Polygynous Ants: Ecological Repercussions of a Multi-queen Colony

Many recent studies of polygynous ants have focused on producing an ecological and reproductive cost-benefit analysis of polygynous systems in order to shed light on how or why it developed over the more common practice of monogyny. Polygyny, the practice of keeping multiple reproductive queens in one colony, has evolved in many ant species, some of which display both polygynous and monogynous strategies. These species are of special interest as they provide important insights into the evolution of this social structure and the ecological pressures that may have caused its development. The following work will outline how polygyny may benefit ants with differing life history and habits, the reproductive risks and benefits, and how morphology and ecological pressures have influenced the social structure of ants. (Hymenoptera: Formicidae) PlantAnts and Polygyny Many ants have survived the various challenges of their environment by exploiting ecological niches and tying their survival strategy to another nearby species. The ant Petalomyrmex phylax displays an exclusive symbiotic relationship with the myrmecochorous plant, Leonardoxa africana ssp. africana (Dalecky et al., 2005). This symbiosis limits the ant’s dispersal to a narrow range where the plant can grow in the rainforest of Cameroon. This 250 x 80 km area contains a continuum of colonies ranging WARD 2 from highly polygynous in the northern end to exclusively mongynous in the southern end. Each tree can be host to a single long-term colony, though several founding colonist queens may temporarily reside on the same tree until they are eliminated by competitors. After mating, queens are faced with the option to seek acceptance in natal nests which may result in rejection, lowered reproductive rates, and shorter lifespan. However, the initiation into a polygynous colony comes with the benefits of nest inheritance for offspring gynes, and the queen’s ability to avoid the incredibly difficult and vulnerable life stages that are associated with independently founding a new monogynous colony. Dalecky et al. (2005) assumed that the variability in colony social structure could best be explained by examining differences in tree quality and density, available nest sites, and nest turnover in each geographic area. To their surprise, they could find no ecological differences that would explain the variation among populations, which suggested that the key to development of polygyny in this species can be found in their colonization strategy. Polygyny results in an evolutionary trade-off between dispersal and colony longevity. By this logic, it would follow that ants in newer, less stable colonies would concentrate energy on dispersal methods as they slowly spread into the southern areas of the rainforest. Meanwhile, in the northern region, already established colonies had the ability to focus less on dispersal habits, and more on colony longevity. Such is the case with many invasive and plant-ant species. The Mesoamerican acacia-ant is another prime example of how polygyny has evolved to help colonies protect food-rich bonanza resources for several generations (Kautz et al., 2009). Pseudomyrmex is a genus of ant that is now known to have one polygynous species Pseudomyrmex peperi living in the same geographic vicinity as their monogynous ancestors. When compared with other Pseudomyrmex species, peperi is a relatively poor colony founder as evidenced by its inability to dominate new ant-free hosts for longer than six months. Despite the significantly lower dispersal rates, peperi ants can maintain ownership of an established colony for over five years and typically aggregate into large, polydomous supercolonies (polydomous ants maintain several residences and allow for free flow between nests). Though each queen typically produces fewer offspring than their monogynous counterparts, the colony is highly invested in brood rearing, with 84% of the colony consisting of brood. In neighboring monogynous colonies, brood comprises only 60% of the colony, limiting the overall growth rate for these species. Supercolonies of the peperi species are thought to have developed from a single queen and successive WARD 3 adoption of new queens within the family line, thus creating a large group of closely related kin that can extend the colony territory by budding. Invasive Ant Species Monogynous colonies tend toward independent colony foundation by a single vulnerable queen, while polygynous ants have adapted to colony stress by splitting colonies often enough to deal with catastrophic damage and an ever-growing population. Frequent movement is more common with polygynous ants, which can benefit invasive species like the polydomous Pharaoh ant, Monomorium pharaonis (Buczkowski & Bennett, 2009). This species solely inhabits urban areas and uses polygyny as a response to frequent environmental stress and nest disturbance. Because they exhibit a low worker to queen ratio of 12.86, the species can reproduce without mating flights and by way of budding. These ants display a high amount of control over caste partitioning in bud nests and tend to bud based on the intensity of nest disturbance, the amount of available nest sites, and a preferred minimum colony size. This high-risk strategy can have considerable payoffs in an urban setting. The Pharaoh ant is just one example of many that shows how an effective dispersal and threat management system can give polygynous species a competitive edge in various environments. This is an important consideration when dealing with biological control agents and pest colonies within buildings. The odorous ant Tapinoma sessile is seasonally polydomous (Buczkowski & Bennett, 2008), highly polygynous, and capable of creating large supercolonies in urban environments. In natural environments, this species tends to form small, monodomous, single-queen colonies that are not particularly dominant over neighboring species (Buczkowski, 2010). This pattern of polydomy, polygyny, and frequent colony budding is common in invasive Argentine ants as well (Silverman & Brightwell, 2008). Linepithema humile is also a highly competitive and aggressive ant once introduced into urban areas. Because supercolonies often contain 16.3 queens for every 1000 workers and can stretch over large areas, treatment with insecticides and biological control agents can be problematic and ineffective. Invasive species often bud off into new colonies at the first sign of danger, isolating the newly relocated ants from mechanical disturbances and toxic baiting systems. WARD 4 While this strategy may give invasive ant species the advantage over many insecticide control measures, it can be a hindrance for some species. The red imported fire ant (RIFA), known as Solenopsis invicta, was first introduced into the United States in the early 1900’s and was established for years before the first polygynous colonies were reported in the 1970’s (Lofgren & Williams, 1984). In 1973, researchers became interested in a microsporidian pathogen, Thelohania solenopsae, for its potential as a biological control agent (Oi, 2006). Because of the high nest densities, infection can be maintained for long periods of time before colony death. This allows for continued transmission of the pathogen to neighboring colonies. A study of pathogen infection rates in Florida pastures in 2004 showed that 83% of polygynous colonies were infected compared with 0% of monogynous colonies. While monogynous colonies can transmit the pathogen through colony raids or nuptial flights, infected colonies only survive for five months and infection rates among other monogynous colonies are not high. Because polygynous colonies habitually accept multiple queens into the nest, queen death is staggered and multiple generations can survive, keeping the colony alive and active for an average of eleven months, or in some cases, more than two years. This trend makes biological control of the predominantly monogynous fire ants in North America impractical. This is one case that further emphasizes how polygyny can significantly extend the lifespan of a colony under stressful conditions. Risks and Reproductive Consequences of Polygyny Many species of wood ants and slave-making ants have been known to kill queens of a foreign ant nest and take over the colony as the new queen (Czechowski & Radchenko, 2006). By masquerading as the original queen, heterospecific gynes can engage in a form of social parasitism and take advantage of local colony resources. This strategy may provide one alternative to independent colony foundation, but many queens instead chose to seek acceptance into foreign colonies by non-hostile means. Studies of colony aggression have shown that for Formica truncorum, Formica exsecta, and Formica paralugubris, virgin queens have a higher chance of acceptance in foreign and natal nests whereas for Formica polyctena, all older queens were accepted regardless of mating status or nest of origin (Holzer et al., 2008). Still, not all ant species are quick to accept social parasites into their society. Pachycondyla WARD 5 luteipes colonies reject 70% of foreign queens while Leptothorax curvispinosus colonies reject all foreign queens. In addition to initial danger, polygynous queens are faced with reproductive drawbacks and shorter lifespans. Holzer et al. (2008) reported that foreign queens accepted into Formica exsecta colonies contributed fewer offspring to the colony while queens in their natal nest produced 1.5 times the number of viable offspring in 80% of tested colonies. One theory that may explain the differences in offspring outputs is Hamilton’s theory of kin selection, where ant workers provide increased care for the brood of their maternal queen. Another idea that has yet to be confirmed is that variations in fecundity may be due to stress from settlement or inexplicit differences in treatment by resident workers. While the study could not produce evidence of nepotism among workers, it could not rule out the possibility that workers provided fewer resources to newly introduced queens, or that introduction itself did not directly affect queens in terms of fecundity. Reproductive benefits of polygyny may also be limited by reproductive specialization (Kummerli & Keller, 2007). In colonies of Formica exsecta, 25% of queens in female-producing colonies specialized in reproduction of males compared with 79% of queens in male-producing colonies. Many species of ants show some amount of reproductive skew among ant classes including Solenopsis invicta, Formica sanguinea, Pheidole pallidula, Leptothorax acervorum, Linepithema humile, Leptothorax regulates, and Pachycondyla inversa. With S. invicta, a positive correlation was established between queen specialization and queen number, suggesting that some queens in polygynous colonies have a better chance of passing down their genetic material. Queens that produce more haploid males (containing 1 set of chromosomes passed down by the mother through an unfertilized egg) and diploid gynes (containing one set of chromosomes from each parent) are more likely to pass down their genetic lines than queens that specialize in non-reproductive female workers. Studies on the neotropical ant species Pachycondyla inversa show that polygynous colonies have dominant queens, that are mostly responsible for caring for the brood, as well as subordinate queens that forage outside the nest (Kellner et al., 2007). Differences in reproductive rate between subordinate and dominate queens decrease as colony size increases, but are still generally present. Because queens are usually unrelated and frequently mate with more than one male, these polygynous colonies can reap the benefits of genetic variation. This low level of relation WARD 6 between nestmate workers raises the question of how polygynous colony members can recognize closely related foreign species from weakly related nestmates.