There’s a buzz in the air these days, a loud one. I’m sure you’ve heard it but it could have easily been mistaken for a malfunctioning drone plane stuck in the trees. The sounds of summer are slowly becoming dominated by the mating songs of male Cicadas, as their relatively long lives culminate in a grand finale. And it sure is noisy.
You may have seen the recent headlines from the media outlets that announced the imminence of a “Cicadapocalypse”, but I can assure you that it will be significantly less dramatic than that. As usual, however, there is some truth behind the warning, only that there may be many more Cicadas around this year than in others, due to the strange periodicity of the Cicada lifecycle.
Cicadas are large-bodied insects, closely related to aphids and plant hoppers, who spend most of their lives as larvae eating roots underground before they pupate to adulthood and crawl up the nearest tree to mate and lay eggs before dying. In Eastern North America there are seven kinds, which are known as Periodical Cicadas, four species of which have 13-year lifecycles and the other three have 17-year cycles.
If you are surprised, either by the length or by the periodicity of these insects’ lifecycles, it would not be an unreasonable reaction, firstly because most insects do not live for much more than a year. Secondly, the odd association with prime numbers is otherwise uncommon among animals, and that is the point. The irregular timing of the mass emergences is the key to its success, as it makes it near impossible for predatory animal populations to track and enjoy the bounty of crunchy and nutritious Cicadas when they become available.
You see, usually predator-prey lifecycles mirror one another to some degree, in that when there is more prey in one year, there can be more predators in the next (more food for them). But as the number of prey decreases, so will the number of predators that can be supported, and so on, leading to a slightly out-of-phase oscillation of predator populations with prey over time. This interaction between the number of predators and the number of prey over time leads to the classic ebb and flow of their populations, as they closely track each other throughout the years.
However, the thing about prime numbers is that they are only divisible by themselves and therefore, prey populations that peak in 13 or 17-year cycles will not overlap with any other peaks in predator cycles, other than during the one rare occurrence of their own mass emergence. This has the effect of preventing a build-up of predator populations in the years prior to the mass emergence and as a consequence, the adult Cicadas find themselves in an environment that is relatively devoid of threats to their survival. As such, many more Cicadas will survive to reproduce than can possibly be consumed. The irregularity of their mass appearances successfully allows the majority of the Cicadas to overwhelm their predators and survive long enough to reproduce.
So if this is all about predator avoidance, what’s with all the noise then? As it happens, the brief duration of adult Cicada lives is occupied by one goal before kicking the bucket, either as predator food or from old age: to attract a mate and to reproduce. It is the male Cicadas who are making the noise and they are doing it to attract and seduce the females into choosing them as a mate based on the quality of his noisy buzz.
The mating call is produced by vibrating two membranes, which are located under the males’ wings and are known as tympanic membranes, resonating at specific frequencies to create the loud buzz. This concept is the same as the one that allows us to make music from a speaker, wherein the speaker’s cone (membrane) is forced to vibrate and the effect is a projection of a loud sound outwards. The Cicadas’ concert may not be music to our ears but it certainly has a charming effect on the females of their species.
Many animals use sound to seduce: crickets and grasshoppers produce a stridulation from scraping ridged combs on their legs or wings, songbirds sing complex melodies, frogs croak and even humans use speech, song and intonation to court and woo one another. In all cases, the underlying principle in the attractiveness relates to the ability of the individual making the sounds.
In nature, courtship behaviours among the animal kingdom typically involve the evaluation of tasks that are not easy to accomplish, therefore a strong performance will tend to imply that it is being made by a strong and healthy individual. These are the qualities that will make the difference between producing average offspring and creating kin that will be inherently more successful at surviving and reproducing in the next generation. Evolution favours those that do better than others and animal reproduction is largely about honing in on those signals that give us hints about the quality of a potential mate’s genes.
Perhaps you don’t think that making a loud and persistent sound really can represent a skill that makes one sexy to others but I dare you to try it. You’ll quickly find that screaming about your sexiness at full volume is really tiring…. I’m sure your neighbours will agree too! The point is easily made though: if you are a big male Cicada, who can make a big and loud noise at the right frequency, all while avoiding the myriad of predators out there, you must have the right stuff for survival and a female Cicada will find that to be irresistible.
So as we sit around the lake, sipping on a cold beverage and try to make the most of our last days of summer, let’s listen to the chorus coming from the trees and acknowledge that the symphony of sound is a remarkable marvel of nature, one marked by a unique time signature and an impressive reed section.
Dr. Adam Oliver Brown