Polyergus breviceps

A midwestern and western North American species that is common in wet to mesic prairies and mesic or wetter old fields.

Identification
Trager (2013): Polyergus breviceps is here restricted to a broad-headed and short-limbed species, one most easily distinguished by its abundant pilosity. In addition to its abundant pilosity and short scapes, it shows marked preference for Formica cinerea group hosts and their typically open, moist grassland or sedge meadow habitat. More pilose examples of Polyergus mexicanus found both at the northern and southwestern portions of its range are those most likely to be confused morphologically with breviceps. Where mexicanus and breviceps occur together (in close parapatry) in the West, mexicanus occurs in well drained soils, often in conifer forests on podzols, while breviceps occurs in wet meadows with organic-rich soils. Even when its pronotal and mesonotal dorsal pilosity is abundant, P. mexicanus lacks the pilosity on the sides of the pronotum, lacks or at least has little vertex pilosity, has a shinier head, and is deeper red in color, with gray rather than yellowish gray pubescence. In the Chicago region, breviceps is readily distinguished, even in the field, from the local version of mexicanus by its clearly smaller size, and its association with Formica montana, contrasting with mexicanus’s larger size, and association with Formica subsericea. In the Dakotas and Rockies, a closer examination may be required to discriminate breviceps from other Polyergus, though these other congeners are more often found in upland prairie, open woodland or forest, rather than the usually moist (including saline and alkaline) meadow habitats preferred by breviceps. True breviceps does not occur in Pacific Coast states or provinces.

Distribution
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.

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States. Neotropical Region: Mexico.

Biology
Trager (2013): The majority of the literature regarding P. breviceps regards either mexicanus or topoffi (Topoff 1982, 1985, Topoff et al. 1984, 1985a, 1985b, 1988a, 1988b, 1989, Topoff and Greenberg 1988, Topoff 1990, Topoff and Mendez 1990, Topoff and Zimmerli 1993, Zimmerli and Topoff 1994, and included references), and true breviceps is in fact little studied.

The raiding of P. breviceps follows familiar patterns described for other species. 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. Polyergus 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. 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.

List of Known Hosts
This species is known to enslave the following species:

Nomenclature

 *  breviceps. Polyergus rufescens subsp. breviceps Emery, 1893i: 666 (w.) U.S.A. Raised to species: Kannowski, 1956: 185; Wheeler, J. 1968: 163. Senior synonym of bicolor, umbratus: Wheeler, J. 1968: 163. Material of the unavailable names fusciventris, montezuma, silvestrii referred here by Creighton, 1950a: 559. See also: Hölldobler, 1985: 225.

Trager (2013): Material of the unavailable names fusciventris, silvestrii, montezuma, referred to breviceps by Creighton (1950), here referred to P. bicolor, P. mexicanus, and P. topoffi.

Type Material


Trager (2013): As indicated in the synonymy above, the “type series” of breviceps included the original Breckenridge, Colorado material, plus two samples collected later at other localities that I identify as bicolor and mexicanus. I restrict the type to the Breckenridge material (CASENT0179559), and exclude the other samples, one being a sample of bicolor (CASENT0179561) and the other a sample of mexicanus (CASENT0179560).

Worker
Trager (2013) - Former syntypes (N=3 on a single pin, including lectotype) HL 1.36–1.40 (1.37), HW 1.32–1.36 (1.35), SL 0.90–0.96 (0.92), ½ VeM 3–6 (5), ½ PnM 12–20 (16), WL 1.96–2.02 (1.99), GL 1.64–1.88 (1.77), HFL 1.32–1.36 (1.35), CI 97–100 (98), SI 66–71 (68), HFI 97–103 (101), FSI 138–53 (147), LI 3.32–3.40 (3.37), TL 4.96–5.28 (5.14).

(N=58) HL 1.24–1.60 (1.46), HW 1.28–1.64 (1.48), SL 0.84–1.08 (0.98), ½ VeM (3, one specimen) 5–17 (9.52), ½ PnM 8–22 (14.97), WL 1.88–2.40 (2.14), GL 1.64–2.60 (2.13), HFL 1.30–1.72 (1.52), CI 0.96–1.08 (1.01), SI 61–78 (67), HFI 94–115 (103), FSI 138–170 (155), LI 3.12–3.96 (3.60), TL 4.96–6.52 (5.73).

Polyergus breviceps is more narrowly defined here than has been conventional in North American ant taxonomy. It 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.

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.

Etymology
Trager (2013) - Emery coined this name from the Latin “brevis” plus “-ceps” to mean short-headed (in contrast to the more elongate head shape of rufescens).