Persistent cooperation between unrelated conspecifics rarely occurs in mature eusocial insect societies. In this dissertation, I present evidence of non-kin cooperation in the Nearctic honey ant Myrmecocystus mendax. Using microsatellite markers, I show that mature colonies in the Sierra Ancha Mountain of central Arizona contain multiple unrelated matrilines, an observation that is consistent with primary polygyny. In contrast, similar analyses suggest that colonies in the Chiricahua Mountains of southeastern Arizona are primarily monogynous. These interpretations are consistent with field and laboratory observations. Whereas cooperative colony founding was observed frequently among groups of Sierra Ancha foundresses, founding in the Chiricahua population was restricted to individual foundresses. Furthermore, Sierra Ancha foundresses successfully established incipient laboratory colonies without undergoing queen culling following emergence of the first workers. Multi-queen laboratory Sierra Ancha colonies also produced more workers and repletes than haplometrotic colonies, and when brood raiding was induced between colonies, queens of those with more workers had a higher survival probability.

Microsatellite analyses of additional locations within the M. mendax range suggest that polygyny is also present in some other populations, especially in central-northern Arizona, albeit at lower frequencies than that in the Sierra Anchas. In addition, analyses of multiple types of genetic data, including microsatellites, the mitochondrial barcoding region, and over 2000 nuclear ultra-conserved elements indicate that M. mendax populations within the southwestern U.S. and northwestern Mexico are geographically structured, with strong support for the existence of two or more divergent clades as well as isolation-by-distance within clades. This structure is further shown to correlate with variation in queen number and hair length, a diagnostic taxonomic feature used to distinguish honey ant species.

Together, these findings suggest that regional ecological pressures (e.g. colony density , climate) may have acted on colony founding and social strategy to select for increasing workforce size and, along with genetic drift, have driven geographically isolated M. mendax populations to differentiate genetically and morphologically. The presence of colony fusion in the laboratory and life history traits in honey ant that are influenced by colony size, including repletism, brood raiding, and tournament, support this evolutionary scenario.