Now we know how they may have done it: not with dazzling color but with radiant heat.
A study published today in Science reveals that cycads, tropical plants that resemble palms, attract beetles using infrared radiation generated by their conelike reproductive structures. Given that cycads are the world’s oldest animal-pollinated plant group, co-senior author Nicholas Bellono, a Harvard University molecular biologist, says the results offer a window into “the earliest form of pollination”—a prototype for what is today one of the most transformative ecological interactions on Earth.
Cycads are thermogenic, meaning they generate serious heat—some species reach up to 15 degrees Celsius (27 degrees Fahrenheit) above ambient temperature. Wondering why they’d expend all that energy, lead author Wendy Valencia-Montoya, a Ph.D. student in Bellono’s lab, devised an experiment: she smeared cycad cones with ultraviolet-fluorescent dye so that incoming beetles would become coated with it and leave visible tracks on the next cone they touched. In the new paper, she and her colleagues found that the beetles preferentially visited the warmest regions of the cones.
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Beetles of the species Rhopalotria furfuracea on a male cone of Z. furfuracea, whose cones produce heat during pollination.
Researchers have established other functions for cycad thermogenesis: the heat increases humidity and disperses scent, both important pollination signals, and it creates a cozy shelter for beetles to mate and reproduce. But this work suggests that infrared light itself serves as a direct cue. Indeed, when the researchers heated 3D-printed cycad cones and covered them with plastic film to prevent heat conduction through touch, making infrared the only possible thermal signal, the beetles were still drawn to them over unheated cones.
To figure out how beetles pick up what cycads are putting down, the team analyzed the insects’ sense organs for thermosensitive structures and found that the tips of the antennae were loaded with TRPA1, a warmth-activated ion channel that also helps snakes and mosquitoes perceive infrared. For both beetle species tested in the study, TRPA1 activation was finely tuned to their respective host plant’s temperature range. Scent, which travels farther, likely directs beetles to the right neighborhood, but infrared seems to be the final beacon guiding them in.
These findings also bear on the longstanding evolutionary puzzle that Charles Darwin referred to as an “abominable mystery”: How did flowering plants, known as angiosperms, rapidly explode into around 350,000 species when cycads and other gymnosperms barely number in the thousands? Maybe, the authors of an accompanying commentary suggest, reliance on infrared may have limited the number of insects cycads could build specialized relationships with. Whereas flowering plants can tweak hue, saturation and patterning, yielding almost infinite combinations to target different pollinators, cycads can only adjust heat intensity.
Irene Terry, a plant biologist who studies cycad pollination at the University of Utah and was not involved in this study, calls it “one of the best, if not the best, cycad papers I’ve ever read.” In terms of evolutionary history, she notes, cycads could have also varied scent-producing compounds to diversify and establish relationships with specific pollinators like flowers do. University of Cambridge plant biologist Beverley Glover, a co-author of the commentary piece, agrees but adds that angiosperms enjoy the best of both worlds—scent and color. “Multiple opportunities for diversification is probably better than one,” she says.
The reliance on detectable temperature also suggests a conservation question: Could global warming make it harder for beetles to distinguish the heat of their hosts? Cycads are already the most endangered plant order, and behavioral ecologist Sean Rands of the University of Bristol in England, who wasn’t involved in this study, says the prospect of communication breakdown adds to the list of threats. “Any information you take away,” he says, “is going to make it harder for pollination to happen.”
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