Primed for Survival

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This is the moment a periodical cicada emerges from the ground for the first time in 13 years.

In the outside world, predators await the near defenseless insect.

But it is not alone.

Millions of periodical cicadas emerge all at once, creating safety in numbers.

This mass emergence fascinates not just biologists, but mathematicians too.

Because, by occurring only once every 13 years, it suggests mother nature is making purposeful use of prime numbers.

Prime Mating

A prime number is any whole number which is divisible only by itself and one.

This means that insects with a cycle of prime-numbered years have the least chance of coinciding with insects born in other cycles.

This is important as it restricts cross-breeding and preserves the unique characteristics of individual species.

Whereas a 13-year cycle only coincides with cycles of 1 or 13 years, a 12-year cycle would coincide with other species born in 1, 2, 3, 4, 6 or 12-year cycles.

Periodical cicadas use this unique property to their advantage.

It allows them to avoid mating with other species of non-periodical cicada, which hatch more frequently.

So the periodical cicada is likely to only mate with others in its own cycle.

Safety in Numbers

By avoiding breeding with other species, the periodical cicada protects its 13-year lifecycle, ensuring millions emerge all at once, producing the survival technique of safety in numbers.

Periodical cicadas have been known to emerge not only in 13-year cycles, but in 17-year cycles, too - another prime number.

Groups emerging in these prime numbered cycles would be destined to meet only once every 221 years.