Some of the earliest plants lured pollinators not with bright colors but by producing heat that made them glow in the infrared, a new set of experiments suggests. Published in Science, the work indicates that long before showy flowers evolved, ancient plants ramped up their metabolism when they had pollen ready, creating an infrared signal nocturnal insects could detect from a distance.
These heat-producing plants, called cycads, still grow 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 look very like modern cycads, she says, so “they haven’t changed much in hundreds of millions of years.”
Cycads are related to pines. Male and female plants each make fleshy, pine-cone-like structures that hold pollen or seeds—unique among these ancient plants and the earliest known structures for pollen. Botanists noticed centuries ago that cycad reproductive structures can be much warmer than surrounding air, sometimes 15 to 25°F hotter or more. Producing heat is energetically costly and rare in plants, but cycads clearly do it.
Researchers have debated why. Heat might be a byproduct of metabolism or could help volatilize scent the way a plugged-in air freshener spreads odor. Previous work showed male and female cones heat up at slightly different times, perhaps guiding pollinators between cones. To probe whether heat itself serves as a signal, Valencia-Montoya and colleagues tracked beetle pollinators by painting them with fluorescent markers and saw beetles arrive when cones heated.
To isolate infrared from other cues like scent, humidity or tactile warmth, the team 3-D printed simulated pollen cones. The hollow replicas were filled with heated sand so they glowed in the infrared like real cones. Deployed at the Montgomery Botanical Center in Coral Gables, Florida, the fake cones produced a striking infrared signature in the field.
The researchers ran tests to separate the visual infrared signal from the physical sensation of heat. In one experiment they wrapped cones with plastic that is transparent to infrared, allowing insects to see the infrared glow but preventing them from touching or feeling the warmth. Hundreds of pollinating beetles were attracted by the infrared glow alone.
When examining those beetles, the team found specialized antennae capable of detecting slight temperature differences—functionally similar to the heat-sensing receptors snakes use to locate prey. Different beetle species visit different cycad species, and the antennae appear tuned to the temperature range of their host plant.
“Infrared radiation is perhaps the oldest discovered pollination signal,” says Nicholas Bellono of Harvard, a coauthor on the paper. Back when pollen first evolved, many insects were nocturnal with poor vision, Valencia-Montoya notes, so a heat-based signal would have been an effective guide. As diurnal pollinators with better sight—like bees and butterflies—emerged, plants evolved visual signals such as color, allowing flowering plants to diversify rapidly.
Peers praised the methods. Roger Seymour of the University of Adelaide, not on the study team, called the combination of techniques “a powerhouse” that convincingly links cone temperature to pollinator attraction and argues the association is ancient. Seymour also suggests heat might offer direct benefits to beetles: warm cones could let cold-blooded insects raise their body temperature, enabling longer visits and greater activity, so heat could serve as both a signal and an energy reward.
Irene Terry at the University of Utah, an ecologist who studies cycads and their insect interactions but was not involved in this work, says it’s only been relatively recently that researchers realized cycads rely on pollinators rather than wind. Odors are clearly important—cycads can smell like bubblegum or bell peppers—so this study, she says, “in the most beautiful way” shows infrared is also key. Terry adds that insects such as mosquitoes already use infrared, so it’s not surprising beetles can too; there’s “a range of things that insects can see that we don’t.”
Bellono asks us to imagine a prehistoric beetle navigating dusk and seeing plants begin to glow red in the infrared. “The infrared is an entirely different world that we don’t experience,” he says. That world existed when dinosaurs roamed, and beetles still use it today to find thermogenic cycad cones.