Safety in numbers

1996 was an exciting year for science stories. In a laboratory in Germany, scientists created a brand new element and named it copernicium, after astronomer Nicolaus Copernicus. In Philadelphia, a computer named Deep Blue beat grand master Garry Kasparov at chess, marking the first time a computer had defeated a human opponent. And in Scotland, Dolly the sheep was born – the first time a mammal had been successfully cloned from an adult cell. But away from the headlines, another notable chain of events was beginning, with the hatching of a tiny insect egg… Or, to be more precise, billions of tiny insect eggs!

The insects in question are periodical cicadas, and they’ve spent the last seventeen years living underground, feeding on sap sucked from plant roots and maturing into adults. Now, in the forests and parks of eastern North America, these 17-year cicadas are starting to surface – in vast numbers!

It’s an incredible display of synchronisation, which helps safeguard the species’ survival in two clever ways. Although many of the freshly unearthed insects will end up being eaten by birds and other predators, the sheer quantity of cicadas emerging in unison creates a ‘safety in numbers’ effect, ensuring that at least some will survive long enough to mate. Furthermore, by only emerging in 17-year cycles, the periodical cicadas minimise the chances of inadvertently mating with other, non-periodical cicada species. This is because 17 is a prime number, which means it can only be divided by itself and one. Therefore, a cicada with a 17-year cycle is mathematically unlikely to have its mating season coincide with that of another species, helping to keep the species genetically distinct.

The next month or so will be extremely busy for the cicadas. They have just 4 to 6 weeks to find a mate, lay their eggs and ready the next generation. After that, this year’s cicadas will all die – with the next wave of adults not due to appear until 2030!