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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!
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