These plants can heat themselves up—scientists finally know why
Since the time of the dinosaurs, cycad plants may have attracted insects using infrared light. It may be the world's oldest form of pollination.
When night falls in the Amazon, seed-filled cones protruding from plants put out a call to local beetles to come pollinate them. That call comes in the form of infrared light, radiated as the plants raise their own temperature.
These plants, called cycads, can heat themselves up to a whopping 27 degrees Fahrenheit above ambient temperature. Scientists report for the first time in the journal Science that the infrared light from that heating attracts pollinating beetles, and that this is among the most ancient pollination signals.
“It is, without doubt, surprising,” says Joe Parker, an evolutionary biologist at Caltech, who was not involved in the study. “It's one of those beautiful systems that, when it's delineated like this, you kind of think, ‘Of course, it's like this. It's so perfect. Of course, nature would evolve this solution, and everything fits together so beautifully.’”
Infrared light is invisible to the human eye, so it may seem like an unusual mechanism for attracting pollinators. But researchers say this pathway to pollination predates the development of color vision in evolutionary history.
“These animals have this totally hidden world that they're using to find plants, and basically facilitate the evolution of life on land,” says Nicholas Bellono, study co-author and biologist at Harvard University.
(This beetle can withstand 40 times more G-force than a fighter pilot.)
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Wayfinding in the rainforest
When Wendy Valencia-Montoya, the new study’s lead author, lived in a tent in the Peruvian Amazon for a six-month undergraduate project, she often got lost in the forest. She marveled at how Rhopalotria furfuracea beetles easily found their way to inconspicuous cycads, which look like small palms or ferns with large leaves and brown trunks, but are related to conifers. Both the plants’ reproductive cones and the beetles that pollinate them are small and hard to find if you don’t know what you’re looking for.
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“Already I was interested in how insects, being so small, they can navigate so well,” she says. “You see one cone, and the next day you just see all the beetles there. I just found that quite magical.”
When Valencia-Montoya started working in Bellono’s lab as a doctoral student at Harvard, she wanted to explore how those beetles found their way to the cycads, which are “living fossils” that represent the oldest lineage of plants that produce seeds through animal pollination.
Scientists know from fossil evidence that beetles have been pollinating cycads at least since the early Jurassic period, some 200 million years ago, when dinosaurs roamed the Earth. From that perspective, understanding how cycads attract beetles is a way to travel back in time to probe the origins of pollination itself.
(Cycads are “living fossils”—and comeback kings.)
Previous research had also established that cycads have a wide range of heating abilities, with some individuals warming themselves by about five degrees Fahrenheit, while the most ambitious changing by more than 25 degrees. The variation in the same plant species indicates that the change in heat is not just a byproduct of a standard metabolic processes, but a signal to attract beetles. Some insects, including these beetles, even specialize in pollinating particular temperature ranges.
“We believe that insects can also discriminate between these different temperatures. So it may work like something to learn, ‘ah, this is this plant, and this is another plant,’” Valencia-Montoya says.
But whether beetles specifically responded to changes in heat—or something else—emitted by the cones, was the mystery Valencia-Montoya wanted to explore.
Experimenting with beetles and cycads
Scientists conducted behavioral experiments with the Rhopalotria furfuracea beetles at the Montgomery Botanical Center in Florida. Initially, they found that male cycad cones—which are full of pollen—heat up and cool down first. The female cones, which receive pollen, take their turn some three hours later, creating a well-choreographed trajectory for the beetles to follow between them.
Subsequent experiments showed that beetles preferred the warmest parts of the pollen-filled cones, suggesting that the heat could be guiding the insects. But scientists suspected there might be a particular aspect of heat attracting these beetles.
The more an object heats up, the more infrared light it emits. Sensing this light is different than feeling warmth through touch or convection.
To distinguish the difference, scientists set up 3-D printed cycads in a controlled laboratory environment and ran a series of experiments. For example, they covered the model cycads in polyethylene film, which is mostly transparent to infrared light about one centimeter from the replica-plant’s surface, and still observed that beetles were more attracted to heated male and female plants than those at ambient temperature.
While previous research has documented other plants that heat themselves up, the going theory has been that this ability would be useful in cold environments to resist frost or even warm up pollinating insects, Bellono says. “There was no real evidence that this would be a signal for pollination itself.”
A prehistoric connection
The scientists then went even deeper and smaller, probing what was happening at the molecular level to enable the beetles to pick up on infrared light from plants.
In neurons in the beetles’ antennae, they found variants of a gene called TRPA1. Previous research had found this particular gene important for infrared sensing in snakes, which helps them find predators, and mosquitoes, which seek out humans and other animals to bite.
The connection surprised Valencia-Montoya. Insects diverged from snakes more than 400 million years ago in evolutionary history, but the same genes still function in similar ways in the case of the beetles and the snakes. “Nature seems to just recycle the same old molecular players and use them again,” she says.
Scientists already knew that some beetles sense infrared radiation, such as fire-chaser beetles, which can detect infrared light from fires hundreds of kilometers away. But this is the first study to show the molecular and cellular basis of infrared sensing in any beetle species.
But what about color variation in the cycad cones? After probing both beetle optical sensitivities and the range of cone colors, researchers determined that the beetles would not be able to pick them out well by color alone amid the rainforest background.
Taking this idea a step further, researchers scoured databases of living plants and their colors, as well as fossil records, and determined that the more color diversity in a seed plant family, the less likely those plants are to be able to turn up their own heat, and vice versa. What’s more, it appears that beetle diversification predates the rise of bees, butterflies, and other pollinators that see in many colors and evolved at the same time as modern flowering plants.
(Dung beetles have a sophisticated navigation system.)
Beetle could be infrared trendsetters
Having a non-color-based way to find the cycads makes sense given that these Amazonian beetles are nocturnal. The beetles also lay their eggs in the cone structure of the plants, linking the insect’s development cycle to these plants. “So without beetles, there's no plants, and without plants, there's no beetles,” Valencia-Montoya says.
While this study shows the importance of infrared light in pollination for the first time, other previously known signals from the cycads—such as humidity and scent—still play roles in attracting the beetles. At short distances scent is not as useful a signal as infrared light for locating the cone entrances, pollen and other rewards, but still draws pollinators in. Valencia-Montoya and her colleagues hope to formally test the possible interconnectedness of heat, humidity, and scent in these plants in future research.
The study also raises the possibility that there are other ecological systems in nature that rely on changes in heat and infrared sensory cues that have not yet been documented, Parker says.
With some 400,000 species known today—and that’s just what’s been described and named so far— beetles represent about a quarter of all known animal species. Given their long evolutionary history, it makes sense that beetles would have been the forerunners of infrared sensing for pollination, Parker says.
“If there's any organism out there that's likely have got in on the act and been able to tune into this form of information that was being emitted by plants and use it as a sensory cue, there's a good chance that will have been a beetle,” he says.
