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Uploaded:2021-06-12
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Certain cicada species in North America emerge from the ground by the millions every 13 or 17 years. But why those specific intervals? Are cicadas secretly prime-number-loving mathematicians?!

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Sources:
https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/04-1175
https://academic.oup.com/aesa/article/104/3/443/22755
https://academic.oup.com/mbe/article/36/6/1187/5372347
https://www.pnas.org/content/110/17/6919
https://peerj.com/articles/5282/
https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/10-1583.1
https://www.jstor.org/stable/10.1086/668596?seq=1
https://www.jstor.org/stable/2463533?seq=1
https://www.smithsonianmag.com/science-nature/what-are-cicadas-180975009/
https://www.aimsciences.org/journals/displayArticlesnew.jsp?paperID=512

Images:
https://commons.wikimedia.org/wiki/File:Magicicada_septendecim_TPopp.jpg
https://commons.wikimedia.org/wiki/File:Cicada_Final_Molt_and_Darkening_timelapse_14_2021-05-27.webm
https://freesound.org/people/dethrok/sounds/272169/
https://www.storyblocks.com/video/stock/sb0129-a-cicada-climbs-a-tree-this-cicada-is-part-of-brood-viii-which-only-appears-in-western-pennsylvania-every-17-years-rjgvnyn6njw15ht2p
https://www.storyblocks.com/video/stock/cicada-nymph-emerging-from-ground-vdzlqfd
https://www.storyblocks.com/video/stock/sb0131-a-cicada-climbs-a-tree-this-cicada-is-part-of-brood-viii-which-only-appears-in-western-pennsylvania-every-17-years-sc1_ni4tvjw1adpud
https://www.storyblocks.com/video/stock/cicada-nymph-climbing-tree-ffugvu6
https://www.storyblocks.com/video/stock/greengrocer-cicada-insect-cicadinae-australasiae-oxc3sbd
https://www.storyblocks.com/video/stock/crow-looking-for-food-on-the-ground-rwex91emio6dwz27
https://www.storyblocks.com/video/stock/brand-new-baby-birds-resting-in-the-nest-blmadpqzfmjiyc3rzi
https://www.storyblocks.com/video/stock/birds-flying-in-blue-sky-tranquil-background-nature-scene-bpl7ub5slkanm1ztg
https://www.storyblocks.com/video/stock/slow-motion-footage-of-a-cute-little-squirrel-looking-for-food-on-a-rainy-day-hwmzas6xdkkv2275r
https://www.istockphoto.com/vector/high-quality-realistic-trendy-no-frame-smartphone-with-blank-white-screen-mockup-gm1224237679-359894798
https://www.istockphoto.com/photo/seventeen-year-cicada-gm178592206-24921996
https://www.istockphoto.com/vector/calendar-deadline-or-event-reminder-notification-gm1195104023-340548700
Thanks to Brilliant for supporting  this episode of SciShow.

Go to Brilliant.org/SciShow to learn  how you can level up your STEM skills. [♪ INTRO]. Once you know what you’re hearing, the  buzzing of cicadas is unmistakable.

And while these insects are famous for  their raucous arrival in the spring, their intensely loud chorus is far  from the strangest thing about them. These insects spend most of  their life cycle underground, burrowing after they hatch and only  popping out once they reach adulthood. Most cicada species emerge on  a cycle every 2 to 5 years, but periodical cicadas take this to the extreme.

In North America, 7 species  from the genus Magicicada emerge by the masses to mate, lay  eggs, and swiftly kick the bucket, after spending either 13 or 17  years underground as nymphs! These periodical cicadas are  split into different ‘broods,’ basically groups that will emerge  in the same place at the same time. And we’re talking millions of adult cicadas  popping up per 10,000 square meters, basically two American football fields.

Then, after all those years chilling  out underground in their immature phase, they have only a few weeks above the  ground as an adult before they die. And even after mulling it  over for about 350 years, we’re still not quite sure  how and why these insects evolved such an incredibly long childhood. Some suggest it might have evolved  through competition between nymphs underground, or interactions with  long-lived insect parasite enemies.

Others researchers think  these long childhoods emerged in response to the Pleistocene ice age, when the climate was less than ideal and  adults were likely few and far between. It could also be that by emerging  so briefly and infrequently, periodical cicadas keep predators from fully taking advantage of the population booms. The fact that their life cycles are  specifically 13 and 17 years long could also help with this.

That’s because they’re both prime numbers, numbers that can’t be  divided equally by any number other than one and themselves. Predators tend to have population  cycles where their numbers fluctuate every few years. But their cycles tend to be 1-10 years long, which won’t reliably sync up with those awkward prime numbers that  govern the cicadas’ lifetimes.

The cicadas’ life cycles could even  make things worse for their predators. Research has shown that the  populations of insect-eating birds are actually lower in emergence years. Which is kind of strange,  since there’s so much food.

As you might expect, most bird populations  increase the year after a big emergence. Well-fed birds do well the following year. But one study looked at population  trends of 15 species of birds across the full 13-year and 17-year cicada cycles.

Turns out, bird populations  dipped and rose over those years, only to crash just in time for the next emergence. While we’re still puzzling over  why exactly this is happening, it might go beyond the birds themselves. Cicadas don’t just feed their direct predators, they affect their environments  for years after their deaths.

Thanks to all the nutrients they leave  behind, plant and tree growth changes, and small mammals thrive in the aftermath, too. And researchers think all these  populations may be influencing each other. But they’re still trying to  work out exactly how it is that bird populations can crash right  when the cicadas are ready to emerge.

When it comes to cicadas, we still  have more questions than answers. As you can probably imagine, it’s tough researching a species that  requires you to wait 13 or 17 years at time. In the meantime, those of us  who live near periodical cicadas can enjoy the ear-splitting shrieks of  millions of bugs looking for a mate.

Cicadas are an example of  how mathematical thinking can yield insights into biology. I mean, who knew prime numbers  might throw off predators? And if you want to flex your math muscles,  you might enjoy a course from Brilliant.

Like Number Theory, if you really  want to master those prime numbers. Brilliant courses are also available  offline using their iOS and Android app. So if you’re traveling or have  a spotty internet connection, you’ll be able to keep learning.

And if you sign up at Brilliant.org/SciShow, you’ll get 20% off an annual Premium subscription. So thanks for your support! [♪ OUTRO].