24 July 2010

The great golden digger wasp, the Concorde fallacy, and free will

Sphex ichneumoneus, the great golden digger wasp, about to enter her burrow.
ResearchBlogging.orgA few weeks ago, I noticed some alarmingly large insects that resembled wasps outside the front entrance of the biology building at Kenyon College. They would fly a few centimeters above the sandy gravel, no doubt surveying the best landing spot. Only a handful of these solitary wasps were here hovering over at least a dozen wasp-diameter holes in the loamy soils under this protected overhang. Suddenly, one landed and disappeared into her burrow. Ah! Digger wasps! I had read about them but never before observed them in the wild. Well, as wild as a well-manicured college campus is in the relative calm of summer. The wasps were a mixture of brilliant orange contrasted with a deep black color. Almost blue iridescent wings fluttered periodically as they danced around the burrows. After consulting available guides and experts in the natural sciences division at Kenyon, it was confirmed these are indeed the great golden digger wasps (Sphex ichneumoneus). These beautiful solitary wasps emerge in the summer and spend about 6 weeks building multiple burrows that they then provision with paralyzed katydids. When the female - the males do not assist in burrow building or provisioning - is satisfied with her stash, she lays a single egg and closes the burrow, commonly completing this process 10 times before her short adult life is over. This species is common to much of the United States, so it's not a surprise to find them here in Ohio.

The sheltered gravel/sand area outside Higley Hall at Kenyon College is the perfect burrow building habitat for the great golden digger wasp.
My usual curiosity of the natural world doesn't typically extend beyond plants, but my interest was piqued. When searching for information on species I have not encountered before, I am often disappointed. Indeed, the great many species out there have been described in full maybe once - the original description - and mentioned a handful of other times in other publications as simply being an associate of other species. It's unusual to find such a detailed account of the life history of a single species, but I was pleasantly surprised with Sphex ichneumoneus. This species and its relatives have been observed in great detail and their behavior has made our species ponder the nature of philosophical fallacies and free will. What a creature!

Investment and return
Over six breeding seasons in the 1970s, H. Jane Brockmann recorded data of wasp behavior from three sites. Typically, each female will work on her own to dig and provision her burrow, but sometimes two females will begin provisioning the same nest in 5-15% of cases. The interloper takes advantage of the other wasp's spent investment and the two will be bringing katydids into the same nest. But because they spend most of their time away from the nest seeking new prey, it is only a coincidence if the two meet and fight over the nest. Fights last between 2 and 16 minutes and often the loser would leave and never return. Because of this one-on-one interaction where both insects have varying degrees of past interest (their future interests would be identical), the data can be thought of in simple game theory mechanics. The founding wasp took the time to dig the burrow and begin provisioning it, while the joiner risked being discovered and the subsequent fight to cheat and not build her own burrow. When faced with a fight, however, each has the same prize and motivation: a well-provisioned nest is worth fighting for, saving the winner days more of additional digging and hunting to lay a single egg.

The "sunk cost fallacy," or Concorde fallacy, so named because the British and American French governments continued to fund the faster trans-Atlantic Concorde flights even when there was no longer any economic incentive to do so, refers to decisions based on past investment because of loss aversion instead of on the rational potential future gains. Brockmann, along with Richard Dawkins, asked the question, "Do digger wasps commit the Concorde fallacy?" in their 1980 publication. The available evidence suggested that the wasp with the least prior investment in a burrow would give up first in a fight and abandon her effort. This result was not skewed by size advantage, which wasp visited the burrow most recently, or whether the winner was the founder or the joiner. Put plainly, the winner was usually the one who brought the most katydids to the burrow. As Brockmann and Dawkins say, "It is hard to resist the suspicion that the wasps are behaving as if following the Concorde fallacy." But are they?

Number of katydids each fight participant brought. Nine fights were over empty burrows. From Dawkins & Brockmann, 1980.
Further, the fight length was strongly dependent on how many katydids the loser brought. Falling into the Concorde fallacy, you might conclude that the loser will fight more vigorously because of greater prior investment in the burrow and less vigorously for those she has barely begun to provision. The losing wasp appears to rationalize: "Fight only as long as is proportional to your individual investment in this burrow." This case study informed the ongoing discussion of whether we humans consider such strategies to be "good" in our assessment relative to their evolutionary stability. The Concordian strategy versus the strict economist (fight based on potential future gain) is fully revealed here in this brilliant case study.

Free will
Just one more quick interesting note about these creatures. In Daniel Dennett's book Elbow Room, he reproduces an account by Woolridge in 1963 about the deterministic behavior of Sphex ichneumoneus. Woolridge watched the wasps return to their burrows with katydids, leaving them just outside while they went inside to inspect. Normally, the wasp is inside for a few seconds, then reemerges and drags the paralyzed katydid backward down into the burrow. He decided to alter the pattern to see if the wasp's behavior changed. When the wasp entered the burrow, Woolridge would subtly move the katydid a few inches from the burrow threshold. The wasp reemerged to find he prey moved, dragged it back to the threshold, then dove back into the burrow alone to inspect again. Woolridge writes, "On one occasion this procedure was repeated forty times, always with the same result." The wasp appears to be an unwilling participant in a free will experiment. She is not a free agent, but instead is driven by environmental cues: once a katydid is near the threshold, I must inspect the burrow and only then can I bring it inside. This property, an apparent lack of free will, was even given the name sphexishness. Dennett notes that publications on free will are rife with fears of sphexishness. Call it genetic determinism or a behavioral loop. Perhaps, though, we're all a little sphexish.

And what of the wasps?
I know their short adult lives will be over soon, but I've enjoyed viewing them through the window these past few weeks. Apparently, though, their lives were meant to be shorter than usual. I walked out the door the other day and noted the acrid smell of pesticides on the air. It got stronger as I approached the burrows and each hole was wet, as if it had been sprayed. Sphex ichneumoneus is a solitary wasp that is not inclined to sting anything but katydids. If you approach them or their burrows, they fly away, bothering no one. I was told our department administrative assistant tried to fill the holes in one day and I suspect she alerted the maintenance department to their presence, thus leading to their demise. Perhaps if people took the time to find out more about the supposed threat before eradicating it, they might change their minds about the course of action. That at least one good motivation for effective science education.

DAWKINS, R., & BROCKMANN, H. (1980). Do digger wasps commit the concorde fallacy? Animal Behaviour, 28 (3), 892-896 DOI: 10.1016/S0003-3472(80)80149-7

05 July 2010

Endosymbiotic bacteria in leafhoppers

Graphocephala coccinea, the candy-striped leafhopper. Source: Wikimedia Commons.
Several weeks ago, [info]urbpan posted about this pleasantly colorful species, Graphocephala coccinea, commonly known as the candy-stripe leafhopper. ([info]cottonmanifesto also took some amazing photos of this species and displays one here.) If you live in North or Central America, you would probably recognize this species as a common visitor of gardens and cultivated areas. Urbpan noted that leafhoppers are frequently vectors of plant diseases including many bacterial infections that are often species-specific.

ResearchBlogging.orgLess frequently realized is a bacterial relationship of another kind: symbiosis. Perhaps by now, more than 40 years after Lynn Margulis' stunning work popularizing it, the endosymbiotic theory of the origins of many organelles within the eukaryotic cell, including mitochondria and chloroplasts, is widely understood enough to continue without comment. But briefly, there is a heap of undeniable evidence that many of the eukaryote's organelles were once free-living bacteria that were engulfed and put to work by other cells: mitochondria respire, chloroplasts photosynthesize. It is reasonable to assume other such relationships exist within certain lineages of eukaryotes.

So here we have leafhoppers, like many other insects including aphids, feed almost exclusively on the phloem or xylem fluid of plants. With such a restricted diet, you are bound to run into nutritional deficiencies. Syrup is tasty, but I wouldn't want to subsist solely on it! What's an insect to do? The clear answer is to harvest those little nutrient-producing biological machines known as bacteria.

An electronmicrograph of the bacteriome of Homalodisca coagulata. The plentiful irregular spheroids (E) are the bacterial symbionts, Baumannia cicadellinicola, the border at the top (B) is the bacteriocyte (single cell) boundary, and the large organelle in the middle (N) is the host cell nucleus. The scale bar is 10 μm. The cell is just packed with bacteria, isn't it? Source: Moran et al. 2003.

The plant fluids usually consist of carbon and nitrogen sources such as amino acids, the building blocks of proteins. Xylem fluid chiefly contains non-essential amino acids such as glutamine and asparagine. Whereas most animals are incapable of modifying those raw materials into every other essential amino acid, vitamin, or cofactor they need thus requiring them to supplement their diet with foods with high concentrations of these necessary items, sap-sucking insects have solved the problem by entrusting the production of these necessary chemicals to their bacterial symbionts. These obligate intracellular bacteria are held within special organs called bacteriomes. The symbiotic bacteria are heritable, often transmitted from mother to egg, leading to interesting evolutionary questions. Some insects even have multiple symbionts which provide different chemicals for the host. For example, the glassy-winged sharpshooter (Homalodisca vitripennis) possesses a Gammaproteobacteria that provides vitamins and cofactors, while the Bacteroidetes (a different phylum entirely from the other symbiont species) synthesizes many of the essential amino acids.

Even more astonishing is the rate of genome size reduction seen in these symbionts. As is the case with the mitochondrion found in humans that harbor only 37 functional genes, the genomes of sap-sucking insect bacterial symbionts is heavily reduced. In the above glassy-winged sharpshooter example, the two bacterial symbionts have complementary genomes; that is, each genome has lost the functional genes that the other is responsible for.

Phylogenetic analysis of the hosts and symbionts reveal common evolutionary relationships, meaning that the relationships among the insect species is highly correlated with the symbiotic bacteria found within them. This indicates that the original infection and symbiosis was an ancient event occurring millions of years ago. Molecular clock data suggests that the common ancestor of the symbionts found in two genera in the subfamily Cicadellinae, Graphocephala and Homalodisca, lived sometime between 80 and 175 million years ago, which correlates well with the data on the divergence of the host insect species.

So next time you see these little garden pests think of all the complex reactions and relationships between host and symbiont when that leafhopper is feeding on your plants and admire the interesting evolutionary history of these organisms before you flick it off your prize raspberry bush.


Moran, N., Dale, C., Dunbar, H., Smith, W., & Ochman, H. (2003). Intracellular symbionts of sharpshooters (Insecta: Hemiptera: Cicadellinae) form a distinct clade with a small genome Environmental Microbiology, 5 (2), 116-126 DOI: 10.1046/j.1462-2920.2003.00391.x

Wu D, Daugherty SC, Van Aken SE, Pai GH, Watkins KL, Khouri H, Tallon LJ, Zaborsky JM, Dunbar HE, Tran PL, Moran NA, & Eisen JA (2006). Metabolic complementarity and genomics of the dual bacterial symbiosis of sharpshooters. PLoS biology, 4 (6) PMID: 16729848