Some of the earliest plants attracted pollinators by producing heat that made them glow in infrared, new experiments show. Published in Science, the work suggests that long before colorful flowers evolved, ancient plants metabolically warmed their reproductive cones when pollen was ready, creating an infrared signal nocturnal insects could detect.
These heat-producing plants, called cycads, still exist in tropical forests worldwide but are among the most endangered plant groups. “Some people call them dinosaur plants because they were much more dominant when the dinosaurs were around,” says Wendy Valencia‑Montoya, a cycad expert at Harvard. Fossils more than 200 million years old show cycads that look much like modern ones. Related to pines, male and female cycads each produce fleshy, cone-like structures holding pollen or seeds—the oldest known pollen-bearing plants.
For centuries botanists noted that cycad reproductive structures can be 15–25°F hotter than ambient air, or even warmer. Plant thermogenesis is rare and energetically costly, and scientists wondered whether heat was merely a metabolic byproduct, a way to volatilize scent, or a direct pollination signal. Previous studies found male and female cones heat at slightly different times, potentially guiding pollinators between cones.
To test whether insects use the heat signal itself, Valencia‑Montoya and colleagues tracked beetle pollinators by marking them with fluorescent dyes and watched their visits relative to cone heating. They also used 3-D printing to make fake pollen cones—hollow models filled with heated sand that glowed in the infrared like real cones. Placing the models at the Montgomery Botanical Center in Florida, they ran experiments that separated heat from other cues. In one test they wrapped cones in plastic that blocked contact but was transparent to infrared, letting insects see the glow without feeling warmth.
The result: the infrared glow alone attracted hundreds of pollinating beetles. Examination of these beetles showed specialized antennae able to detect slight temperature differences—analogous to heat-sensing receptors in snakes. Different beetle species visited different cycad species, and the beetles’ antennae appear tuned to the specific temperature ranges of their host plants.
“Infrared radiation is perhaps the oldest discovered pollination signal,” says Nicholas Bellono of Harvard, a coauthor of the study. Valencia‑Montoya notes that early pollinators were often nocturnal with poor vision, so a heat-based signal would have been advantageous. As daytime pollinators with good vision, like bees and butterflies, evolved, plants expanded into color signaling, enabling rapid diversification.
External experts praise the study. Roger Seymour of the University of Adelaide calls the multi-technique approach “an important contribution,” and suggests heat might also act as a direct energetic reward, allowing beetles to warm up and stay longer in thermogenic flowers. Irene Terry of the University of Utah, who studies cycads and insect interactions, says the research complements evidence that odorants and other cues are important while elegantly demonstrating infrared’s role. Some cycads emit scents ranging from bubblegum to bell peppers, she notes, but infrared adds another sensory channel insects use.
Bellono asks readers to imagine a prehistoric beetle navigating dusk, spotting plants that glow in infrared—“an entirely different world that we don’t experience,” he says. That ancient thermal signal likely guided pollinators long before colorful flowers evolved, and some beetle species still use it today.