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A University of Wyoming assistant professor has generated quite a bit of buzz with his discovery that some alpine bumblebees can fly to heights greater than Mount Everest.
That’s more than 29,000 feet above sea level, by the way. Not bad, for an insect considered to be a poor flier.
“It turns out they can fly very high,” says Michael Dillon, a faculty member in UW’s Department of Zoology and Physiology. “We did not expect that. It’s a pretty remarkable feat to be up there.”
Dillon’s paper, titled “Surpassing Mount Everest: Extreme Flight Performance of Alpine Bumblebees,” was published in this month’s issue of Biology Letters, a Royal Society journal that publishes short, highly innovative, cutting-edge research articles and opinion pieces accessible to scientists from across the biological sciences. Dillon wrote the paper with Robert Dudley, an ecologist at the University of California-Berkeley and the Smithsonian Tropical Research Institute in Panama.
“Though commonly considered poor fliers, bumblebees regularly forage in high alpine regions where thin air should make flight difficult,” Dillon says. “We challenged wild bumblebees, caught at 3,250 meters, to fly at higher simulated altitudes in a field flight chamber.”
Higher and higher
All of the bees tested in the Plexiglas chamber were males captured at flowers near Rilong, in Sichuan, China. During the experiments, the bees were challenged to fly at continually rising altitudes. To simulate this effect, Dillon dropped the pressure in the chamber to mimic 500-meter climb intervals in altitude to see how the bees would react.
The bees’ flight in the chamber was often erratic at first, but that struggle would eventually cease. The bees would then make repeated, controlled vertical ascents. All flights were recorded by a digital video camera mounted directly above the chamber.
It was found that all of the bees tested could fly above 7,500 meters, an elevation higher than six of the seven highest mountain summits in the world. Three flew above 8,000 meters. Two bees flew above 9,000 meters (29,527 feet), revealing that some bumblebees can fly over the top of Everest, the world’s highest mountain peak.
The two bees that flew the highest also had the largest thoraces (approximately 36 percent of body mass) compared with the other bees, whose thoraces were 31 percent of their body mass.
Conventional wisdom would have one think that the heavier bumblebees would have a more difficult time flying at higher heights. But Dillon says the thorax contains 95 percent of a bee’s “flight muscle.” Thus, the bees with larger, more powerful muscles can fly better.
But the overall key to flying at higher altitudes was the bees’ ability to stay vertical and beat their wings at a wider arc.
“To fly at these extreme altitudes, bumblebees increased the maximum angle through which they beat their wings while keeping the wing-beat frequency constant,” Dillon says.
It may be years away, but the research could have future practical applications in micro-vehicle and micro-air technology.
“The technology is based on mimicry of insects,” Dillon explains. “Right now, micro-air vehicles are limited, but Amazon is talking about using Quadcopters to make (home) package deliveries.”
The findings were surprising because animal flight is constrained at high altitudes due to significant reductions in air density and oxygen, Dillon says. These constraints, combined with reduced air temperature, explain partial or total loss of flight at high elevations in many insect groups.
However, Dillon’s research shows alpine bumblebees possess substantial aerodynamic reserves, and can sustain hovering flight under hypobaric conditions at elevations in excess of 9,000 meters.
The bees’ capacity to modify their force output during flight -- at varied elevations and different terrains -- is essential to foraging, as bees can double in size when they fill up with nectar. But Dillon surmised the extra power also may be relevant for short-term bursts for mating or to escape predators.
While bees would have no reason to fly to heights matching Mount Everest unless there were flowers available, Dillon says, he added it would be more difficult for a bee to ascend to that height than a human climbing the world’s highest peak.
“For a person to walk up Everest, it’s athletically challenging,” he says. “But, it’s nowhere near the effort it takes for a bee to fly there because of the oxygen demand. A bumblebee needs to supply its tissue oxygen 10-15 times that of an elite athlete exercising.”
That ability may be due to bees having a tracheal system that is completely independent of circulation, Dillon says.
Bees also have a more difficult time staying warm, Dillon says. Humans have higher body temperatures than bees, and it’s difficult enough for adult humans to stay warm in cold temperatures. It’s even harder for human babies to keep warm because their body mass is much less than adults, so they lose heat more easily. Bumblebees, which are significantly smaller than babies, would be even more challenged, Dillon says.
However, bumblebees do have an ally. Because of the tremendous energy required to flap their wings, bumblebees can generate a significant amount of heat during flight.
While much has been learned, Dillon says it is not yet known if bees that live at different altitudes have different flight capacities.
“We’re at this discovery phase,” Dillon says. “We don’t know enough about how they deliver oxygen to their tissue.”
His research has drawn the attention of Smithsonian magazine, Nature News, National Geographic News and Live Science, among others.
An alpine bumblebee flies at a simulated height of 6,000 meters in a field flight chamber. The height is simulated by dropping the air pressure in the chamber.