Forced associations by young queens of the harvester ant Messor semirufus during colony founding

After landing at the end of their nuptial flight, young queens of the harvester ant Messor semirufus search for a suitable nesting site and dig a burrow. After 3 months in the burrow, they start laying eggs, and nurse their first brood of workers alone. Field observations indicate that a few newly dug burrows contain more than one queen. Laboratory experiments were conducted in order to discover why these young queens’ associations form. We found that groups do not exhibit any productive advantage over single-founding queens, either with respect to progeny number, or with respect to the time until the first eggs are laid. Groups have a slower rate of nest digging than single queens, and mortality rate is considerably higher for queens in groups than for single queens. From the initiation of the group, queen interactions involve aggression and a behavioral hierarchy, with a prior-residence advantage. The tendency to form groups is stronger if queens density is greater and if digging conditions, characterized by soil hardness, are less favorable. We conclude that foundress associations in M. semirufus are in fact the result of nest invasions in an attempt to displace the resident queen. These are motivated by the high cost of the search for a suitable nesting site and of the digging of the nest.