A century ago, William Morton Wheeler inked this iconic illustration of the striking polymorphism displayed among members of an ant colony. You may have seen it; Andrew Bourke and Nigel Franks used it as the cover for their 1995 text Social Evolution in Ants.
I always assumed Wheeler’s figure depicted some exotic tropical marauder ant, a voracious jungle species with massive soldiers for slicing up hapless prey. I don’t read captions carefully enough, I guess, because I learned recently that this charismatic creature is actually a local harvester ant, Pheidole tepicana. Not only that, but the lab downstairs from mine keeps several captive colonies for research on caste development. Obligingly, last week they let me stop in with my camera to take some photos.
The largest workers have absolutely massive heads:
Eli Sarnat, the reigning expert on the Ants of Fiji, has just published a lovely taxonomic revision of a group of Pheidole that occur on the islands. Pheidole are found in warmer regions worldwide, but Fiji has seen a remarkable radiation of species that share a bizarre set of spines on the mesosoma. Eli sorted through hundreds of these things to determine that the group contains seven species, five of which had not previously been described. Pheidole pegasus is largest and among the most distinct of the group. It was collected only once, from the summit of Mt. Delaikoro.
After reading a couple times through Corrie Moreau’s hot-off-the-press Pheidole evolution paper, I am pleased to give it a thumbs-up. The paper is behind a subscription barrier, so I have distilled the results into an informal summary:
On the Indian subcontinent there is a species of ant with a distinct nest entrance flanked by a raised series of concentric clay rings, as though to prevent flooding. The ant seems reasonably common- at least, it is commonly photographed (see here). Locals call it a “harvester ant”. I have not seen any photographs where the ants are visible enough to identify.
The internet has two different ideas about the identity of this mystery mini-architect. One is Pheidole sykesii, which is possible, but I fear that idea may be one of those self-reinforcing internet citation circles with no verified literature behind it. The other is that this is a species of Trichomyrmex, a suggestion on twitter from several others, including at least one professional myrmecologist.
My Google-Fu has run dry, though. Do any of you know what makes these lovely mounds?
We’re only halfway through the year, but already 2014 will be remembered as pivotal for studies of ant evolution and classification. Following right on the heels of Schmidt & Shattuck’s massive ponerine revision comes an important new study from the Ant Tree of Life group. Ward, Brady, Fisher, and Schultz (2014) have reconstructed the first thorough genus-level phylogeny of the great ant subfamily Myrmicinae.
How important is this study?
Roughly half of all ants are myrmicines, both in abundance and in species diversity. Their numbers include fire ants, harvester ants, leafcutter ants, big-headed ants, acrobat ants, and so on, to the tune of some 6,000+ species.
So… Boom! Suddenly, we’ve been given a detailed picture of the evolution of half the ants. This is big. It is so big I cannot cover the paper in detail. Instead, I’ll just give a few preliminary thoughts, as follows:
1. This is a well executed study, as we’ve come to expect from the Ant Tree of Life team, applying a thorough analysis to over 250 carefully selected taxa and 11 genes. It’s also a shining example of an older generation of genetic techniques, alas, and while I am confident the stronger results will mostly endure, be aware that an incoming next-gen tide of full genomes, and the 6,000 yet-unsampled myrmicine species, may yet overturn some of the findings.
2. The deep history of Myrmicinae, starting 100 million or so years ago, mostly occurred on those continents that drifted to become the Americas. Echos of these earliest divisions are heard in six clear, genetically distinct groups that Ward et al have formally set the up as a new system of tribes, replacing an earlier, messier scheme. The six groups are listed here in their order of divergence: Myrmicini (Myrmica & Manica), Pogonomyrmecini (Hylomyrma & Pogonomyrmex), Stenammini (Aphaenogaster, Messor, Stenamma, and relatives), and three sprawling groups with thousands of species: Solenopsidini, Attini, and Crematogastrini.
3. The news is not all good. The clarity deep in Ward et al‘s tree fades for slightly younger events. Early relationships within some of the the six tribes are discouragingly ambiguous. This study has resolved some problems, myrmicine taxonomists face a difficult road ahead. Many of the world’s greatest genera do not form natural groups and will have to be redone. These include Aphaenogaster, Pheidole, Tetramorium, and especially Monomorium, which splatters almost comically across the Solenopsidines.
Distressingly, fuzzy resolution in a data set with this many markers and taxa means achieving proper resolution, if at all, will likely be expensive. Myrmicines may have speciated so explosively that we may never be able to reconstruct what happened with confidence.
4. The authors correct a few of the more obvious instances of paraphyly. Notably, the New World “Messor“, being unrelated to their old world doppelgangers, were moved to a revived Veromessor, and several social parasites like Protomognathus and Anergates have been sunk into the host genera from whence they evolved: Temnothorax and Tetramorium, respectively. There are other changes, too; they are listed in the abstract.
Most of the identified problems- such as what to do with Monomorium and Aphaenogaster–were left for targeted future research.
5. Remember the dispute over Pyramica vs. Strumigenys? The argument was fundamentally over how ant mandibles evolve. Apparently, high energy trap-jaws arise easier than anyone imagined. According to Ward et al, not only is the assemblage of trap-jaw ants formerly included in dacetini a polyphyletic splatter, even within the genus Strumigenys the trap jaw has arisen at least twice.
6. The rare and bizarre African myrmicine genus Ankylomyrma is not a myrmicine at all! Rather, Ward et al‘s results unambiguously tie it to the equally bizarre Tatuidris of the Neotropics, sitting on a distant branch of the ant tree. Peas in a poneromorph pod…
Ultimately, Ward et al have crafted a sobering view of how little we still know about ant evolution, and how much remains to be done.
disclosure: I received my Ph.D. from Phil Ward’s lab where much of this study was completed, and I contributed a few of the samples, but I was long gone by the time the study was initiated and have had no other involvement with the research.
Ant guy Ed LeBrun has a paper out in Science today documenting a novel defensive use for formic acid: detoxifying the venom of competing fire ants:
Abstract: As tawny crazy ants (Nylanderia fulva) invade the southern USA, they often displace imported fire ants (Solenopsis invicta). Following exposure to S. invicta venom, N. fulva applies abdominal exocrine gland secretions to its cuticle. Bioassays reveal that these secretions detoxify S. invicta venom. Further, formic acid, from N. fulva venom, is the detoxifying agent. N. fulva exhibits this detoxification behavior after conflict with a variety of ant species; however, it expresses it most intensely after interactions with S. invicta. This behavior may have evolved in their shared South American native range. The unique capacity to detoxify a major competitor’s venom likely contributes substantially to its ability to displace S. invicta populations, making this behavior a causative agent in the ecological transformation of regional arthropod assemblages.
Normally, formic acid from the ants’ venom gland is applied to an opponent as a potent volatile weapon, but this sort of self-medication is novel. I haven’t read the paper in depth, but it looks fascinating.
I’ve long been interested in the invasion of North America by a suite of highly-competitive species from the same region of Argentina. Some of the most dominant ants in along the gulf coast are imports: fire ants, Argentine ants, tawny crazy ants, Pheidole obscurithorax, and rover ants all know each other, so to speak, from their interactions along the banks of the Paraná river. Thus, this detoxification behavior likely originated long before any of these ants hitchhiked to North America.
Goodness, it’s already Saturday and I haven’t answered the Monday Mystery! Sorry to keep you hanging. The correct answers to Monday’s Challenge are:
A=5 Oecophylla longinoda (South Africa) B=1 Pogonomyrmex occidentalis (California) C=3 Pheidole bergi (Argentina) D=6 Camponotus termitarius (Argentina) E=2 Crematogaster sp. (South Africa) F=4 Atta vollenweideri (Argentina)
Points are awarded as follows: 10 to Rodolfo for getting all the correct answers first, while 1 consolation point each goes to Igor Nascimento, Sanford Porter, Cody Cutter Cardenas, and MrILoveTheAnts for providing more specific location/taxonomic information.
The impressively moustached Pheidole bigote was described from Chiapas by Jack Longino in 2009. What’s up with the bizarrely plush facial adornment? No one knows. Seems like there’s a lot of that going around these days.
Since we’re on the topic of common names, a peeve of mine is the use of “Carpenter Ant” to refer to the genus Camponotus. While it’s true some Camponotus– including the common North American wood-destroying C. pennsylvanicus– do carve chambers in wood, a great many others nest in the ground where woodworking is not much of a possibility.
An accurate common name for Camponotus is not an arcane problem. This is the single most widespread and abundant ant genus worldwide. If I were to recommend just one genus that non-specialists should recognize, it is this one. Camponotus is found nearly everywhere, often in abundance, and the genus is arguably the most species-rich in the world. With the possible exception of Pheidole big-headed ants, of course.
Australians call theirs “Sugar Ants”. Camponotus novaehollandiae is the “Northern Common Sugar Ant“, for example. Insofar as all Camponotus like sugar that moniker is decent, but a sweet tooth is hardly a unique behavior among formicids. Still. It’s more accurate than “Carpenter Ants”.
Unlike for Polyergus, where I have a decidedly unpopular suggestion, I don’t really know where to start for Camponotus.
What were last night’s army ants targeting? This challenge was tricky, as it involved identifying ant pupae rather than the more taxonomically accessible adults. Nonetheless, you did well.
Most of the brood in the booty cache belonged to the same species as this ant:
Sprinkled in with the others were some smaller Pheidole of an undetermined species.
Points are awarded as follows: 5 to James for picking the Pheidole, and 5 more to Julio for getting the Dolichoderus, not just to genus, but to species. Many of you did well speculating that the mystery pupae were dolichoderine. I thought it was Azteca in the field, but close examination of the photos showed that the propodeal profile was off, matching only D. lutosus, a reasonably common ant in the same habitat.