Cicadian Rhythms: Why Does the 17-Year Cicada Emerge Like Clockwork?

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2021-05-25 06:00:09

The 2013 Swarmageddon is here! After years of their absence, cicadas are overrunning parks, forests and communities all across the central-eastern United States. Periodical cicadas (from the genus Magicicada) are known for their synchronized emergence at 13- and 17-year intervals. Simply the fact that they can live this long is extraordinary: periodical cicadas have the longest life span of all insect species! But their precise 13- and 17-year emergence cycles have long been an evolutionary enigma.There are seven species of periodical cicadas: four 13-year cicada species and three 17-year cicada species. Periodical cicadas emerge in groups called broods, which include multiple species that all emerge at a specific synchronized time within a specific range. This means that although all periodical cicadas generally emerge in the same year for any given region, cicadas in different regions are not synchronized with each other and there are cicada emergences somewhere nearly every year (for a table of brood emergences, go here ). This leaves us with some pretty intriguing questions: How do periodical cicadas know when to emerge (and where are they before that)? How did different species living in the same regions get synchronized to the same cycle? And what evolutionary pressures led to life cycles that are precisely 13- and 17-years long?Cicadas spend most of their lives underground, feeding on fluids produced by plant roots. During this time (which is the better part of either 13 or 17 years, depending on the species), they undergo multiple juvenile development stages. In the year of their emergence, they come to the surface as the soil warms, emerging generally between late April and early June, depending on temperatures. Once they emerge, they climb up the nearby vegetation where they molt into their adult form. Freshly molted, they are white and soft-bodied, but within a week their bodies darken and harden, allowing them to fly and make that distinctive cicada sound with their thorax. With just weeks left to live, the males sing a chorus of species-specific songs to attract females of their own species. After mating, the females lay their eggs in twigs, which hatch many weeks later. The hatchlings drop to the ground and burrow their way into the soil to start the next 13- or 17-year cycle. Because everyone that emerged in the same year reproduced at roughly the same time and their offspring develop at roughly the same rate, the offspring will all emerge synchronously in the year determined by their developmental rate. To see an amazing short film on the life of periodical cicadas (and support the making of a full-length cicada documentary), go here.Despite their loud rattle, beady eyes and large bodies, cicadas are practically defenseless. They don't bite or sting and they're not poisonous. Their primary defense against predators then, seems to be their sheer numbers during their synchronized emergence. If they come out in greater numbers than can be possibly consumed by the predators present at that time, then more of them will be likely to avoid predation and find a mate. This is exactly what they do, emerging at densities of over a million per acre.Lots of available prey generally stimulates predator reproduction, increasing predator populations. Cicada predators with life cycles that match the cicada life cycle would be at a great advantage. By having a long life cycle, cicadas can prevent predators from matching their reproductive timing. By having a life cycle that is a prime number (as are 13 and 17), they can also prevent predators from developing a life cycle that is a factor of that number. (For example, a 12-year cicada species could be wiped out by any predator species with a 2-, 3-, 4-, or 6-year life cycle because each cicada emergence would be met with a boom in the predator population). But these same factors also make periodical cicadas difficult for scientists to study. As a result, data on periodical cicadas is lacking compared to other species.A new study by Walter Koenig with Cornell University and Andrew Liebhold with the USDA Forest Service investigated the interaction between the emergences of periodical cicadas and many of their avian predators. They collected data from the North American Breeding Bird Survey on 15 insectivorous bird species whose populations had previously been shown to be affected by cicada emergences. For each bird species, they used population sizes and metabolism to estimate that species' predation pressure on the cicadas at a given location. They then looked at the total predation pressure of all of the species for a given location over the life cycle of the cicadas that live there. If the bird populations track the cicada emergences, then we would expect to see an increase in bird populations before and during cicada emergences.Surprisingly, for both 13- and 17-year cicadas, the predation pressures were lower during cicada emergence years than in any other year! Koenig and Liebhold proposed three possible explanations for this phenomenon: (1) The birds weren't less abundant, but were simply harder to find in the chaos of the cicada choruses and were thus harder to count; (2) The birds avoided areas with lots of loud cicada calls; or (3) Periodical cicadas have a long-term effect that reduces the number of predators during the next cicada emergence. To compare these three competing explanations, they compared bird populations in cicada-emergence years to the bird populations the year before the cicada emergence. They did this both in areas with high cicada densities and in nearby areas that did not have cicadas. If the birds were present but simply harder to count with cicadas around (hypothesis 1), then we would expect a decrease in the bird counts in cicada-emergence years in areas with cicadas but no effect in areas without cicadas. If the birds were avoiding the cicadas (hypothesis 2), then in cicada-emergence years we would expect a decrease in the bird counts in areas with cicadas but an increase in areas without cicadas. And if cicadas truly cause a decline in bird populations in cicada-emergence years (hypothesis 3), then we would expect a decline in bird numbers in both areas with and without cicadas.Bird populations consistently fell in the cicada-emergence years both in areas with lots of cicadas and in areas without them. This indicates that there is a true decline in bird populations: The cicada calls are not simply driving the birds away or making it harder for scientist to find them. Although this isn't nail-tight proof that periodical cicadas are somehow manipulating future bird populations to be low during their next generation's emergence over a decade later, it does indicate that this is a possibility... which would be pretty remarkable! There is much left to learn about how this process would work. Hopefully we'll have a better understanding of this system when they next emerge in 2030.--Further reading: Koenig, W.D. and Liebhold, A.M. Avian predation pressure as a potential driver of periodical cicada cycle length, The American Naturalist, 181, 145-149 (2013). DOI: http://www.jstor.org/stable/10.1086/668596.Koenig, W.D., Ries, L., Olsen, V.B. and Liebhold, A.M. Avian predators are less abundant during periodical cicada emergences, but why?, Ecology, 92(3), 784-790 (2011). DOI: http://www.ncbi.nlm.nih.gov/pubmed/21608486. Image Credits: 17-year cicadas emerging in 2004 are shown in 2004May21-Cicada_(6), by Greg Hume at Wikimedia Commons.The periodical cicada distribution map, called Distribution_magicicada_usa, is by Alfred at Wikimedia Commons.A cicada close-up, in a photo called In the early spring of 2013 the eastern half of the U.S. will see and hear the emergence of the 17-year Cicada (Magicicada) (Pic 2).jpg, is by the USDA at Wikimedia Commons.