Carrying a handful of sand is never easy. But imagine a change of scale - I have a pile of river pebbles and cobbles (garden, ornamentation, for the purpose of) that I need to move, and fortunately, I have a wheelbarrow. But if all I had were my own limbs, carrying an armful of cobbles is nigh-on impossible; now imagine that you're an ant - every cobble is a sand grain and it needs to be moved up and out of the deep subterranean nest that you have been busy excavating. Now of course if the sand is damp, then the grains tend to stick together and carrying a clump of grains is feasible, but, if the sand is dry, then the problem is that of an armful of cobbles, and a conscientious ant is not going to be satisfied with one grain at a time. And the luxuriously-named pogonomyrmex is an ant that's solved the problem.
I am constantly fascinated by the inventiveness and adaptability of creatures, particularly very small ones, in their relationships with sand - endless examples of nature's creativity. Not being in any sense an expert on such things, I just keep encountering more extraordinary examples, of which pogonomyrmex is one. This encounter is thanks to the blog by Alex Wild, a biologist at the University of Illinois with a particular enthusiasm for ants (his website, http://www.myrmecos.net/index.html, includes two categories listed as "ants" and "not ants"); furthermore, Alex is a superb photographer of his subjects - the images above are from his site and used here with his kind (and instantly granted) permission.
The Greek word pogon means beard, and pogonomyrmex is a genus of generally desert-dwelling harvester ants - bearded ones. The beard is a basket-like structure of stiff hairs below the head that is extremely handy for carrying seeds, eggs - and sand grains; it's called a psammophore, "sand-carrier," and can clearly be seen in the portrait of pogonomyrmex rugosus, below (also courtesy of, and copyright, Alex Wild).
It was the psammophore of pogonomyrmex desertorum in action that first struck me - these are the images at the top of this post; the hairs can be seen clutching a load of fine sand grains, and, as the ant cleans herself off after dumping them (right), the whole structure becomes clear. And yes, we know it's a female because males don't dig - it's the females, the bearded ladies (a phrase I have unashamedly borrowed from Alex) who do the work. And, as Alex remarks,
What most intrigues me about psammophores is that lots of completely unrelated groups of desert ants have them. Apparently psammophores evolve at the drop of a hat, although this is perhaps not too surprising. Most ants already have hairs of some sort on the underside of the head.
The excavating efforts of the pogos (as, it seems, these ants are affectionately known) are monumental - these ladies can dig. Research at Florida State University used innovative techniques to reveal the subterranean dimensions and architecture of nests of pogonomyrmex badius, the Florida harvester ant. The sheer size of an excavated pile of sand next to a nest entrance is a fairly good indication of the extent of the underground workings, but their design is difficult to see. Walter R. Tschinkel developed a method of filling a nest with a thin slurry of orthodontal plaster (not to mention molten zinc and aluminum) and then excavating the cast; the sandy Florida soils are sufficiently permeable that air is easily forced out of the small structures of the nest, allowing them to be completely filled with the plaster. The results are incredible, and a full report is available at the BioOne site here. Large nests can be three meters deep, and the architecture is exquisite (photo below).
Shafts descend in a regular helix form, with chambers excavated at varying intervals off them. The number of chambers at shallow levels is much greater than at depth, with different populations of the colony occupying different depths and performing different functions. It would seem that the senior citizens live and work in the relatively less crowded upper levels, with the youths consigned to the depths. The demography and dynamics of the colony are complex, the details - and mysteries - revealed in the article fascinating. As Tschinkel comments,
The nest's architecture represents a considerable investment of energy and time, and provides shelter, microclimate choice, and defendability. In addition to these obvious services, nest architecture is likely also to be a mechanism for integrating the colony members into an efficient functioning whole, the superorganism. The large amounts of time and effort invested in nest construction surely must pay dividends in fitness, for this energy could have been invested in other fitness-enhancing functions instead. An as yet unmet challenge is to identify how particular features of the architecture serve particular colony functions, and how this service contributes to colony fitness.....
The functional significance of the top-heaviness is mysterious. This large space seems under-used, typically harboring very low worker density. Perhaps this area is important for regulating brood development rate during the cooler months, for these chambers are exposed to the greatest diurnal variation in temperature. Alternately, a unit volume of upper chamber is cheaper to excavate than deeper ones because the soil is brought up over smaller vertical distances, but if this space is not heavily used, why create it? If top-heaviness is not functional, but merely a non-adaptive epiphenomenon of some other behavior, it is an expensive one.
This is but an initial excavation of the extraordinary world of pogos and their relatives - there's more to come.
[All pogo photos courtesy of and © Alex Wild, http://myrmecos.wordpress.com/2008/03/19/the-bearded-ladies/; nest cast photo from The nest architecture of the Florida harvester ant, Pogonomyrmex badius: Walter R. Tschinkel, Department of Biological Science, Florida State University, Tallahassee, FL 32306-4370]
The photo of the colony cast is very cool. It reminds me of a clip I posted last fall. That cast shows an amazing sensibility regarding climate control. It's difficult to imagine ants "thinking," but their work demands a consideration of what "thinking" means.
Posted by: Lockwood | July 16, 2009 at 10:31 PM
Absolutely - and when you consider the phenomenon of "megacolonies" there would seem to be some sort of "communal thinking" going on. What's the link to the clip that you posted?
Posted by: Sandglass | July 17, 2009 at 08:09 AM