This spring, when the soil temperature reaches 64 degrees Fahrenheit, trillions of cicadas will burrow from underground across the Southern and Midwestern United States. In a rare so-called double emergence, two different cicada broods – one in a 13-year life cycle and the other in a 17-year one – will take to the trees to sing, eat and mate.
And even if we prefer not to think about it, considering their lodgings on the branches above, cicadas will also remove waste in the form of urine. Despite their size, cicadas have an impressively strong stream, scientists reported in a article published on Monday in the journal Proceedings of the National Academy of Sciences.
The researchers adapted a fluid dynamics framework based on features such as surface tension and the effects of gravity to map how animals of various sizes, from mosquitoes to elephants, can urinate. .
“It’s a nice physics-of-life perspective” to see all the data laid out in a single graph, said Saad Bhamla, a bioengineer at the Georgia Institute of Technology, who was a co-author of the study.
The jets of urine produced by cicadas, according to the research, have speeds of up to 3 meters per second — the fastest of all animals analyzed in the new work, including mammals such as elephants and horses.
Scientists have extensively studied how creatures eat and drink throughout the animal kingdom, but few have discovered the mysteries of liquid excretion. However there are many reasons to explore how different animals urinate, says Dr. Bhamla. Understanding how animals’ bodies have evolved to solve their waste problems can offer new ideas for nozzle design, for example.
There are also ecological implications to the research. Cicadas drink 300 times their body weight in xylem, a nutrient-poor plant sap, each day. All that liquid has to go somewhere. But the environmental impact of a large flush of cicada urine is completely unknown.
For Dr. Bhamla, the spirit of questioning is motivation enough. “We are a curiosity-driven lab,” he said. And what first piqued his curiosity about insect urine was a strange observation of a group of bugs called sharpshooters.
Dr. Bhamla and a doctoral student, Elio Challita, videotaped the sharpshooters releasing their urine one at a time, then using a special appendage to catapult each drop away from their bodies at high speed. that speed.
That finding lines up with a study from a decade ago, which showed that mammals larger than about 6.6 pounds peed in jets, while smaller ones couldn’t produce enough. pressure and therefore only leaks.
Sharpshooters are small, so they can’t create jets. But as xylem feeders they have a lot of liquid to dump, the researchers reasoned, so they evolved an energy-efficient method of dripping.
But while doing field research in the Peruvian Amazon, researchers spotted a cicada emitting a jet of urine that broke the rule of thumb.
Dr. Challita, who co-authored the new study and is now a postdoctoral researcher at Harvard, studied the bladder emptying habits of as many insects as she could find, both in real life and from YouTube videos. , and tried some calculations.
Because of surface tension forces, pushing fluid out of a tube becomes more difficult as the tube becomes smaller. Cicadas are about four to eight times larger than sharpshooters, so their plumbing is not subject to the same constraints. But they still have to use energy to overcome those forces.
Cicadas take the record for the strongest jet stream relative to their size, though butterflies and bumblebees can also produce jets. Mosquitoes, aphids and flies, however, must settle on the drip.
Adapted by Dr. Challita and Dr. Bhamla took two steps for mapping the urinary feats of 15 animals of different sizes. These measures trace the roles of surface tension, gravity and inertia in how fluids are expelled from a tube such as the urethra. For larger species, including humans, gravity and inertia are central to how quickly the body can push urine, and surface tension forces are easily resisted.
“But on a small scale, gravity is not that important,” explains Dr. Challita. “That’s where biology comes in.” Surface tension dominates, making jetting a more expensive process in terms of energy, even though the cicadas are large enough for inertia to provide assistance. Their bodies can cope with the cost of strong urination, the researchers speculate, and evolution considered it well-spent energy.
“Cicada urine remains in a unique region in fluid dynamics, where both inertia and capillary forces play significant roles simultaneously with gravity,” said Sunghwan Jung, a biological and environmental engineer. at Cornell, which was not involved in the work.
said Dr. Bhamla that there is a lot of room for future research in the area of drip or flush excretions. Understanding the fluid dynamics at play will allow researchers to examine more closely why an animal uses one solution over another.
“I think it’s very cool,” he said. “Elio and I were glad to know this.”