Trager (2013): "The raiding of Polyergus breviceps follows familiar patterns described for other species.
[Instigated by a successful scout, a dense swarm of raiders heads off, slowly at first, then organizing into a column and hastening the pace. The columns are periodically interrupted as the raid progresses. When this occurs, there is the appearance of the group’s not knowing where to go next. Then, a worker that apparently had run ahead of the main group (a scout/leader?) may be seen to return to the group and recruit the column in the right direction once again. Upon arriving at the host nest, Polyergus workers hesitate and amass outside the entrance, often clearing pebbles, twigs, and other impediments from the entrance of the host nest before they enter. Shortly after the first Polyergus enter, they begin emerging and head home bearing pupae, prepupae, or less commonly, last-instar larvae. Sometimes, especially early in the raiding season, raiders return one or more times the same afternoon to pillage additional pupae from a particularly productive host colony. Less often, a Polyergus colony raids more than one host colony in an afternoon, either simultaneously or sequentially].
Near Taos New Mexico, I once observed raiding columns from two colonies of this species cross paths, resulting in a battle lasting two days, including over night, with high mortality. One of the colonies disappeared after this.
I have not directly observed mating and colony foundation, but have seen alates fly from the nest several hours before the late afternoon raids, and I also have seen a lone, dealate gyne wandering near a mound of Formica montana in a prairie near Chicago. It would seem such lone gynes are capable of colony foundation, even with this rather aggressive host and its populous colonies.
P. breviceps is naturally a species of wet and mesic prairie and meadow habitats, though it persists in drier, but formerly wet, locations after habitat degradation and hydrological disruption, if the host remains abundant (I have observed this both in CO and IL). Wheeler (1910, p. 477) describes a situation near Florissant, Colorado, of Polyergus (which I surmise to be breviceps) living with “F. neocinerea” (Formica canadensis) in conspicuous mounds raised above the moist soil of a mountain meadow, and what he took to be the same species (but which I surmise to be mexicanus) living with Formica argentea in less conspicuous nests on the wooded slopes above this meadow. In the Chicago region, tallgrass prairie restoration plantings are colonized by Formica montana in just a few years, and breviceps seems to arrive almost or indeed concurrent with them, just a few years after conversion from plowed crop land. This may occur through breviceps gynes teaming up with young F. montana gynes or incipient host colonies, as has been reported for Polyergus topoffi.
Polyergus breviceps normally parasitizes members of the F. cinerea complex; Formica montana in the humid prairies of the Great Lakes and northern Plains states, and Formica canadensis in western mountain meadows. Some samples studied also included Formica altipetens or less often, the less closely related Formica neoclara or Formica occulta. A few samples have been found from drier western grassland sites with these less pilose hosts, and these breviceps seem to average a bit less pilose than those with cinerea group hosts, but still have telltale pronotal lateral pilosity. These also differ in proportions (narrower head, slightly longer limbs) from typical P. breviceps, and may represent another species or a hybrid".
This ant is common in wet to mesic prairies and mesic or wetter old fields of northwestern Indiana and the Chicago Region, and is distributed west to the Rocky Mountains and south to the White Mts. of northeastern Arizona (Trager, 2013).
Trager (2013): "Polyergus breviceps is a broad-headed and short-limbed species, but most easily distinguished by its abundant pilosity. This is among the two smaller Nearctic species, though averaging somewhat larger than partially sympatric bicolor, and larger than some isolates of Polyergus mexicanus.
Worker description: Head suborbicular to (less often) subquadrate, its length and breadth about equal, or not uncommonly the breadth a bit greater, sides quite rounded, outer margins of eyes not or at most slightly extending beyond sides of head; vertex flat or broadly and shallowly concave, the flat portion or concavity about as wide as the space between mandibles; vertex pilosity conspicuous and abundant, usually 16–24 (6–30) macrosetae; scape not reaching vertex corners by about twice its maximum diameter, clavate in the apical third; pronotum usually with 22–36 (16–44) erect setae, including a few shorter ones near the lower margins; mesonotal profile flat or very weakly convex for most of its length; propodeum evenly rounded; petiolar dorsum rounded and shallowly emarginate; first tergite densely pubescent; first tergite pilosity flexuous, basally suberect and distally subdecumbent, about as dense in posterior half of tergite as in its anterior half, appearing to be in 5 or 6 transverse arrays.
Head matte; mesonotum matte; gaster matte; slightly shining lateral portions of all tagmata in some specimens. Color usually dull red with infuscation of dorso-posterior portions of tergites. Pilosity matching color of body to slightly darker, pubescence yellow gray".
Broad-headed and short-limbed Polyergus species; easily distinguished by its abundant pilosity. In addition to its abundant pilosity and short scapes, it shows marked preference for Formica cinerea group hosts. Typical habitat: open, moist grassland or sedge meadow (Trager, 2013).
Taxonomic history
Subspecies of Polyergus rufescens: Wheeler, 1901e PDF: 714; Wheeler, 1910a PDF: 571; Santschi, 1911d PDF: 7; Wheeler, 1914c PDF: 56; Wheeler, 1915b PDF: 419; Wheeler, 1917a PDF: 555; Emery, 1925d PDF: 269; Essig, 1926 PDF: 867; Cole, 1936a PDF: 38; Cole, 1942 PDF: 385; Wheeler & Wheeler, 1944 PDF: 269; Buren, 1944a PDF: 310; Smith, 1947g PDF: 157 (redescription); Creighton, 1950a PDF: 558; Smith, 1951c PDF: 874; Cole, 1954g PDF: 285; Smith, 1958c PDF: 161; Smith, 1967a PDF: 374; Kutter, 1968c: 208.Status as species: Kannowski, 1956a PDF: 185; Wheeler, 1968 PDF: 163; Hunt & Snelling, 1975 PDF: 23; Francoeur, 1977b PDF: 208; Wheeler & Wheeler, 1977b PDF: 2; Yensen et al., 1977 PDF: 185; Wheeler & Wheeler, 1978b PDF: 396; Smith, 1979: 1466; Allred, 1982: 504; Hölldobler, 1985: 225; Wheeler & Wheeler, 1986g PDF: 95; DuBois & LaBerge, 1988: 150; MacKay et al., 1988: 120; Wheeler et al., 1994 PDF: 308; Bolton, 1995b: 342; Mackay & Mackay, 2002 PDF: 407; Coovert, 2005 PDF: 161; Ward, 2005 PDF: 30; Trager, 2013 PDF: 511 (redescription).
Polyergus breviceps is a species of ant endemic to the United States. It is a social parasite of other ants, namely of Formica gnava but also of Formica occulta and Formica argentea.[2] Polyergus is an inquiline parasite, having lost its ability to take care of its young and themselves. "The workers do not forage for food, feed the young or the queen, or even clean up their own nest".[3] To survive, Polyergus workers raid Formica nests to steal the pupae—which, once hatched, become workers of the mixed nest. This sort of relationship is not unique, of the approximately 8,800 species of ants, at least 200 have evolved some form of symbiotic relationship with one another.[3] What makes Polyergus special is the way a newly mated queen can, all by herself, take over a Formica nest and start a new colony.
Polyergus workers emerge from their nest (a mixed nest where Formica workers are already enslaved) and forage for a suitable raid target. If they find one, the scouts return to the nest, rally the other Polyergus, and head out in a raiding column. If the new target is a Formica nest, the Polyergus incite a "panic-flee" response from the Formica by releasing formic acid and take the Formica pupae back to their nest as slaves. Occasionally, a new queen goes on a raiding column with the other Polyergus. She then, after mating, leaves the column and forages for a suitable Formica nest.[3] Howard Topoff evaluated how the queen takes over a colony. After finding a Formica nest, she finds an entrance and is immediately attacked by Formica workers. The queen responds by biting with her sharp mandibles and releasing a pheromone from her enlarged Dufour's gland that, unlike many other parasitic ants, has a pacifying effect. The queen quickly searches for and locates the Formica queen and, with her adapted mandibles, proceeds to bite and lick various parts of the Formica queen for an average of 25 minutes.[4] "Within seconds of the host queen's death, the nest undergoes a most remarkable transformation".[3] The Formica workers cease to be aggressive to the intruder and start to groom the Polyergus queen as if it were their own.[3] The takeover now complete, the Polyergus queen gains not only a nest, but a worker caste as well. She then lays her eggs and the cycle continues.
Nearly all slave-making ants, including Polyergus, have mandibular adaptations that help them attack others.
Specific to Polyergus, when the queen first enters a Formica nest she releases a pheromone from her enlarged Dufour’s gland. Topoff did experiments to show that this pheromone has an important facilitative effect in colony usurpation; it reduces the aggression of the defending Formica workers. The researchers took the Dufour’s, pygidial, and poison glands from freshly mated Polyergus queens, using water as a negative control. These were crushed in distilled water to make a solution containing their extracts. Because it was impossible to prevent a Polyergus queen from secreting her own pheromones during a live encounter, they used the harvester ant Pogonomyrmex occidentalis (naturally attacked by Formica) as the subject. These ants were dipped in the above solutions, one ant/solution/test at a time, and placed in a petri dish with three Formica occulta "attackers". They were observed for three minutes to see for how long the Pogonomyrmex was attacked.[5] The results were as follows: "the mean duration of aggression by the Dufour’s gland treatment was 53.3 seconds...The mean duration of aggression for the water, pygidial gland, and poison gland controls were: 143.5 seconds, and 137.2 seconds and 132.2 seconds respectively".[5] Apparently, at some time, Polyergus queens evolved the capacity to passively facilitate colony takeovers.
Topoff and Ellen Zimmerli also did experiments to prove that the Polyergus queen "tricks" the Formica colony by obtaining chemicals from the Formica queen in the process of killing her. In one test, the Formica queen was killed (by flash freezing then thawing) prior to Polyergus contact. Even though it was already dead, the Polyergus queen bit, stabbed, and licked the queen just as if it were alive: and the Polyergus was consequently accepted by the colony.[4] Another test showed that if no Formica queen was present, then Polyergus had little chance of a successful takeover. Clearly, the Formica queen is providing some sort of chemical(s) to the Polyergus queen, however unintentionally. That takeover can occur even if the host queen is dead, but not if she is not present, proves that chemical absorption is important. The Polyergus queen needs only kill one host queen to be accepted. If the colony was polygynistic, the Polyergus queen can take her time finding the other queens. "Hour by hour, day by day, she methodically locates and kills every Formica queen, sometimes taking several weeks to clear out all remnants of opposition".[3]
Considerable work has been published on the evolution of ant dulosis and almost all of it has at least some empirical evidence to defend its positions. Notably, that nearly all raiding ant species have is an adaptation of their mandibles that make them big or piercing or both. It makes sense that the very thing that helps an ant be a better raider makes that same ant depend on others. The parasite loses the use of its mandibles for actual work. On this and other evidence, some propose that predation is the precursor to slavery.[6] It has also been suggested that colony multiplication by adoption and budding followed by temporary parasitism can lead to dulosis. While observations indicate that colony multiplication can lead to temporary parasitism, it is apparently very questionable that temporary parasitism leads to dulosis.[6]
In addition, about 100 years ago, Carlo Emery observed that, "The slave-making temporarily and permanently parasitic ants originate from closely related forms which serve them as host."[7] Jurgen Heinze did a series of experiments that prove this observation, known today as Emery's Rule. Rather than just relying solely on morphological cues, he used enzyme gel electrophoresis assays to create a detailed phylogenetic picture of host and parasite connections.[8] His experimental results support a loose version of Emery’s rule. On one end of the spectrum, they saw near identical electromorphs between species in most host and parasite pairs studied. However, it also appears that at least one example does not support Emery’s rule in that (Leptothorax paraxenus) differed from its host in several electromorphs.[8] The selective forces involved in the evolution of social parasitism are not fully understood, and it is unlikely that one model fits all relationships.
"Any hypothesis explaining the evolution of slavery in Polyergus must account both for the origin of group raiding with brood capture, and for non-independent colony foundation by queens".[9] Topoff gives an adequate hypothesis to the evolution of Polyergus by integrating three processes: queen takeover, olfactory imprinting, and territorial fighting.[9] Here are his main points. A free-living ancestor of Polyergus, that is a scavenger, would band together with others for intraspecific contest. Occasionally a queen invaded colonies of Formica. Initially the queen drove off the Formica queen and workers and appropriated the Formica pupae (he gives as example the queens of Formica wheeleri that do this now). The killing of the Formica queen and adoption by the workers evolved from this. His previous work (which was presented earlier) shows what would have evolved for adaptation to occur: "(1) sharp mandibles for killing the Formica queen, (2) a pheromone that reduces aggression from resident workers, (3) a tendency to hold onto the dead Formica queen long enough to absorb her odors...".[3] The next step was olfactory imprinting between the two species, a consequence of living one’s life from start to finish in the presence of both species. Polyergus’ ancestor then forages and encounters a colony of the same species. Again, they recruit nestmates to a territorial raid. In a brilliant insight, Topoff suggest this trait explains the raiding behavior. The Polyergus worker, raised by Formica as conspecifics, "identifies individual of Formica as belonging to its own species, another territorial raid is incited. Thus from the standpoint of Polyergus... slave raids are equivalent to territorial raids".[9] The captured brood is imprinted as well and they become workers in the original nest. As time went on "Our ancestral Polyergus could easily slide in the direction of facultative parasitism".[9] Eventually, Polyergus ancestors lost the ability to take care of themselves and become the inquiline Polyergus we see today.
Polyergus breviceps is a species of ant endemic to the United States. It is a social parasite of other ants, namely of Formica gnava but also of Formica occulta and Formica argentea. Polyergus is an inquiline parasite, having lost its ability to take care of its young and themselves. "The workers do not forage for food, feed the young or the queen, or even clean up their own nest". To survive, Polyergus workers raid Formica nests to steal the pupae—which, once hatched, become workers of the mixed nest. This sort of relationship is not unique, of the approximately 8,800 species of ants, at least 200 have evolved some form of symbiotic relationship with one another. What makes Polyergus special is the way a newly mated queen can, all by herself, take over a Formica nest and start a new colony.
