How Unusually Cool Ice Age Summers Probably Shaped Periodical Cicada (Magicicada) Evolution

It sounds like everybody’s burglar alarm is blaring in Evans, Georgia.  But the noise doesn’t originate from annoying, malfunctioning security systems.  Instead, the 13 year periodical cicadas have emerged.  For over a decade these insects have lived a foot underground, well below the frost line, but now they’re ready to mate–a frantic affair that takes place within a timespan of 3-4 weeks.  Almost their entire lives, they’ve lived as nymphs, surviving on the xylem fluids of deciduous trees.  The urge to mate causes them to dig tunnels to the surface which they crawl through.  Sometimes they continue to crawl, making it halfway up a wall or a tree trunk before the winged adult bursts through the back of the shell of its thorax.  The holes from which they’ve emerged are visible, their molted shells scattered under foot like discarded shrimp exoskeletons at a Cajun seafood boil.

Photo from google images of a periodical cicada.

The unusually long period between emergences among the 7 species of periodical cicadas in the genus Magicicada puzzles scientists who hypothesize about its evolution.  Magicicada emergences in large numbers are obviously a defense mechanism known as predator satiation.  Like passenger pigeons, they occur in such a high population that they overwhelm the ability of predators to consume them.  Unlike other types of cicadas which are strong, fast fliers, periodical cicadas are clumsy and slow, but so many appear at once that predators are unable to consume most of them before they have a chance to mate and lay eggs.  Some scientists think their emergences every 13 or 17 years (depending on the species) is a way to keep predators from increasing their own populations after exploiting them as a food source, thus avoiding a cycle when predator numbers eventually escalate enough to decimate cicada numbers.  This seems an unlikely explanation to me for 2 reasons: cicadas are only available for a few weeks a year which is not enough time to have a significant long term impact on predator populations.  More bird nestlings may survive at first due to the abundance of cicadas, but then for the rest of the year, they must adapt to the normal supply of food.  Moreover, the average wild bird only lives 2 years–far shorter than 13 or 17 years.  Cicadas could avoid upswings in predator populations with much shorter periods between emergences.  Other scientists believe the odd high prime numbers emergences can be explained by a combination of predator cycle avoidance and interspecific competition among nymphs.  Although several entymologists have derived statistical models supporting this theory, I think the paleoclimate explanation proposed by R.T. Cox, C.E. Carlton, and independently by Yoshimura is more plausible.

Periodical cicadas depend specifically upon deciduous forests.  During the coldest stages of Ice Ages, deciduous forests north of the southern Appalachians were rare relics outnumbered by other environments such as spruce forests and prairies.  The bulk of deciduous forests then occurred south of the Appalachian mountains.  Even here, summer temperatures occasionally were too cold for cicadas in the Magicicada family.  They require temperatures above 68 degrees F for a period of 3-4 weeks for flight and mating.  Drs. Cox and Carlton assumed that during the coldest stadials (which lasted on average 1500 years) 1 in 50 summers failed to reach this temperature, and cicada reproduction failed.  Using a statistical formula, they estimated that over a 1500 year stadial, cicadas emerging every 6 years had a 4% chance of avoiding unusually cool summers; cicadas emerging every 11 years had a 51% chance of avoiding unusually cool summers; but cicadas emerging every 17 years had a 96% chance of avoiding unusually cool summers.  Cicadas emerging after shorter periods were eventually eliminated from the gene pool, while those with genes for longer cycles became dominant.

Map of Magicicada ranges from the below referenced paper.  The distribution of 13 and 17 year periodical cicadas supports the paleoclimatic explanation for their high prime number emergences.

17 year cicada species tend to live north of 13 year cicada species, even though the shorter cycle is a dominant gene.  Summers too cool for breeding would’ve occurred more frequently in the norther parts of their range, so those with 17 year cycles would’ve had a greater chance of avoiding them than those with the 13 year cycle.

Reference:

Cox, R.T. and C.E. Carlton

“Paleoclimatic influence in the evolution of periodical cicadas (Insects: Homiptra:Cicidae: Magicicada spp.)”

The American Midland Naturalist 120: 183-193 1988

Notes on my observations of periodical cicadas

–Periodical cicadas are slow.  I was able to catch one by simply picking it off the ground after if fell in flight and landed on its back.

–Birds are feasting on them.  Crows are catching the nymphs as soon as they crawl to the top of their tunnels.  I also saw a Canadian goose nab one that fell in a small lake.

–Wow! They are loud.

–They must be mole food during their nymph stage.

–Evans has become heavily developed.  Cicada habitat has been greatly reduced and covered with blacktop parking lots from which they can never emerge and escape.  I wonder if this is another creature that survived for millions of years, until the actions of Homo sapiens eventually renders them extinct.

Tags: 13 year cicadas, 17 year cicadas, American Midland Naturalist, C.E. Carlton, cicada range map, Evans, evolution, Georgia, Magicicada, periodical cicadas, R.T. Cox, stadial, statistics, Yoshimura