F. sanguinea, are facultative slave-makers, meaning colonies can live either alone or be . This allows them to be a good to study the origins of ant slave-making. A fertilized F. sanguinea will enter the of the host ant species and kill their queen. She then takes advantage of the workers who tend to her and her . F. sanguinea workers will also raid nearby nests, stealing and to become future workers for F. sanguinea. The raids are also not exclusively for acquiring slave workers but are sometimes predation events.
F. sanguinea has not been observed to have division of labor in which certain individuals raid or forage.
This excludes them from the groups of . However, some individuals possess more compounds than others, which presumably would be more successful in slave raids.
Formica sanguinea uses scouting individuals to locate the nest that will be parasitized. Once a nest has been scouted, the raid will happen. The activities that go on during a raid are composed of digging and fighting at the target nest. Both slaves and slave-makers are observed to be carrying the brood back to the slave-maker nest. If a member of the colony is killed in the war, individuals will take that carcass back to the nest to be eaten later. Raids were also never observed on rainy or overcast days. This is thought to be because of the detriment rain might have on terrestrial conditions and the effectiveness of .
Most colonies are , meaning they have only one queen. F sanguinea are commonly , meaning the singular queen mates with more than one male. In studies observing this, 70% of queens mated with multiple males.F. sanguinea is observed to have higher levels of polyandrous than any other Formica species. However, when colonies engage in polygyny, having more than one queen, the rates of polyandry drop. It is assumed that colonies utilize either polyandry or polygyny, to increase the of the colony. Therefore, two different strategies can be employed. One strategy is that a female mates with many males and then disperses to start a new colony. The second strategy is for a female to mate with a fewer number of males, and stay close to the natal colony with other females who have mated with a fewer number of males. In the case of polygyny, it has been observed that there is one dominating queen who is more sexual reproductive than the others. Formica also is observed to have a paternity skew, meaning that the mated males offspring are not represented equally in the population. Whether this is because of internal selection or is unclear.