Colony founding by pleometrosis in the fire ant, Solenopsis invicta
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- Tschinkel, W.R. & Howard, D.F. Behav Ecol Sociobiol (1983) 12: 103. doi:10.1007/BF00343200
Newly mated queens of the fire ant, Solenopsis invicta, found colonies either alone (haplometrosis) or by joining with other newly mated queens (pleometrosis). Surveys after mating flights showed that nests and queens were usually aggregated in space, that queens were aggregated among occupied nest chambers, and that the occurrence and degree of pleometrosis was related to the mean queen density. Queens and nests were strongly associated with slightly higher ground, away from rainwash areas and puddles.
The effects of queen density and microtopography (small hills) on pleometrosis were tested in a two-factor factorial experiment. A 64-fold increase in applied queen density resulted in a 2.19-fold increase in mean queens/nest (pleometrosis). Variation in queen density accounted for 70% of the variation in the mean queens per nest, as well as 78% of the aggregation of queens among the available nest chambers. Queen density also accounted for 86% of the aggregation of queens in area. Thus, at all densities, queens are moving into areas and nests of higher density, increasing both the local mean densities and the level of aggregation. Microtopography had no significant effect. Lab experiments suggest that the interactions leading to association take place on the surface.
A mechanism is proposed in which the central causal factor regulating pleometrosis is local queen density, local being one to a few square meters, and a variety of factors affect pleometrosis by their action upon the local queen density.
In the laboratory, groups of 5 foundresses produce more workers than do groups of 10 or 15, or single queens. Nests founded by groups begin the growth period with about 3 times as many workers as do those founded by single queens, and the former remain about three times as large for at least the first 100 days of growth and probably more. Higher worker production rate probably confers an advantage in survival and competition throughout colony growth. These differences between haplo- and pleometrotically founded nests may be among the factors favoring foundress associations.
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