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September 29, 2009

For the Faithful, Eusociality

 by Elizabeth Pennisi

As social as humans are, their cooperative nature pales in comparison to that of ants, bees, wasps, and termites (see hill, left). Colonies of these insects can number in the millions and function seamlessly as “superorganisms.” In their book, The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies, Burt Hölldobler and E. O. Wilson point out that this way of life makes for very successful living. These insects represent a mere 2% of the insect species yet take up two-thirds of the insect biomass. In tropical rainforests, ants outweigh all the mammals and land vertebrates combined.

Yet the scores of entomologists and evolutionary biologists who have marveled at the efficiency of superorganisms have yet to sort out for sure how superorganisms evolved. True superorganisms are highly eusocial: Typically, one or a few queens lay all the eggs, which are tended to by nonreproductive workers. Their fecundity can be astonishing: In their more than 10-year life span, queens of Atta ants (see right) can produce 150 million daughters, for example. Multiple generations live together; and because workers are sterile, very few conflicts arise, and the colony runs quite efficiently.

Less extreme versions of this lifestyle exist, leading some to suggest that eusociality evolved in stages, starting with a female that set up communal nests with other females, with some forgoing reproduction to help provision and protect the young. A few have suggested that it’s not even that critical that the founding females be all that closely related.

In my Origins essay on cooperation, I  did not touch upon the origin of these truly social insects. But Jacobus Boomsma of the University of Copenhagen, Denmark, has thought extensively about this question and rejects the stepwise progression from cooperative breeding to eusociality, asserting that not just close kinship but also lifetime monogamy is critical to incipient eusociality. Early eusocial species “have a very special form of strict monogamy that has been unappreciated,” he says. This idea has been suggested before, but “Boomsma has performed a valuable service in reviving it and extending it,” says Andrew Bourke of the University of East Anglia, United Kingdom.

Termites mate for life, with a single queen and “king” producing generations of siblings, all equally related to one another. Once in their lifetime, wasps, bees, and ants leave the nest on a mating frenzy, with the queens returning with enough sperm to last the rest of their reproductive years. The consequence of having just one mate for life is that the many generations of offspring are all siblings that on average share half their genes. That number of genes in common is the same as they would have in common with their own offspring should they try to reproduce. Thus, if there is even a small survival advantage to group living, that advantage would be a strong enough selective force to encourage the evolution of sterile castes and true eusociality, Boomsma argues. “When a parent refrains from mating with any additional mates, their offspring are free to stop mating at all,” he explains. However, strict monogamy is rare, particularly over evolutionary time scales, and thus, so is eusociality.

In the late 1950s, Kansas University entomologist Charles Michener suggested that eusociality could arise in one of two ways. By the subsocial route, parents associated with offspring; by the parasocial or semisocial route, females joined forces with their peers in communal settings. Yet even today, that latter scenario lacks any hard evidence, says Boomsma. Such cooperative breeding setups never lead to permanently sterile helper castes; there are too many conflicts of interest. Those conflicts disappear where queens have a brief mating interval and then settle down for a life of reproduction using the sperm acquired during that one fling.

A 2008 phylogenetic analysis of mating systems in ants, bees, wasps, and termites supports Boomsma’s hypothesis. In 2008, William Hughes of the University of Leeds, U.K., and his colleagues looked deep into the past of 267 eusocial bees, wasps, and ants. They found ancestors of these lineages of eusocial insects were monogamous. Only later, once sterile castes had evolved, have some groups begun to mate more than once they reported.

To hear more about Boomsma’s hypothesis, listen to a talk he gave earlier this year at the Evolution of Society meeting sponsored by the Royal Society.


Credits: (termite hill) Summi; Atta ants: Adrian Pingstone


A nice read, but one small note. I believe part of the close relationship amongst these social insects is that they share more than 1/2 their genes. Ants, wasps and bees can share up to 3/4 of their genes between siblings and termites inbreed to achieve higher than 50% shared genes. A small point, but I think an important one. As the closer the genetic relationship, the more cooperation is encouraged as you are helping a greater % of your genes. to say they only share 50% of their genes with siblings is leaving out the key reason as to why these "superorganisms" are different than siblings in many other species.

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