Bachelor Thesis from the year 2021 in the subject Biology - Behaviour, grade: 1,3, Johannes Gutenberg University Mainz (Institute of Organismic and Molecular Evolution), language: English, abstract: This paper deals with the hypothesis, that there is a mechanism in Lasius niger queens giving them the ability to assess the fecundity of another queen, probably through CHCs. The thorax length likely does not contribute to this mechanism. Cooperation between unrelated individuals provides benefits that could ensure the survival of species. This work investigates on pleometrotic choosing behavior in Lasius niger ant queens. Here, two or more unrelated and usually monogynous queens found a colony together right after their nuptial flight and cooperate up to the emergence of the first workers. Then they fight until only one of them remains. Since the surviving queen is usually more fecund and heavier than her former nestmates, it is hypothesized that she will choose smaller queens with a lower fecundity to cooperate with. Therefore, queens in special arenas with two opposite arranged branches were given different choices. Here, either both branches contained a queen of higher or lower fecundity or only one of them. Two different experiments were performed, where the queens were either grouped random or based on their fecundity. It was found that queens that were chosen over another queen later laid significantly less eggs than the rejected ones. Additionally, queens that chose a lower fecund queen were significantly faster at choosing than queens that chose a higher fecund one. Queens that chose a queen over an empty nest site also did this faster than queens that chose an empty nest site over a queen. There was no significant difference in thorax length between chosen and rejected queens. The differences in relative thorax length were only significant in one of two collection locations, where chosen queens were larger than rejected ones.