Flower mites spend their lives slurping nectar and nibbling pollen in flowers throughout the tropics. To travel from one blossom to another, these tiny, eight-legged creatures hitch rides on the beaks of hummingbirds, taking shelter in the birds’ nostrils during flight.
When a speedy hummingbird arrives at a flower to drink nectar, mites run toward its beak to get onboard before eventually transferring to another blossom. But the poppy-seed-size mites are basically blind and can’t jump, said Carlos Garcia-Robledo, a biologist at the University of Connecticut. How do they sense the bird’s presence and attach to it so quickly?
While doing research at La Selva Biological Station in Costa Rica, Dr. Garcia-Robledo and his colleagues decided to try to answer this question.
In a study published Monday in Proceedings of the National Academy of Sciences, the team discovered that flower mites can sense the same kinds of modulated electric fields that hummingbirds create when their wings rapidly flutter next to a flower. Moreover, these electric fields can also rapidly lift mites across a small air gap.
This is the first time that the ability to sense electric fields has been found in mites, and it suggests that this “electroreception” may be widespread and ecologically important, said Daniel Robert, a biologist at the University of Bristol in England who has published many studies on electroreception.
In the study, Dr. Garcia-Robledo and the biologists Diego Dierick and Konstantine Manser devised experiments to assess the mites’ abilities.
In one, they placed mites near an electrode above a grounded copper plate. When the electrode was off or imbued with a nonmodulated electric field, all but one of the mites walked away.
When it was turned on and vibrating within the range of electric fields that emanate from hummingbirds, almost all mites stayed and lifted their two front legs toward the electrode.
In the first test, the mite reacted instantly, Dr. Garcia-Robledo said. “I was surprised the response was so evident and fast,” he said.
In another experiment, the animals were placed in a glass “arena” that had negatively and positively charged ends. When the current was switched on, the mites ran to the positively charged side, much as they rush toward positively charged hummingbirds in nature.
The scientists looked closely at the mites’ front legs and discovered they contain structures similar to Haller’s organs, sensory hairs that help ticks sense chemical cues and heat. On each leg, they also found three hairs that closely resemble those that spiders use to measure electrical fields.
More experiments showed that mites with both front legs removed did not appear to be attracted to the modified electric field but that those with one leg were.
They also anesthetized mites and brought an electrode toward them until the electric field was sufficient to lift the animals across an air gap of between 0.5 to 3 millimeters. The mites could travel 150 body lengths per second, one of the highest speeds ever measured among land animals.
“They are super, super fast,” Dr. Garcia-Robledo said.
The study strongly suggests that mites are indeed glomming onto birds using these fields in nature, he said, briefly moving more swiftly than their flying hosts ever do.
Dr. Robert, who was not involved in the study, said the finding raised other intriguing questions. By sensing signals embedded within a hummingbird’s electrical field, mites might be able to learn something about the animal itself. Could this include species-level recognition, as different birds vary in size, shape and flapping frequency?
Electroreception is widespread in aquatic animals, but is less common on land. Previous studies have shown that bumblebees can sense the electrical fields of flowers and use them to assess whether the flowers have been recently visited by other pollinators. Hoverflies appear capable of doing this as well.
Spiders can also sense electric charge in the atmosphere, which helps them with a behavior called ballooning. Another arachnid, the castor bean tick, uses electrostatic charge to attach to hosts.
This paper is the first to show electric fields being used in phoresis, the technical term used when one creature temporarily hitchhikes on another (a habit that is distinct from the parasitism practiced by ticks).
“This elegant study is really exciting because it introduces yet another ecological context in which animals use electroreception,” said Sam England, a biologist at Museum für Naturkunde in Berlin, Germany.
Flower mites are parasites of their host blooms, depleting some of the same nectar that ever-hungry hummingbirds consume. But the birds don’t seem to mind and don’t seem to try to get rid of them.
“Most hummingbirds have these mites on them,” Dr. Garcia-Robledo said.
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