Read our COVID-19 research and news.

Pollen droplets like these glisten in the moonlight, attracting nocturnal insects to Ephedra foeminea.

Pollen droplets like these glisten in the moonlight, attracting nocturnal insects to Ephedra foeminea.

Gideon Pisanty

Plant releases its pollen in the full moon

Full moons are often thought to be a catalyst for romance. Something about that dim white light in the dark sky has for centuries inspired humans to compose sonatas, paint pictures, and explore the cosmos. But it seems we’re far from being the only organisms inspired by the moon’s beauty. A new study, published online today in Biology Letters, suggests that a small scrubby shrub called Ephedra foeminea releases its pollen only when the moon is full.

Found on cliffs, ravines, and bare rocks from Italy to Yemen, Ephedra is a gymnosperm, meaning, like pine trees, it houses its reproductive structures within cones. Some varieties depend on wind to spread its pollen around, but others form pollen drops to attract flies and moths. What scientists didn’t know was precisely when—or how—insect pollination took place.

The realization that E. foeminea waits for the full moon came almost by accident. In the summer of 2014, Stockholm University botanist Catarina Rydin and her team traveled to the Balkans to study why some lineages of the plant had switched from wind to insect pollination. But they arrived too early in the summer and couldn’t find any actively pollinating plants. Looking over past records and other literature, Rydin realized that many pictures of the plant’s pollen-laden droplets oozing out from their tiny cones were beautifully lit by moonlight. “I am not sure why, but all of a sudden we experienced a eureka moment!” she says.

Just a few weeks later, on the night of a full moon, E. foeminea shrubs in the study area began to release drops of pollen-rich fluid. The sugary liquid is especially attractive to a host of nocturnal flies and moths, many of which are known to navigate using the polarized light from the moon. Rydin and her team thus speculate that Ephedra evolved to release its pollen when the moon was brightest so that its pollinators could be as efficient as possible. “It’s an interesting study. I think that there’s a clear correlation there,” said Chris Cutler, an entomologist at Dalhousie University, Halifax, in Canada.

The researchers then went back to analyze historical data on E. foeminea’s pollination periods in previous years and compared them with lunar cycles from the same year. After controlling for fluctuations in temperature and precipitation, the team found that 2014 was not an anomaly: The shrub appeared to be consistently entering its weeklong pollination phase on the night of the full moon in July. To further bolster their theory, the team looked at species of Ephedra that are pollinated by wind and discovered that these close relatives did not appear to sync their pollination phases with the lunar cycle.

It seems odd that a plant would forego pollination opportunities while waiting for the moon to reach its maximum size, and indeed, full-moon pollination has garnered justified skepticism before. But Rydin points out that the cost of producing pollen is high enough that Ephedra might conserve energy for the July full moon, which stays in the sky longer than partial moons. “We think it is all about maximizing the efficiency, not only regarding the preciseness of pollinator navigation and attraction to the cones, but also in terms of number of efficient dark hours. Only at full moon do the insects have a moon to navigate by during the entire night.”

How the plants sense the full moon remains a mystery, but Rydin’s early guess is that they are actually aware of the minute differences in tidal force that the moon exerts on Earth. The moon’s orbit is elliptical, meaning that at some points it’s closer to our planet than others, causing its gravitational pull to fluctuate ever so slightly. How the plants might perceive such miniscule forces is unknown, but Rydin hopes to get a chance to investigate further. Likewise, an investigation into the insect species might help shore up the findings. “I think what’s needed next is to find what the insect fauna is in those habitats and if they have cyclic activity with full moons. That’s the missing link so far with the paper,” Cutler says.

The other possibility is that Ephedra is detecting the moonlight itself. But this theory presents its own challenging questions. How does the plant differentiate between moonlight from an almost full and a truly full moon? What happens if it’s cloudy on the night of the full moon in July? Oddly enough, the second question is hard to answer using historic data because the weather in Ephedra’s distribution region is so mild, and cloudy summer nights are so rare.

Whatever the case, the result is spectacular: Amid the moon’s cool glow, thousands upon thousands of silvery droplets coalesce on Ephedra’s cones to reflect the moonlight into the compound eyes of passing insects, signaling the start of a weeklong feast for the arthropods and another mating season for E. foeminea.

*Correction, 2 April, 1:07 p.m.: An earlier version of this article stated that Ephedra was a flowering plant, when, in fact, it is a gymnosperm.