Mixed-colony records together with nest densities and gyne morphology suggest temporary social parasitism in Tetramorium (Hymenoptera: Formicidae)

Abstract

Two types of permanent social parasitism, dulosis and inquilinism, are well known in the ant genus Tetramorium Mayr, 1855. In contrast, temporary social parasitism has never been reported for these ants. A recently detected mixed colony of Tetramorium immigrans Santschi, 1927 and the putative social parasite Tetramorium aspina Wagner et al., 2018 make it thinkable that additional social parasites exist in the genus with more than 500 species known. Here, we extended our research to species putatively related evolutionarily to T. aspina, and detected two mixed colonies of Tetramorium moravicum Novák & Sadil, 1941 and Tetramorium ferox Ruzsky, 1903. The hypothesis that the latter is a parasite of the former is tested via three lines of indirect evidence using literature and self-collected data: 1) The distributional area of T. ferox is congruent with or a part of that of T. moravicum; 2) Nest densities of T. ferox are lower than those of independently founding Tetramorium species; 3) T.-ferox-complex gynes show a morphology fitting the parasitic syndrome. The alternative hypothesis to social parasitism is that T. ferox is free-living. Line 1 does not fully support the hypothesis of social parasitism but requires multiple host species as extra assumption to justify the hypothesis. In contrast, the two other lines of evidence support the hypothesis: The known maximum nest densities of T. ferox are significantly lower than in independently founding species (Line 2). Gynes of the T. ferox complex are smaller, have a deeper clypeal excision, smaller mesosoma, wider waist, and smaller gaster than independently founding species (Line 3). Moreover, a stronger positive allometry of waist width in species of the T. ferox complex indicates an adaptive value of this specific structure. We consider obligatory temporary social parasitism in the T. ferox complex as the most likely colony-foundation strategy. Temporary social parasitism might represent the evolutionary link between independent and dulotic species, and our findings may foster better understanding the evolutionary pathway of social parasitism in ants.