Polyergus breviceps

Polyergus breviceps is a social parasite that specializes in raiding other nests for their immature brood. It is one of five species that make up the genus, which is distributed across the Northern Hemisphere, in Europe (Polyergus rufescens Latreille), Japan and Eastern Siberia (Polyergus japonica Yano), North America (Polyergus breviceps Emery, Polyergus lucidus Mayr) and the former Soviet Union (Polyergus nigerrimus Marikovsky). The brood from raided nests is brought back to the slave-maker nest, where it matures and then carries out all the foraging, nest cleaning, and brood care tasks of the slave-maker nest. P. breviceps establish new colonies by taking over colonies of their host species. After entering a nest, a newly mated P. breviceps queen fatally attacks the resident queen, at which point the workers of the nest almost immediately begin grooming the social parasite. This dramatic change in host worker behavior suggests that there is a change in parasite queen chemistry that helps her to be adopted by the host workers. Examination of chemical profiles from newly mated P. breviceps queens before and after having killed a Formica queen revealed that 'killer queens' developed a hydrocarbon profile that was virtually identical to the queen profile of species killed. (For more information see below.)

Classification: Formicidae: Formicinae: Formicini: Polyergus breviceps, Formica occulta, Formica gnava

Southwestern Research Station

http://research.amnh.org/swrs/

Much of my work with Polyergus was completed at the Southwestern Research Station in the Chiricahua Mountains of southeastern Arizona, where researchers are provided with laboratory space, access to a variety of supplies for experimental chamber construction, and access to a well-equipped technical equipment laboratory (TEL). Communal living quarters and designated mealtimes foster useful discussions among graduate students, research assistants, volunteers and established scientists. Evening seminars allow students to present their work to a knowledgeable though (sometimes) not overly critical audience. In turn, students are exposed to research that has been ongoing for 20 years or more. Most importantly, there is generally an abundance of eager beavers who want to go into the field and experience fieldwork. These individuals tend to be bright and energetic undergraduates from all over the world that are willing to volunteer their dishwashing, food-preparing, rock-pile moving talents for the opportunity to experience nature and work with researchers. The volunteers have provided me with an invaluable service, even if most decline a second day of turning over large rocks in search of Formica nests and then digging them up. At 1646 meters in elevation, 15.5 kilometers west of the New Mexico boarder, and 46.6 kilometers north of the Mexico border. SWRS is located in the Upper Sonoran Life-zone, where much of the dominant vegetation is alligator juniper (Juniperus deppeana), scrub oak (Quercus turbinella), and pinyon pine (Pinus edulis).

Chiricahua Mountains

The Chiricahua Mountains are just one home to Polyergus breviceps Emery, the western representative of two Polyergus species found in North America; Polyergus lucidus Mayr, the second species, is restricted primarily to the eastern half of United States. The Chiricahuas reach about 3048 meters and are particularly species rich and diverse as the range serves as a shelter from the surrounding deserts for northern ranging southern species (of plants and animals) and southern ranging northern species. Mountain ranges that are separated from other mountain ranges by desert often are biotically distinct islands, as they are isolated from similar plants and animals. Weldon F. Heald, from whom AMNH purchased the Painted Canyon ranch and much of the land that now constitutes the research station in 1954, appropriately referred to the Chiricahua Mountains as "Sky Island".

Polyergus breviceps

Ripe for the killing? -The purpose of my Ph.D. studies was to elucidate processes underlying three stages of colony founding in Polyergus breviceps. Alate Polyergus breviceps queens are recruited along with their non-reproductive nestmates to a target Formica nest. The young queens mate en route and enter the raided Formica nest soon after arriving. Presumably, in nature as in laboratory tests, the newly mated P. breviceps queens subsequently find the resident Formica queen and kill her. The objective of my first experiment was to identify the cue that elicits aggression from P. breviceps queens. Providing newly mated Formica gnava queens with a supply of immature and mature offspring did not contribute to the emergence of aggressive behavior in the newly mated P. breviceps queen. Examination of cuticular hydrocarbon profiles of newly mated and mature F. gnava queens revealed no qualitative differences in hydrocarbon constituents or in relative proportions, and are unlikely to produce this kairomonal effect.

Evidence of murder - Once the invading P. breviceps queen ceases attacking the resident Formica queen, resident Formica workers begin grooming the P. breviceps queen. My second investigation examined whether the P. breviceps queen acquires some Formica queen chemicals that are critical to nestmate recognition and queen attractiveness during these aggressive interactions. Profiles of cuticular hydrocarbons changed significantly after a P. breviceps queen had killed a Formica queen, and the change was specific to the species of Formica queen killed.

Do these eggs smell funny? - The third and fourth experiments were designed to ascertain whether Formica are inclined to rear immature P. breviceps pupae and eggs respectively, and whether patterns of surface hydrocarbons could be correlated with the rearing tendencies of Formica. Because Polyergus evolved from Formica or both evolved from a common ancestor, it is often presumed that Polyergus shares chemical characteristics with Formica and may thereby avoid being rejected by Formica. Aggression on the part of Formica against invading Polyergus queens and between workers of both genera indicates that for adults this is not true. Past research has indicated that Formica have a natural tendency to care for P. breviceps pupae. However, if post-pharyngeal chemicals, which are considered a source of nestmate recognition cues, are transferred among individuals through allogrooming, the tendency of Formica to rear breviceps pupae may reflect a response to conspecific cues. By observing Formica responses to Polyergus pupae taken from nests with different slave species, I concluded that Formica were not necessarily inclined to rear P. breviceps pupae. Thus, using pupae in adoption test was not necessarily reflecting the mechanism underlying the integration of P. breviceps offspring during colony takeover by a P. breviceps queen. When Formica were provided with breviceps or heterospecific Formica eggs, surprisingly they rejected almost all of them. Less than 3% of all heterospecific eggs were reared to adulthood, including those that were presented to newly enslaved Formica. And, newly enslaved Formica did not begin rearing breviceps eggs in laboratory tests until 5 – 6 months after having been taken as slaves, suggesting that a time-dependent modification in Formica, the Polyergus queen (and her eggs), or both is involved in adopting P. breviceps offspring.

Polyergus: General Biology & Collecting Methods

Polyergus breviceps are found on the station grounds down in elevation to the base of the mountains just shy of the desert of the Lower Sonoran Life-zone, where they enslave Formica gnava Buckley, and up in elevation to the Canadian Life-zone, where they enslave Formica occulta Francoeur. P. breviceps begin their slave-raiding forays sometime in May, and by June they are raiding daily. SWRS populations begin raiding between 15:00 - 19:00 hours. Raids by the Barfoot populations (el. 2750 m) begin as early as noon and cease around 16:00 hours. Alate females first appear at nest entrances and in raiding columns at the end of June, and become increasingly abundant once the monsoon season began in early July. Reproductives continue to be produced throughout July and some colonies produced alates as late as August 10th. Polyergus is protandric: males began appearing at nest entrances slightly earlier in season and in time than female reproductives. Mating of P. breviceps reproductives on slave raids makes collecting newly mated P. breviceps queens very easy, and sometimes 25 newly mated queens were collected at a raided nest on a single day. Collecting newly mated F. gnava queens is much more unpredictable. There are no known reports of their mating swarms and during my six years in the field I have never observed one. The majority of newly mated F. gnava queens collected during 1995 - 1998, were found on one day of each summer, between the hours of 13:00 to 15:30 after several days of medium rainfall or one day of very heavy rainfall of the seasonal rains. Newly mated occulta queens were similarly found by searching the ground once the monsoon rains began.

Nesting Sites

Mixed nests of P. breviceps/F. gnava and of P. breviceps/F. occulta are relatively uncommon and their nesting sites are somewhat non-descript, found under logs, large rocks, or just a bunch of grass. Disturbance of a mixed-species nest generally produces a defense swarm of only Formica, and the Polyergus remain below, deep within the ground. Polyergus nests are more frequently found by stumbling across a slave-raid. Over the years members (immatures, workers, reproductives, slave-makers, slaves) from ten colonies of P. breviceps/F. gnava and from four colonies of P. breviceps/F. occulta were collected for use in experiments in addition to new colonies I created in the laboratory. F. gnava and F. occulta nests, on the other hand, are much more common and generally easier to find, particularly the larger, polygynous nests, which are often disclosed by the Formica activity surrounding the nest. Smaller nests are less obvious, and raiding Polyergus often reveal their whereabouts. Providing a forager with food and following it back to its nest is also useful method for finding nests. Using both methods, I was able to get an idea of the general density of F. gnava nests in 100 X 100-meter area around a large P. breviceps nest.