When I first saw a milling swarm of these dark army ants pushing across the road last month at our lodge in Belize, I did a double-take. Their behavior and form were reminiscent of the familiar Eciton burchellii army ant I’ve seen in Panama and across South America, yet these ants were completely black. I’m used to swarm-raiding army ants having distinctly reddish backsides, like so:
I vaguely recalled Jack Longino having written about black Eciton burchellii in Costa Rica, so when I arrived home I looked it up:
There are numerous subspecies of E. burchellii. In Costa Rica there are two, E. b. foreli and E. b. parvispinum. The workers of foreli have a red brown metasoma, such that minor workers appear bicolored in the field, while the workers of parvispinum have a black metasoma, and the minor workers are entirely black. I can find no other morphological or behavioral feature that correlates with the color difference. The color forms could easily rest on a very small genetic difference, perhaps a single gene. However, the males also exhibit a difference. Males from the Atlantic slope, corresponding to the range of foreli, have a number of long flexuous setae on the scutellum; males from the Pacific slope and the range of parvispinum have the scutellum bare. In most cases this character difference is discrete, but I collected one male that was intermediate. Among a series of males from Monteverde, which is close to the zone of contact of the two forms, one had long setae on one half of the scutellum and the other half was bare (differing across a sagittal plane). All the rest of the males and all of the workers I have seen from Monteverde have the parvispinum phenotype.
The distributions of foreli and parvispinum are very sharply parapatric and do not seem to correlate with particular habitats or thermal environments. Eciton b. parvispinum has the broadest distribution, occurring across all of the Pacific slope, from the dry forests of Guanacaste to the rainforest of the Osa Peninsula, from sea level up into the mountains, and dropping a short distance down onto the Atlantic slope, where it meets foreli. For example, on the Barva Transect of Braulio Carrillo National Park, foreli is common from La Selva up to 500m elevation. Above that elevation Eciton burchellii has lower density, but I have three collections from 1400-1500m elevation, about 10km further up-slope, and they are parvispinum. In the Peñas Blancas Valley, east of Monteverde, only foreli occurs at Refugio Eladio, at 800m elevation. At Refugio El Aleman, 950m elevation and 5km further up the valley, I have only collected parvispinum. West of El Aleman, continuing up to Monteverde, only parvispinum is found. Like other examples of step-clines in morphological or genetic characters, it begs the question of what mechanism is maintaining the sharp boundary between these two forms, especially for such large, nomadic organisms.
Apparently I had seen my first Eciton burchellii parvispinum! The majors and submajors still had light-colored heads while retaining the dark metasoma:
If you’ve spent time trying to identify ants, you’ll be familiar with the caution applied to using color to diagnose species. The lack of confidence in such an obvious trait can be frustrating to ant newbies (“But this one is orange! That one is black! Why are they the same species?!”), and could be just another case where a single species encompasses substantial variation. Not unlike our own species, of course.
What does the color difference mean? Is the dark form, as Jack mentions, just a single allele that’s become fixed in some Central American populations? Or is it just the tip of a larger genetic iceberg, such that the dark and light forms are separate, non-interbreeding groups?
The two forms do not appear to coexist. Taxonomists typically interpret this lack of sympatry as indicative of a single species that varies across space, but in this case I’m not so sure that conclusion is appropriate. I am not an army ant taxonomist, so take this with a truckload of salt, but my opinion is that E. burchellii parvispinum is a good biological species and should be elevated to E. parvispinum.
My reasoning is as follows:
1. Genetic studies on E. burchellii in Mexico and Panama show that males of the species disperse well enough to maintain genetic diversity across reasonably large areas. Thus, even if the flightless queens are limited in their dispersal, their genes are broadcast widely on the wings of males. I doubt these forms are completely geographically isolated.
2. Queens are polyandrous- that is, they mate with many males- and if E. burchellii is just a single, interbreeding species we should find at least some colonies of mixed color variations in regions where dark and light forms occur. The two species don’t coexist locally, but they do broadly overlap across large swaths of Mesoamerica, enough so that males should jump populations at least occasionally.
3. Jack’s observation that males of E. b. parvispinum and E. b. foreli are consistently different in other traits suggests a more complete genetic separate than just a single color difference in workers.
The lack of local co-existence among forms might have another explanation beyond mere genetic isolation by distance. The raiding biology and dietary habits of both forms are so similar that the two would certainly enter into intense competition, so one or the other ends up winning out in a particular patch of habitat. A forest might just not be big enough for the both of them.
In any case, I’m just speculating. A proper study would involve a great deal more genetics and the measuring of male bits.
Berghoff SM, Kronauer DJC, Edwards KJ, Franks NR (2008) Dispersal and population structure of a New World predator, the army ant Eciton burchellii. J Evol Biol 21:1125–1132. doi:10.1111/j.1420-9101.2008.01531.x
Jaffé R, Moritz RFA, Kraus FB (2009) Gene flow is maintained by polyandry and male dispersal in the army ant Eciton burchellii. Population Ecology 51: 227–236. doi:10.1007/s10144-008-0133-1
Longino, JT (2007) Eciton burchelli parvispinum Forel 1899 (website) - http://academic.evergreen.edu/projects/ants/Genera/eciton/species/parvispinum/parvispinum.html accessed online 10/11/2013.