The milky specks known as hummingbird mites are among the most skilled hitchhikers in the world. They live their lives inside flowers and inflorescences, where they feed on nectar and pollen, mate, and produce their young. But all flowers eventually wither and die, and a mite, approximately the size of a grain of sugar, is far too tiny to set out into the world in search of a new flower. Instead, it awaits the beak of a hungry hummingbird to dip into the chute of their flower. When one arrives, the mite leaps from petal to beak, where it crawls inside the bird's nostrils to hitch a ride and hop off whenever the bird visits another flower of the same species.
These mites, however, lack wings, as well as the ability to jump. So how do they manage to leap from beak to flower and back? A new paper reveals the arachnids actually harness the power of electrostatic attraction to make the leap. Once the mites detect the signature electrical charge of a hummingbird—an ability known as electroreception—they whoosh through the air, their itty-bitty bodies riding the charged air. The new paper is published in Proceedings of the National Academy of Sciences.
Carlos Garcia-Robledo, an evolutionary ecologist at the University of Connecticut and an author on the paper, generally researches how climate change affects the interactions between arthropods and plants. He first learned about hummingbird mites when he was a student, as the interaction between the mites and the birds is one of the classic stories of tropical biology. The mites "are taking advantage of the hummingbirds in two ways," said Víctor Ortega Jiménez, a biomechanics researcher at the University of California, Berkeley, who was not involved with the research. Not only are the arachnids hitching a free ride, but they are also feasting on the same reservoir of nectar that the birds need, he said.
Garcia-Robledo has a lab in Costa Rica at the La Selva Biological Station, where he often opens flowers pollinated by hummingbirds to find petals teeming with dozens of mites. "It was kind of a mystery how these mites are able to hitchhike on these hummingbirds that are super fast, right?" Garcia-Robledo said. "Because they are almost blind. They don't jump. They are not particularly fast either."
Last year, Garcia-Robledo was thinking about a paper from a group of researchers at the University of Bristol that found ticks can use static electricity to jump on people. Static electricity is a form of electricity that results from the imbalance between positive and negative electrical charges. We experience static electricity when we rub two objects together, such as scuffing our feet on the carpet. In this exchange, one material gives up electrons, becoming positively charged, and the other collects electrons, becoming negatively charged.
Animals naturally accumulate static electricity whenever they rub up against objects in their environment, such as grass or other animals. But the larger an animal is, the more forces like gravity will outweigh forces like electrostatics, said Sam England, a sensory ecologist at the Natural History Museum of Berlin who was not involved in the new paper. "With these mites being so tiny, the potential for electrostatics to have an even more profound influence on their ecology is very high," England wrote in an email. In the tick paper, England and other researchers watched as ticks defied gravity and flew across gaps of air several times longer than their bodies, attracted to the static electricity of a host.
Serendipitously at La Selva, Garcia-Robledo met Kosta Manser, a sensory ecologist at the University of Bristol working in the lab that produced the tick paper. Garcia-Robledo pitched Manser his idea: Hummingbirds generate positive electrical charges while flapping their wings in flight, and plants rooted in the ground carry negative charge because the ground is negatively charged. What if static electricity helped ferry the mites from bloom to beak?
Soon, the researchers and Diego Dierick, a researcher who provides tech support for biologists at La Selva and an author on the new paper, took the mites to the lab for some tests. They needed a spherical electrode, and Dierick remembered seeing viral videos in which "people take a sheet of aluminum foil and roll it into a shiny foil ball," he wrote in an email. Using those methods, Dierick assembled a tinfoil conductive sphere, which was then suspended over a grounded copper plate. The researchers turned on the electrode to vibrate at one of the frequencies hummingbirds are reported to vibrate. "Instantaneously we saw that the mites put their hands up, and they were mesmerized with this aluminum disco ball," Garcia-Robledo said. This waving behavior is called questing, and it increases the animal's chance of finding a host. "This was our big surprise," he added. If the sphere was not vibrating at a hummingbird's frequency, however, the mites walked away.
In the next test, the researchers placed the mites in a glass tube that was positively and negatively charged at either end. "Because hummingbirds are positive, we wanted to test if they are attracted to positive or negative fields," Garcia-Robleo said. When the current was switched on, the mites scurried to the positive end. Garcia-Robledo watched videos from doomsday preppers, who often build Faraday cages to block electromagnetic fields, to build the contraption. "So that's in my algorithm now," he said.
It was clear the mites were attracted to electrical fields. But were these forces acting upon the mites, or could they actually sense them? To figure this out, the researchers wanted to examine the anatomy of their legs. "These are .6-millimeter organisms," Garcia-Robledo said. "So imagine the size of those legs, right? They are super, super tiny." Garcia-Robledo had to become a microscopic surgeon overnight. He anesthetized the mites with ice, brought the arachnids under the lens of a microscope, and amputated the tips of their legs with a hypodermic needle. "I eventually became really good at amputating those legs," he said, adding that by the end of the experiments he could lop off a mite's leg tips in around five minutes.
Once the legs were under the microscope, the researchers discovered sensory hairs similar to structures in ticks called Haller's organs, which help the arachnids sense chemicals and heat. And each mite leg was tipped with three hairs similar to those used by spiders to detect electrical fields. Mites with both their front legs amputated had no interest in the vibrating electrode, all electing to walk away.
Mites moving of their own accord are not exceptionally fast. But the researchers wondered just how fast they became when whizzing through the air on electrical field lines. They anesthetized the mites with a balloon filled with CO2 and placed the critters on their backs below the charged tinfoil ball. While filming, they nudged the electrode closer to the mite until the charge raptured the arachnid. When the researchers played back the footage, they saw the mites traveled at an astonishing average speed of 150 body-lengths per second.
Ortega Jiménez found the paper's results in the lab "fantastic," he said. But he expressed reservations over how much can be concluded from experiments that focused on the mites alone. "If they want to really say that this is really happening, they need to use a hummingbird and the mites," he said. Ortega Jiménez's wondered if the mites' walking speed—nearly 20 body-lengths per second—might be fast enough for them to simply run from the flower to the beak. He added that he hopes to collaborate with the researchers in his own lab at Berkeley; although most flower mites are found from Mexico to South America, one species lives on Indian paintbrush flowers in southern California.
England said it was "really nice to see that the mites are actually able to detect the electric field" of their hummingbird hosts, as this could lend credence to his hypothesis that ticks, which are close relatives of mites, may have similar powers of electroreception. He had never heard of hummingbird mites before this paper, despite having done a lot of fieldwork at the same station at La Selva. But this, all the researchers pointed out, is the beauty of the world of electrostatic ecology. "The whole interaction is governed by a force completely invisible to humans," Manser said.
