Tuesday, May 30, 2017

Brooding on Stragglers

I’ve always defined straggler as “someone who falls behind, who fails to keep up.”  So I was intrigued to find that in the literature on periodical cicadas those members of a brood who arrive before or after their expected year of arrival are called stragglers.  Clearly, my definition is much too narrow, missing the essence of what it means to straggle.  The Oxford English Dictionary defines straggler variously, but its first meaning is:  “One who wanders or roves without fixed direction; one who strays from his companions or from the regular route; a gadabout; a camp-follower, a tramp, a vagabond.”  The heart of this as it applies to periodical cicadas is “one who strays from his companions.”  It doesn’t matter whether the straggler arrives early or late, just that the straggler is separated from his or her companions.

In the past several weeks, neighborhoods near me in the Maryland suburbs of Washington, D.C. have seen the emergence of a massive (well, it seems like that) number of periodical cicadas, those of the genus Magicicada.  Lord, what a beautiful name for this genus.  Entomologist William T. Davis named this genus in 1925 but didn’t pause to explain his thinking.  (Cicada Tibicen, A South American Species, With Records and Descriptions of North American Cicadas, Journal of the New York Entomological Society, March, 1925.)  (Periodical cicadas are different from their brethren who emerge annually after relatively brief underground sojourns.)

Streets near me became killing fields with the smashed bodies of cicadas strewn everywhere, testament to the implacable force of the automobile.  Winds deposited in the gutters many, many shells from which the insects had crawled after their extended stay underground sipping on tree sap.  Vertical surfaces, such as trees or telephone poles, were the scenes of amazing transformations as the ghostly cicadas emerged from their shells and, shortly, took on color and substance, before launching into the air in brute-force flight, nothing graceful about it.

Delve into the life history and paleontological history of cicadas and you fall down Alice’s rabbit-hole.  No other insect has a longer total life cycle than the periodical cicadas who have two cycles, one of 13 years and the other of 17 years.  Complicating this story is the fact that there are three “species groups” of periodical cicadas: Decula, Decim, and Cassini.  Each of these groups independently evolved into forms that live under ground for either 13 or 17 years (well, for the most part that’s true, an issue I’ll return to momentarily).  These 13- and 17-year forms have organized themselves into “broods” with different years of emergence and different geographic ranges.  Underlying this developmental (and mathematical) complexity is one astounding attribute, what entomologist Stewart H. Berlocher has described as “the real kicker”:  nearly every brood includes members of the three “species groups.”  (Regularities and Irregularities in Periodical Cicada Evolution, PNAS, April 23, 2013.)

The idea of a “species” seems under some duress here.  Although the Decula, Decim, and Cassini groups interbreed probably rarely enough to be considered separate species, the species designation for the Magicicada, so current thinking goes, drops down one stage to the 13- and the 17-year cohorts of each species group.  This results in the following species of periodical cicadas:  Magicicada tredecim, M. neotredecim, M. tredecassini, M. tredecula, M. septendecim, M. cassini, M. septendecula.  The first four are 13-year species (M. neotredecim came into being recently, evolving from a cohort of M. septendecim, a 17-year species); the last three are 17-year species.  Berlocher notes, “All broods of each of these species have occasionally been proposed as species.”

In Maryland, our periodical cicadas are only members of 17-year broods:  Brood II (last appeared 2013, next expected in 2030), Brood V (in western Maryland – last appeared in 2016, next expected in 2033), Brood X (last appeared in 2004, next expected in 2021), and Brood XIV (lasted appeared in 2008, next expected in 2025).  (17 & 13 Year Cicadas, Cicada Mania website.)  In Maryland, each time one of these broods crawls out of the ground and sets the trees singing, each of the 17-year species, M. septendecim, M. cassini, and M. septendecula, is represented.

Stragglers.  Herein lies another great source of confusion in this already complex life cycle.  The bulk of a brood emerge as expected at the end of their defined 13- and 17-year cycles, but varying numbers of so-called stragglers can emerge before or after their expected arrival year.  Generally, stragglers emerge four years early, one year early, or one year late.  In Maryland, stragglers from Brood X were awaited this year (four years before their 2021 due date) and, apparently, they’ve arrived.  Or, have they?  The verdict is not in.

A bit to my south, in North and South Carolina, and Georgia, Brood VI, a 17-year brood, has emerged on schedule.  Are the periodical cicadas here in Maryland a northern extension of Brood VI?  Entomologist Jonathan Neal raises this question on his blog Living with Insects in the post titled The 2017 Cicada Mystery (May 21, 2017), writing,
Brood VI emerged in NC, TN and GA earlier this year. Brood VI in the past has had heavy emergence in IL, MI and WI. The question cicada sleuths are asking: “Is the current emergence in MD Brood part of Brood X that is emerging 4 years early? Or could the emergence be part of Brood VI. If it is Brood VI, are there factors that cause it to increase its range or cause populations to fluctuate over time? If it is Brood X, what is causing large numbers to emerge early[?”] Many studies and tests will be needed to arrive at a conclusion.
Are they simply stragglers of Brood X arriving four years early?

Or are they the products of the Brood X group that emerged in 2000 (four years before their 2004 due date)?  Biologist Gene Kritsky, on the Mount St. Joseph University’s MSJ Cicada Website, observes that, at least in southwest Ohio, these Brood X stragglers came out in 2000 in numbers that were sufficient to overwhelm predators and allow some of them to mate and reproduce.  It’s those stragglers that he expects to be emerging now.  Are they becoming a separate brood, no longer stragglers from another brood?

So we’re left with a big unknown.  Absent data on the numbers of periodical cicadas that emerged in 2000 in Maryland, it’s hard to know which of these possibilities – stragglers from Brood X, range extenders from Brood VI, or products of the stragglers that emerged in 2000 – is the case here.

Other aspects of these life cycles have attracted lots of scientific interest.  The hypotheses about why these periodical cicadas have so synchronized their life cycles that all three species groups nearly always emerge in enormous numbers at the same time tend to center on safety in numbers.  Predators are overwhelmed, so many individual cicadas survive to reproduce.  That evolution has moved to 13 and 17, prime numbers, is often explained by reference to the challenge those periods pose to predators trying to harmonize their own shorter life cycles to those of their prey.  It has also been suggested that these periods ensure that cicadas breeding on different cycles will very seldom emerge at the same time, avoiding the prospect of crossbreeding that would doom the offspring.  (See Susan Milius, Mystery in Synchrony:  Cicadas’ Odd Life Cycle Poses Evolutionary Conundrums, Science News, July 13, 2013.)

Further, straggling is a longstanding phenomenon.  Some periodical cicadas are always early or late to the party.  Opinion about the current straggling raises the prospect that climate change has had an impact with warmer conditions prompting faster growth and earlier emergence.  As writer Scott Dance describes it, “[S]cientists say there are always some subsets of the 17-year cicada broods that don’t wait the full cycle before emerging.  These experts think cicadas ‘count’ in fours, and if they are big enough after 13 years, some crawl out sooner.”  (This Isn’t a Cicada Year, So Why Are They Now Showing Up Across the Mid-Atlantic?, Washington Post, May 20, 2017.)  But research by entomologist Richard Karban shows that the members of any specific brood do not develop in lockstep, some complete certain stages before their compatriots, but, in general, the early finishers, as science writer Susan Milius puts it, “end up waiting for the signal to emerge, giving the laggards time to catch up.”  (Mystery in Synchrony citing research by Richard Karban.)

Apparently, these 13- and 17-year cycles are relatively recent phenomenon in geological and paleontological terms.  Based on genetic studies, biologist Teiji Sota and his colleagues, conclude that the three species groups (Decula, Decim, and Cassini) separated some 3.9 million years ago, during the Pliocene Epoch.  They posit that this speciation process occurred when populations of their common ancestor became geographically isolated and evolved into separate species groups (allopatric speciation) and then, later, came back together to evolve their synchronized life cycles.  (Independent Divergence of 13- and 17-Y Life Cycles Among Three Periodical Cicada Lineages, PNAS, Volume 110, No. 17, April 23, 2013.)

Does the work by Sota et al. mean that no cicadas before, say, 3.9 million years ago had such long life cycles?  I don’t know.  They suggest that because these species have shown repeated shifting between 13- and 17-year cycles during their existence, there is probably a genetic basis for these life cycles that predates the separation into the three species groups.  Was it manifest before that point?  Again, I don’t know.  Cicadas, themselves, (not necessarily periodical ones) do go back, far earlier than the Pliocene.  The wonderful fossil cicada (unidentified as to genus and species) shown below is from the Eocene Florissant site in Colorado.  It's some 34 million years old.

(This picture is reproduced from the National Park Service’s Florissant website.)

Apparently the earliest cicada fossil that has garnered general agreement that it is, in fact, that of a cicada dates from the Early Cretaceous (more than roughly 100 million years ago).  (George Poinar, Jr., and Gene Kritsky, Morphological conservatism in the foreleg structure of cicada hatchlings, Novicicada burmanica n. gen., n. sp. in Burmese amber, N. youngi n. gen., n. sp. in Dominican amber and the extant Magicicada septendecim (Fisher) (Hemiptera: Cicadidae), 2012, accepted manuscript subsequently published in Historical Biology, Volume 24, Issue 5, 2012.)  But Poinar and Kritsky believe that the cicadid group goes back much farther, perhaps to the Permian Period.

I’ll close with some wonderful speculation (I assume that's what it is, speculation) by entomologist Scott Richard Shaw in his book Planet of the Bugs (reviewed previously in this blog).  He posits that insects played an essential role in the development of dinosaur diversity by being the source of protein for smaller, herbivorous dinosaurs which, in turn, were a source of protein, along with other insect-eating animals, for carnivorous dinosaurs.  Efforts in the insect world to deal with dinosaurs' incessant munching on its denizens prompted, Shaw observes, various kinds of avoidance strategies among insects.  Some turned to “behavioral escape mechanisms” in which case, writes Shaw, 
[I]t’s certainly possible that mayflies’ and cicadas’ mass synchronized emergences adapted and were fine-tuned in response to intense dinosaur predation.  (p. 121)
A neat thought.
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