Pogonomyrmex

These ants are common New World ants that are found in arid habitats. Most species are seed harvesters, others feed on a variety of items, especially dead or dying insects. Large quantities of seeds may be stored in their nests. Most species have a large psammophore or beard of coarse hairs on the ventral surface of the head, which are used to carry fine sand during the excavation of the nest. Large mating flights occur in late summer, usually after a rain on the previous day (Mackay, 1981). Some species build large, conspicuous mounds and clear the vegetation from around the nest. Most species in this genus can deliver very painful stings. (Mackay and Mackay 2002)

Identification
There is currently one species known from Central America (i.e., as defined beginning south of Mexico; Taber's key to North America species includes Mexican species): Pogonomyrmex guatemaltecus. This ant is presently known from Guatemala, El Salvador and Mexico. There are three validly named species from the Caribbean. All of these species are Hispaniola endemics and all are considered to be in the subgenus Ephebomyrmex (see this key couplet: 11E).

Pogonomyrmex species groups

Distribution
Johnson and Cover (2015) - The seed harvester ant genus Pogonomyrmex is a moderate-sized New World group that currently consists of 68 described species that occur as three disjunct faunal groups, one in North America, one in South America, and one on the Caribbean island of Hispaniola (containing Haiti and Dominican Republic); no species are common to any two areas. North American species of Pogonomyrmex were the focus of a revision by Cole (1968), which set the stage for numerous studies of ecology, biogeography, territoriality, mating behavior, communication, caste determination, and foraging behavior that have greatly facilitated our understanding of ant biology (Anderson et al. 2006; Gadau et al. 2003; Gordon & Kulig 1996; Hölldobler 1976a, 1976b; Johnson 2000, 2001, 2002, 2006; Taber 1998).

Biology
The genus Pogonomyrmex is the preeminent group of harvesting ants in North America, where it nearly blankets the arid regions of Mexico and the western United States. As is true of other harvesting ants, the workers of this genus collect seeds for food, "harvesting" the plants in their nesting areas by snipping off the seeds with their mandibles. Husked within the nest and stored in subterranean or mound chambers, these highly nutritious seeds become the paramount food for the society, sustaining the ants even through the winter. Seeds are not their sole food, however, because the ants are scavengers as well as harvesters. Although their victims are chiefly arthropods, the workers may transport a varied array of dead booty to their nests.

The nests themselves are constructed in the soil, generally in areas fully exposed to the sun. Some are beneath stones, whereas others are surmounted by soil craters or by small to huge mounds with or without coverings of gravel. In addition, the workers of some species alter the area peripheral to the nests by clearing away the plants-felling them, bit by bit, with their powerful mandibles. Such mounds with their surrounding denuded area, characteristic of the western harvester, are familiar sights to travelers in the arid West.

Although the workers of some species of Pogonomyrmex are extremely hostile and will vigorously attack and sting an invader, other species are contrastingly inoffensive and retiring. The effects of a sting can be very painful. Localized swelling and inflammation ensue rapidly. Soon thereafter a throbbing pain, which may last for several hours, extends to the lymph nodes of the inguinal, axillary, or cervical area, depending on the location of the sting. Frequently, the skin around the wound becomes very moist. Multiple stings will produce excruciating pain and may induce systemic disturbances of considerable severity. Victims who are hypersensitive to the ant venom may collapse from anaphylactic shock and be in grave danger if not given prompt medical attention.

Considerable economic importance has been attached to this genus not only because of the stinging characteristics but also because some of these harvesters may procure seeds in abundance from cultivated crops and may also damage range lands when their nests are numerous. In the United States four species are so outstanding in these respects that the Entomological Society of America has approved common names for them. These are: the western harvester ant (Pogonomyrmex occidentalis), the red harvester ant (Pogonomyrmex barbatus), the California harvester ant (Pogonomyrmex californicus), and the Florida harvester ant (Pogonomyrmex badius).

Bob Johnson has been studying Pogonomyrmex for many decades. The following provides a summary of some of the more intriguing aspects of Pogonomyrmex biology. These topics have been studied in the past and deserve further examination both to better understand Pogonomyrmex and ants more generally.

Worker morphology
Worker polymorphism is relatively rare among ants, having evolved approximately 15–20 times in the more than 300 extant ant genera (Bolton, 2014; Brady, Schultz, Fisher, & Ward, 2006; Oster & Wilson, 1978). Despite the relative rarity of this trait, polymorphism has evolved two times within Pogonomyrmex, once in South America (Pogonomyrmex coarctatus) and once in North America (Pogonomyrmex badius), and both species produce supermajors (Cole, 1968; Kusnezov, 1951). Head width for P. badius ranges from 1.27–3.74 mm, while that for P. coarctatus ranges from 1.97–3.99 mm. In addition, the size-frequency distribution is bimodal in P. badius, with nests containing numerous minors and few majors, with intermediate-sized workers being rare. Alternatively, the distribution is continuous in P. coarctatus, with nests containing numerous minors, and abundant medias and majors (Tschinkel, 1998, C. Smith, unpub. data). Majors of the two species also concentrate in different parts of the nest and appear to specialize at different tasks; P. badius majors occur deeper in the nest and rarely forage, leading to the suggestion that these individuals are seed-milling specialists (direct data are meager to support this hypothesis), while those of P. coarctatus concentrate in upper parts of the nest and appear to be foraging specialists (Tschinkel, 1998, C. Smith, unpub. data). These differences suggest that different evolutionary pressures selected for polymorphism in these two species.

The ecological release hypothesis has been suggested to explain the evolution of polymorphism in P. badius and other species of ants (Oster & Wilson, 1978). This hypothesis suggests that polymorphism evolved to expand dietary breadth in species that experience reduced interspecific competition (P. badius is the only large harvester ant in the eastern United States). However, this hypothesis was not supported for P. badius because workers did not display a significant correlation between worker size and seed or prey size harvested (Ferster & Traniello, 1995; Traniello & Beshers, 1991). Neither does the ecological release hypothesis apply to P. coarctatus, which is sympatric with two to four congeners throughout its geographic range (this study). One commonality between P. badius and P. coarctatus is that both species live in mesic, low-elevation habitats where resources appear to be abundant (pers. obs.; C. Smith, pers. comm.). Moreover, factors selecting for the evolution of polymorphism in these two species remain unknown, but mechanisms that determine caste (at least in P. badius) involve an interaction of nutrition, social environment, and genetics (Rheindt, Strehl, & Gadau, 2005; Smith, Anderson, Tillberg, Gadau, & Suarez, 2008).

Variation in queen phenotypes
Species of Pogonomyrmex display most of the wide array of queen phenotypes known among ants, including species with alate queens, brachypterous queens, and ergatoid queens. In some species queens are dimorphic, i.e., winged and ergatoid queens or brachypterous and ergatoid queens within the same species (Heinze et al., 1992; Johnson et al., 2007; Peeters, 2012; Peeters et al., 2012). The most striking pattern related to queen phenotypes in Pogonomyrmex is the large number of South American species that have only ergatoid queens (five species in three species-groups; Pogonomyrmex bispinosus [bispinosus-group], Pogonomyrmex mayri [mayri-group], and Pogonomyrmex cunicularius, Pogonomyrmex pencosensis, and Pogonomyrmex serpens [cunicularius-group]), whereas no species with only ergatoid queens occur in North America. The most worker-like ergatoid queen species are those in the P. cunicularius-group, in which queens and workers are morphologically similar (the mesosomal segments are fused as in workers), except that queens are slightly larger, possess ocelli and a spermatheca (lacking in workers), and contain more ovarioles than workers (Johnson, 2010). The ergatoid queens of P. mayri appear to lack ocelli. The sister genus Hylomyrma (Ward et al., 2015) also has several species with ergatoid queens (Kempf, 1973). Ergatoid queens are absent in the P. angustus-group, which consists of three species that will be placed in a new genus (Ward et al., 2015). Poor dispersal ability likely precluded migration to North America by species that possess only ergatoid queens.

Queen dimorphic species occur in both North and South America. Little is known about mechanisms that cause queen dimorphism, but a mitochondrial phylogeny and a comparison of microsatellite allele frequencies in the North American species Pogonomyrmex pima inferred that the two queen phenotypes (alate and ergatoid) belong to the same gene pool (Johnson et al., 2007). Ergatoid queens of both P. pima and Pogonomyrmex imberbiculus (another queen dimorphic species) possess all the reproductive structures of alate queens (except for wings), and both phenotypes have a similar reproductive capacity; this same pattern is probable for Pogonomyrmex naegelii, but dissections have not be done.

In another type of queen dimorphism, the South American species Pogonomyrmex laticeps produces ergatoid and brachypterous queens; so far, only one queen phenotype has been found in a colony (Peeters et al., 2012). Current data also suggest that ergatoid and brachypterous queens have discrete geographic ranges—ergatoid queens are only known from southern portions of the range, whereas brachypterous queens are only known from northern portions of the range. In all queen dimorphic species, ergatoid queens are intermediate in size between that of worker and alate/brachypterous queens (Heinze et al., 1992; Johnson et al., 2007; Peeters et al., 2012).

Queens of Pogonomyrmex are also known for their dramatic variation in body size. Queen dry mass varies by approximately 20-fold across species in both North and South America, from approximately 1.5 mg dry mass (P. pima and P. imberbiculus in North America, P. naegelii in South America) to those that approach 30 mg dry mass (Pogonomyrmex wheeleri in North America, Pogonomyrmex coarctatus in South America) (unpub. data).

Mating frequency
In social insects, obligate multiple mating (effective queen mating frequency >2) is only known to occur in vespine wasps (Foster & Ratnieks, 2001), the leaf-cutter ant genera Acromyrmex and Atta (Villesen, Murakami, Schultz, & Boomsma, 2002), army ants (Denny, Franks, Powell, & Edwards, 2004; Kronauer, Johnson, & Boomsma, 2007), honeybees (Moritz et al., 1995; Palmer & Oldroyd, 2000), the ant Cataglyphis cursor (Pearcy, Aron, Doums, & Keller, 2004), and harvester ants in the genus Pogonomyrmex (Gadau, Strehl, Oettler, & Hölldobler, 2003; Overson, 2011; Pol et al., 2008; Rheindt, Gadau, Strehl, & Hölldobler, 2004; Wiernasz, Perroni, & Cole, 2004). Multiple mating is widespread in species of Pogonomyrmex as it has been documented in both North America (Pogonomyrmex barbatus, Pogonomyrmex rugosus, Pogonomyrmex badius, Pogonomyrmex californicus, Pogonomyrmex occidentalis) and South America (Pogonomyrmex coarctatus, Pogonomyrmex inermis, Pogonomyrmex mendozanus), suggesting that this trait evolved early in the history of the genus (Pol et al., 2008). In addition, both alate and ergatoid queens of Pogonomyrmex pima mate with only one male, such that Pogonomyrmex is the only ant genus known to contain species with both single- and multiple-mating queens (Holbrook, Strehl, Johnson, & Gadau, 2007). Consequently, Pogonomyrmex is an ideal genus to examine the evolution of mating frequency

Colony founding
North American species of Pogonomyrmex exhibit a range of colony founding strategies (Johnson, 2002, 2004, 2006, 2010), and available data suggest that those in South America display an equally wide range of strategies (Johnson, 2010; Kusnezov, 1951). There are several species with ergatoid or brachypterous queens that are known to employ independent colony founding (e.g., Pogonomyrmex cunicularius, Pogonomyrmex pencosensis, Pogonomyrmex mendozanus, and both ergatoid and brachypterous queens of Pogonomyrmex laticeps). This is unusual because ergatoid and brachypterous queen species typically reproduce via dependent colony founding, i.e., with a group of nesmate workers (Peeters, 2012; Peeters & Molet, 2009). Dependent colony founding is associated with producing few queens, short-distance dispersal on foot, and relatively high survival rates. Alternatively, independent colony founding is associated with producing many queens, long-distance dispersal via wings, and very low survival rates.

The few data available indicate that these four ergatoid and brachypterous queen species are exceptions to these two reproductive syndromes. Haplometrotic foundresses of the ergatoid queen species P. cunicularius and P. pencosensis have been excavated (n = 1 for each species) (Johnson, 2010), and colonies typically produce >100 queens (pers. obs.). Haplometrotic foundresses have also been excavated for ergatoid (n = 1) and brachypterous queens (n = 1) of P. laticeps, and colonies of both queen phenotypes produce >50 queens (Peeters et al., 2012). Brachypterous queens also have been observed to employ independent colony founding in P. mendozanus, in which colonies can also produce approximately 100 queens (pers. obs.). More data are needed to better document nest founding strategies in these and other ergatoid queen species of Pogonomyrmex.

These species clearly provide an opportunity to examine previously unknown tradeoffs in nest founding syndromes. All of these species probably employ queen foraging, which allows queens to adjust brood production to environmental conditions. When food is high in abundance and/or predictable in occurrence, foraging queens can produce more minim workers than fully claustral queens that have fixed energy reserves (Johnson, 2006). A granivorous diet has been proposed as one explanation—many arid and semi-arid habitats, such as those occupied by species of Pogonomyrmex, experience relatively predictable pulses of seed production that result from seasonal rainfall. These seeds can remain dormant in the seed bank for several or more years (Lopez, 2003; Reichman, 1984; Tevis, 1958), and have the additional advantage that they can be stored for extended periods by the ants. However, seed use may not be the only explanation because these four species have different diets and two of them, P. pencosensis and probably P. cunicularius, harvest more insects than seeds (Aranda-Rickert & Fracchia, 2011, 2012).

Nomenclature

 *  POGONOMYRMEX [Myrmicinae: Myrmicini]
 * Pogonomyrmex Mayr, 1868b: 169. Type-species: Formica badia, by subsequent designation of Wheeler, W.M. 1911f: 170.
 * Pogonomyrmex senior synonym of Ephebomyrmex, Forelomyrmex (and the junior homonym Janetia Forel): Lattke, 1991a: 305; Bolton, 1994: 106; Bolton, 1995b: 28; Fernández & Palacio, 1998: 1649; Bolton, 2003: 223.
 * EPHEBOMYRMEX [junior synonym of Pogonomyrmex]
 * Ephebomyrmex Wheeler, W.M. 1902c: 390 [as subgenus of Pogonomyrmex]. Type-species: Pogonomyrmex naegelii, by subsequent designation of Wheeler, W.M. 1911f: 163.
 * Ephebomyrmex subgenus of Pogonomyrmex: Wheeler, W.M. 1910g: 140; Emery, 1921f: 48; Wheeler, W.M. 1922a: 660; Gallardo, 1932b: 94; Kusnezov, 1949c: 293; Creighton, 1950a: 132; Kusnezov, 1951a: 245; Smith, M.R. 1951a: 794; Kusnezov, 1956: 118; Smith, M.R. 1958c: 115; Kusnezov, 1964: 56; Smith, M.R. 1967: 351; Cole, 1968: 22; Smith, D.R. 1979: 1357; Mackay, Mackay, Dominguez, et al. 1985: 42.
 * Ephebomyrmex as genus: Creighton, 1957a: 54; Kusnezov, 1960b: 350; Kempf, 1972a: 106; Brown, 1973b: 180; Snelling, R.R. 1981: 395; Wheeler, G.C. & Wheeler, J. 1985: 257; Dlussky & Fedoseeva, 1988: 79; Hölldobler & Wilson, 1990: 13; Taber, 1998: 146.
 * Ephebomyrmex junior synonym of Pogonomyrmex: Lattke, 1991a: 305; Bolton, 1994: 106; Bolton, 1995b: 28; Fernández & Palacio, 1998: 1649; Bolton, 2003: 223.
 * FORELOMYRMEX [junior synonym of Pogonomyrmex]
 * Forelomyrmex Wheeler, W.M. 1913a: 80 [as subgenus of Pogonomyrmex]. Replacement name for Janetia Forel, 1899c: 61 (footnote). [Junior homonym of Janetia Kieffer, 1896: 236 (Diptera).]
 * Forelomyrmex raised to genus: Taber, 1990: 307.
 * Forelomyrmex junior synonym of Pogonomyrmex: Lattke, 1991a: 305; Bolton, 1994: 106.