In September of 2004, my wife and I were driving through the Rincon Mountains on our way home to our house in Tucson. We’d been out anting all afternoon, and it was dark as we drove along the gravel road.
In the glow of our headlights, I saw a column of small ants running along the edge of the road – I locked up the brakes and slid to a stop.
Army Ants!
They were running along the edge of the road in a column perhaps 5 or 6 inches wide. They continued along the road for perhaps 25 yards, I’d guess, before the column left the road and disappeared into the surrounding oak scrub.
It’s funny. I saw Army Ants in Costa Rica in the 1980s. Since then, I had read about North American Army Ants for decades. I’d always imagined them as being the same size as the Costa Rican Army Ants – with big, impressive major workers.
Most North American Army Ants are surprisingly small, as you can see in the photo below.
You may notice that the caption on a lot of my photos in this post are “Neivamyrmex texanus workers running.” That’s because they ran…a lot. I saw very few of them pausing to groom, or pausing at all. This contrasts with my previous post about Neivamyrmex nigrescens, in which I found some of those Army Ants pausing to groom, or to gape their jaws threateningly at the sky.
I’m not sure why the behavior was different. Is it a difference in the behavior of the two species of Army Ants? Is it because I was watching these ants at night, in the dark, and I watched the Neivamyrmex nigrescens during the light of day?
It was also interesting how many of these Army Ants were, well….grubby. A lot of them were covered with dust and gravel. That shouldn’t have been a surprising observation, it’s my understanding that Army Ant workers are famous for being dirty. I guess that comes from being mostly subterranean, which many of the North American Army Ants are, and just popping up every now and then on the surface of the ground like an artesian well of ants.
I noticed that some of the Army Ants were carrying what appeared to be ant larva and ant pupa. These ants were running in the opposite direction to most of the Army Ants. I back-tracked these pupa-laden ants to see if I could find where they were getting their prey.
There was a small side branch off of the main trail where the pupa-laden Army Ants were entering the main stream of Army Ants. Within three or four feet of the main trail, I found a Pheidole nest that the Army Ants had broken into.
The Pheidole nest seemed to be in a shambles. There were Pheidole minor workers running around carrying larvae and pupa. Presumably they were trying to abscond from the nest with the larvae, in the hopes of saving at least some of them. Other Pheidole minors were running around frantically. They did not seem to be defending against the Army Ants in a meaningful way.
There were three size classes of Pheidole workers – a small worker, a major, and a much larger super major. Based on my photos and the three size classes, I believe the Pheidole nest was Pheidole obtusospinosa.
An interesting thing about Army Ants and Pheidole majors. Remember how I mentioned the tri-phasic worker castes (minor worker, major worker, and super major worker) in Pheidole obtusospinosa? At least some researchers (Huang 2010, for example) suggest that the evolution of the large head size in Pheidole, and specifically the development of a specialized extra-huge-headed caste of super-majors, may be related to defense against Army Ants. Circumstantially, most of the Pheidole species that have a super-major caste of worker have a geographical overlap with the range of Army Ants.
Also, Huang (2010) watched Pheidole obtusospinosa defend against Neivamyrmex texanus Army Ants. This was in a mountain range that was pretty close to where I currently was. They described the Pheidole super majors behaving in an organized fashion, blocking their nest entrances against the raiding Neivamyrmex texanus.
As Huong watched, the Pheidole successfully repulsed the Army Ants from the main nest entrance. Then the Pheidole majors were able to counter-attack, and disrupt the trail of the Army Ants such that further reinforcements of Army Ants failed to arrive.
I wonder if that ability to disrupt the Army Ant trail was related to skatole. Skatole is the odorous chemical that at least some Army Ants use as part of their trail marking. I know that some Pheidole also produce skatole (as an alarm pheromone?), so I wonder if the Pheidole alarm pheromone may also serve to make the Army Ant trail less clear.
I obviously came upon the Pheidole obtusospinosa/Neivamyrmex texanus drama at a later point than Huang did in his observations of a successful defense. I missed the defense, and was present just for the sacking of the Pheidole obtusospinosa nest.
Looking back, I wonder if I had misinterpreted the “hiding” behavior of the super majors. Perhaps what I was interpreting as their “hiding” was really a misplaced attempt at blocking a nest entrance. The super majors were instinctively trying to put their heads into a cavity, any cavity, in an effort to block the Army Ants…but the nest had been so disrupted and the super majors so scattered by the Neivamyrmex successfully breaking in that there was nothing left to block.
Wow. Yet again…what a fabulous evening this was.
Sources:
Huang, Ming H.(2010). Multi-Phase Defense by the Big-Headed Ant, Pheidole obtusospinosa, Against Raiding Army Ants. Journal of Insect Science. 2010; 10: 1.
