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Generally speaking, we humans and other animals weighing more than 3 kg will eject a column of urine when urinating, while some lighter, smaller animals (such as many insects and small mammals) do not emit a column of urine, and their urine is drop-by-drop.
But cicadas are an exception! These insects, which weigh only 2 grams, squirt out elongated columns of urine, just like large mammals.
蝉撒尿，图片来源：Wilson Walker
Not only that, but the latest research also found that the cicada's water flow when urinating is powerful – it can squirt urine at speeds of up to 3 meters per second, the fastest of all the animals evaluated in this new study, including mammals such as elephants and horses!
The behavior of cicadas to urinate is difficult to study because they are usually carried high in trees, and even if people find them, they can easily alarm them, causing them to fly away. Image source: Jelbuilder
While conducting a field trip in Peru, a team of researchers at the Georgia Institute of Technology in the United States came across a tree known locally as the "Crying Tree." Someone thought there was liquid on their heads, and then everyone looked up and saw that the tree was indeed "crying", but the liquid that was spilled was not sap, but cicada urine - many cicadas on the tree had been peeing.
If the researcher finds a treasure, he quickly moves a table and stands on it to shoot a high-speed video, collect research data and go back to slow down the analysis. The results of the experiment found that the urine ejected by the cicadas was in the shape of a water column. The researchers thought this was strange because cicadas suck up a lot of xylem sap on plants, and insects with such feeding patterns generally produce a small urine bead "biu~".
大多数以木质部汁液为食的昆虫尿的是小水珠。 图片来源：Georgia Institute of Technology
Prior to this, the research team found that leafhoppers urinate a small droplet of water, which is then violently ejected. This is because, for the tiny leafhopper, its orifice is too small, limited by surface tension, it cannot remove more and heavier things at a time, and the way of urinating by projecting water droplets can save the body's energy.
投射小尿珠,图片来源:Saad Bhamla, Tedmontery
In contrast, the cicada is a larger insect, about 4~8 times the size of the leafhopper, and the intestine in the body is also wider. In addition, cicadas drink xylem sap 300 times their body weight every day, and xylem sap is very poor in nutrients, 95% of which is water, which means that cicadas have to process a lot of sap to extract enough energy and nutrients, so there is a lot of liquid waste to be excreted. These jets can also be used for self-defense. When threatened, cicadas spray liquid waste at invaders.
排尿的方向也很灵活。 图片来源：Georgia Institute of Technology
"It takes a long time to pass one drop of urine at a time, and it's not very efficient," said Elio J. Challita, one of the paper's authors and a Ph.D. student in chemical and biomolecular engineering at the Georgia Institute of Technology. ”
"蝉雨",图片来源:Jelbuilder
For larger species, including humans, gravity and inertia are essential for the speed at which the body excretes urine and can easily counteract surface tension. However, the study found that the cicada is the king in this regard, setting a record for the strongest jet stream relative to its size. Butterflies and bees can also squirt urine columns, but the liquid sprays not as fast as cicadas.
蝴蝶排尿,图片来源:Thomas Marent, David Weiller
Compared with mammals, cicadas urinate 2~3 times faster than elephants and humans. Lab mice, as they are known, weigh about 100 times as much as cicadas, but they can only tick when they urinate.
Urine of rats, Image source: Reference [10]
Saad Bhamla, an assistant professor in the School of Chemical and Biochemical Engineering at Georgia Tech who was involved in both insect excretion studies, said cicadas have a special muscle that ejects liquid waste out of their orifices.
In 2014, a team of researchers at the Georgia Institute of Technology also found that mammals weighing more than 3 kilograms emptied their bladders in about 21 seconds. Yes, elephants urinate for about the same time as kittens. This research later won the Nobel Prize in Physics.
The 21-second pee rule, Image Credit: References[10]
In the past, scientists were more concerned with the eating behavior of animals, and the understanding of excretion is not very thorough. While the team's initial study of insect excretion was motivated by curiosity, the findings may be applicable to the fields of soft robotics and manufacturing, leading to the development of more advanced nozzles, a device used to control the flow of fluids.
"Insects are the perfect laboratory to explore fluid processing at the microscopic scale," said Anne Staples, a fluid dynamicist at Virginia Tech.
There are more than 3,000 species of cicadas, which can be divided into two categories: annual cicadas and cyclic cicadas. The annual cicada matures every summer and is found all over the world. There are only 7 species in the genus Cicada, which has a longer life cycle, of which 3 species have a life cycle of 17 years and 4 species have a life cycle of 13 years. The Georgia Tech research team is not studying cyclic cicadas, but their Amazon-based relatives.
Image source: References[1]
This spring and summer, trillions of cyclical cicadas will emerge from the soil of the southeastern and Midwestern United States. Some have been dormant underground for 13 years, while others have been for 17 years. When the time comes, for a few short weeks of life on the ground, they will sing loudly and pee to their heart's content.
Resources:
[1] Challita, E. J., & Bhamla, M. S. (2024). Unifying fluidic excretion across life from cicadas to elephants. Proc. Natl. Acad. Sci. U.S.A., 121(13), e2317878121. doi: 10.1073/pnas.2317878121
[2] Bhamla, S. (2023, February 28). The fascinating physics of insect pee. Retrieved from https://www.ted.com/talks/saad_bhamla_the_fascinating_physics_of_insect_pee/transcript
[3] Katsnelson, A. (2024). Cicadas’ Emergence May Feel a Little Wet From the Power of Their Pee. N.Y. Times. Retrieved from https://www.nytimes.com/2024/03/11/science/cicadas-pee-stream.html
[4] Cicadas’ Unique Urination Unlocks New Understanding of Fluid Dynamics | Research. (2024, April 02). Retrieved from https://research.gatech.edu/feature/cicada-urination
[5] Daniel, A. (2024). Scientists studied how cicadas pee. Their insights could shed light on fluid dynamics. NPR. Retrieved from https://www.npr.org/2024/03/20/1239529315/cicadas-pee-urination-jets-new-research-microfluidics-3d-printing-experiments
[6] Of Engineering, G. T. C. (2024, March 11). The Unique Urination of Cicadas. Youtube. Retrieved from https://www.youtube.com/watch?v=6KBL7HHd1-o
[7] Borenstein, S. (2024). Cicadas are nature's weirdos. They pee stronger than us and an STD can turn them into zombies. Phys.org. Retrieved from https://phys.org/news/2024-04-cicadas-nature-weirdos-pee-stronger.html
[8] Jelbuilder. (2023, March 24). Cicada Pee: The bizarre phenomenon I experienced in the bush. Youtube. Retrieved from https://www.youtube.com/watch?v=u1BodTnp9os
[9] Press, S. B. |. A. (2024). Cicadas are nature's weirdos. They pee stronger than us and an STD can turn them into zombies. NBC10 Philadelphia. Retrieved from https://www.nbcphiladelphia.com/news/national-international/cicadas-facts-zombies-sexually-transmitted-disease/3818240
[10] Yang, P. J., Pham, J., Choo, J., & Hu, D. L. (2014). Duration of urination does not change with body size. Proc. Natl. Acad. Sci. U.S.A., 111(33), 11932–11937. doi: 10.1073/pnas.1402289111
[11] Super-fast Insect Urination Powered by the Physics of Superpropulsion | Research. (2024, April 03). Retrieved from https://research.gatech.edu/super-fast-insect-urination-powered-physics-superpropulsion
[12] Tamisiea, J. (2024). Why Cicadas Power Spray Their Pee. Sci. Am. Retrieved from https://www.scientificamerican.com/article/why-cicadas-power-spray-their-pee