April 18, 2021

Weekly Emails

Cicada awakening

Brood X surfacing for its 17 year cycle

Read Time 4 minutes

In the coming weeks, concentrated regions in the northeast will experience a cicada takeover, culminating in a horde with staggering numbers in the order of billions.* If you were located in certain regions of the northeast like I was in 2004 (in the mid-Atlantic city of Baltimore, Maryland), the last time a group of cicadas called Brood X surfaced from beneath the soil, you may remember seeing and hearing the swarming red-eyed insects. At the time, my wife Jill and I were getting married and we had to account for the cicada awakening in our wedding plans. There was concern about how many of them would be in attendance. Brood X, also known as The Great Eastern Brood, is one of the largest populations of periodical cicadas, characterized by their 17-year life cycle. For those currently residing in parts of about 15 states on the east coast, expect to see the largest density in May and June (here is a great interactive map of cicada sightings from past cycles). Their temporary takeover is not an easily forgotten event. (For a short overview, here is a 5-minute BBC documentary on the cicada emergence with commentary by Sir David Attenborough, whose calming presence may counterbalance the frenetic event. Or if you prefer audio, here is a 27-minute Vox segment on the event. I like both.) 

*  This estimation is based on a study that reported cicada density ranging from 133,000 to 1.5 million cicadas per acre. It is reasonable to predict an upper threshold density of 960 million cicadas per square mile in some places and, given the varied density across topography and region, an overall emergence number in the late hundreds of millions.

When I saw the news that cicadas will be “back in town,” I remembered how much they intrigued me last time they surfaced (and I was also amazed how fast 17 years went by). Around the time I was planning to get married (and planning for the possibility of additional cicada headcount at the reception) I read this article in the Baltimore Sun. I remember the article 17 years later because of how fascinated I was by the evolutionary enigma that is the periodical cicada genus named Magicicada, comprising 7 species with either a 13- or 17-year prime-numbered life cycle. There are an infinite amount of prime numbers, which are whole numbers only divisible by 1 and themselves; 13 and 17 are two such examples. I was intrigued by the mathematical connection and the many questions that remain in the scientific field about the cicada emergence pattern. I have two questions, among many, that stand out to me about the life cycles: (1) What drove the two unique 13- or 17-year emergence patterns and (2) Why did the genus settle on the particular number of years that it did? 

Part of what makes the Magicicada genus so captivating is how it provokes more questions than answers. In short, there is not yet a consensus explanation for either question posed. However, there are two compelling theories for the evolved emergence pattern. One maintains that the preliminary driving force was climate-related and a second theory investigates causal predator-prey dynamics. 

The climate-related theory asserts that cooling temperatures during ice ages and subsequent decreased population size selected for population synchronicity and optimal emergence cycles. The optimal life cycles were those least likely to co-emerge with other cicada populations. As the theory goes, a 13-year cycle was the most favorable among the shorter range of ancestral emergence patterns and the 17-year cycle was optimally selected out of ranges with longer life cycles. Synchronous cicada populations that did not co-emerge with others had better survival as it limited the chance of intergroup mixing, which would produce a subset of cicadas (with a hybrid emergence pattern) that deviate and emerge before or after the larger group, a term referred to as stragglers. By remaining a population separate from others, the cicadas belonging to that population were timed together: there is safety in numbers, which ecologists call predator satiation

A second theory posits that the 13- and 17-year periodicity reflects an optimal strategy to negotiate predation cycles above ground and ecological pressures for the brood (e.g., food sources and parasites) down below. Notably, the periodical cicada prime numbered life cycle is by definition only divisible by the number 1 and itself, limiting the probability of predator life cycle overlap. Predators don’t have yearly cycles or patterns as long as 13 or 17 years and have not otherwise co-evolved to match the prime-numbered life cycles of the species. 

Consider the example adapted from a seminal 1977 essay that considers the timing of cicada brood cycles: if a brood’s emergence pattern was every 15 years, divisible by 3 and 5, and its predator life cycle was every 5 years, then each emergence would coincide with a predator cycle. With a prime number life cycle like 17 years and a 5 year predator life cycle, the brood would only encounter its predator every 85 years (by multiplying 5 by 17). 

However, explanations for the mystifying prime-numbered emergence patterns that emphasize predator-prey relationships do not sufficiently explain why 13 and 17 are the specific prime numbers selected. A spatial-temporal simulation study of the predator-prey relationship confirms that a prime-numbered life cycle is an optimal strategy for survival and population maintenance. (With predation pressure, life cycles peak between 11 and 19 years, where 19 occurred most frequently). The study confirms there is a prime-numbered sweet spot for periodical cicada emergence patterns but does not serve to explain why the life cycles do not naturally occur as 11 or 19 years. The theory does, however, support that brood periodicity minimizes predator overlap and optimizes predator satiation. 

A good evolutionary puzzle leaves something to solve. Regardless of the rationale, the periodical cicada life cycle is a marvel that will come to light soon for about 8 weeks’ time, to close out a cycle and begin anew. If you’re lucky enough to live in an area where Brood X emerges, resist the potential for irritation due to their noise or their numbers. Instead, treasure the incredible intersection of biology and mathematics they illustrate. 

 

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  1. I was doing a postdoc at UPenn in Philadelphia when the 17-year cicadas were supposed to emerge in ’96, but they didn’t. It seems they had been paved over in the previous 17 years 😉

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