High Oxygen Levels Spawn Monster Dragonflies
Biologists have grown super-size dragonflies that are 15 percent larger than normal by raising the insects, from start to finish, in chambers emulating Earth’s oxygen conditions 300 million years ago.
The research, presented Nov. 1 at the Geological Society of America’s annual meeting in Denver, Colorado, provides more support to the idea that big ancient animals and high-oxygen concentrations weren’t coincidental. It may also offer an instrument to help gauge Earth’s ancient atmospheric conditions.
“No one has been successful growing dragonflies under controlled laboratory conditions before, at least to my knowledge,” said paleobiologist John VandenBrooks of Arizona State University, leader of the work. “This has allowed us to ask the question, ‘how have oxygen levels through time influenced the evolution of insects?’”
During the Paleozoic era, around 300 million years ago, huge dragonflies zipped around with wingspans stretching more than two and a half feet, dwarfing modern relatives. Back then, however, the planet’s atmosphere had roughly 50 percent more oxygen than today.
To explore the effects of ancient oxygen levels, VandenBrooks’ team raised 11 other “living fossils,” including beetles and cockroaches, in three habitats with different oxygen concentrations — one at the late Paleozoic’s 31 percent oxygen level, another at today’s 21 percent level and the third at 12 percent from 240 million years ago (Earth’s lowest oxygen level since complex life exploded onto the scene half a billion years ago).
They found that dragonflies and beetles grew faster, as well as bigger, in a high-oxygen environment, while cockroaches grew slower and remained the same size. All but two bug species grew smaller than normal at low concentrations of oxygen.
Measurements of insect breathing-tube volume from the experiment could be correlated with that of insects trapped in amber, VandenBrooks said, providing a solid tool to determine oxygen levels in poorly understood eras.
“We started out with insect physiology to understand the fossil record better, in light of data from modern species,” he said. “Then we realized we might have a biological tool to estimate ancient oxygen levels — a proxy — using that physiology in specimens trapped in amber.”
Dragonflies are born as water-loving nymphs and spend about half a year wolfing down small worms, crustaceans and, eventually, larger prey such as guppy fish. When adults emerge as speedy terrestrial fliers, they begin breathing through a network of tracheal air tubes and live only for a couple of weeks.
“It wasn’t quick, but it paid off,” VandenBrooks said of raising the critters in the lab, adding that 225 nymphs (75 per atmospheric habitat) had to be hand-fed worms and guppies every day for almost half a year.
After the dragonflies and other bugs molted into adults, the researchers measured their breathing-tube volumes. They discovered that high oxygen concentrations lowered tracheal volume, while low oxygen concentrations boosted it. VandenBrooks said tracheal volume may be tied to prehistoric dragonfly body size.
“As you become a larger insect, more of your body is taken up by tracheal tubes. Eventually you reach a limit to how big you can be,” VandenBrooks said. “The more oxygen that is available, the smaller that system needs to be and the bigger you can grow.”
Dragonflies in the modern habitat grew normally, with wingspans of about 3.5 inches, while the hyperoxic chamber spawned dragonflies with 15 percent larger bodies and 4-inch wingspans. Beetles also grew proportionally larger but, conversely, cockroaches didn’t swell to monsters in rich oxygen levels. Instead, they remained the same size and developed more slowly.
“We’re not sure why this happened,” VandenBrooks said, adding that cockroach tracheal volume, however, still decreased along with most of the other bugs.
“We might be able to correlate this modern tracheal data with tracheal volumes we measure in amber fossils to find out what oxygen concentrations were during some contentious periods in history,” VandenBrooks wrote in an e-mail. He also noted that oxygen levels around 300 million years ago are better known than from 120 to 65 million years ago, a period with “conflicting and poorly resolved” oxygen models.
“One model out there says levels were lower than now, another says higher-than-present levels,” he said. “We need a good proxy to estimate historic conditions. Amber fossils are promising if we can more tightly correlate breathing-tube volume to oxygen.”
VandenBrooks said he’d like to “take a more in-depth look at the fossil record and expand forward to the present and backward to the past” to see if amber is a viable proxy. In addition, he wants to repeat the oxygen-level experiment focusing more tightly on dragonfly behavior.
“We want to know how it affects their metabolism,” VandenBrooks said. “How does it affect their ability to perform? Their speed? Their efficiency? I’d love to know these things.”
Images: 1) Flickr/Al Power. 2) Micrograph of a modern honey bee’s internal tracheal tubes, the means by which all insects exchange oxygen with their body’s tissues./USDA 3) A dragonfly raised in one of VandenBrooks’ hyperoxic habitats./John VandenBrooks.
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