Dense clump of fighting Tetramorium immigrans. They looked like a seething mass of oil to me.
I was walking through one of the city parks in Cortez, Colorado a couple of days ago and came across what looked like a black, shiny liquid that had been poured onto the edge of the sidewalk.
Looking a little more closely, I could see that it was a mass of fighting ants.
The mass of ants followed along the edge of the sidewalk for perhaps six feet or so. The mass was very linear - the ants did not spread more than six inches or so into the grass or onto the sidewalk.
The Tetramorium were fighting in dense clumps.
These are warring Tetramorium immigrans colonies. These wars usually break out in the Spring and early Summer, when the colonies are trying to establish their territories.
The ants pour out of their nests in masses and fight semi-formalized battles, looking like living oil spills.
“Formalized” because a lot of the fighting is ritualized - the ants pull and tug at each other, but don’t go at each other in a full-throated attempt to kill.
“Semi-” formalized because some of the ants do, indeed, get killed in these competitions. Just not as many as you would expect, given the masses of fighting ants.
A closer view of one of the dense clumps of fighting ants.
Ants from one colony will grab ants from another, and pull. The victim ant will get (at best) spread-eagled, and at worst, pulled to bits.
Close-up of fighting workers.
Still more fighting workers.
This particular territorial dispute went on for at least two days.
Group of workers from one nest attacking worker from another nest. They seemed to want to spread-eagle the opposing ants.
Tetramorium immigrans is very much a city ant in the United States. They seem to like building their nests in the cracks in sidewalks - hence the name “Pavement Ants”.
Fighting Tetramorium immigrans workers.
Pavement Ants were introduced to the New World from Europe.
Originally, the scientific name for these ants was Tetramorium caespitum. Scientists have long suspected that there was more than one species of “Pavement Ant”, though, and that calling them all “Tetramorium caespitum” was actually spackling over a lot of taxonomic….uh….difficulties.
In recognition of this problem, and I’m not making this up, the Tetramorium that we have in the United States was renamed “Tetramorium species E” a few years ago.
This renaming struck me as being immensely humorous. I wondered if perhaps Tetramorium sp E had been involved in a legal dispute with its record label.
Close-up of one of the Tetramorium immigrans workers biting my thumbnail. You can see that the ants are not very large. You can also see that I should probably use some type of moisturizer.
Fortunately, in 2017 Wagner etal (hopefully) resolved a lot of the taxonomic issues with the group. They used a bunch of DNA techniques, as well statistical treatments of the ants’ morphological characters, to split the Tetramorium caespitum cryptic-species complex.
I won’t even pretend that I walked through their keys to identify these warring caespitum complex ants.
Instead, I used the species ranges listed in the paper.
A rose by any other name, I guess.
Sources:
Antwiki.org’s Tetramorium immigrans page.
Alex Wild’s posts on Tetramorium. This gentleman takes spectacular photos of ants, as well writing compelling and informative accounts of them.
Wagner, Herbert C, Arthofer W, Seifert B, Muster C, Steiner FM, Schlick-Steiner BC. 2017. Light at the end of the tunnel: integrative taxonomy delimits cryptic species in the Tetramorium caespitum complex (Hymenoptera: Formicidae). Myrmecological News, 25, 95-129.