Fig. 1

Setup for mate choice experiment (modified after Lüscher and Wedekind, 2002). A focal worm was placed together with two stimulus worms into a fork-shaped mesh bag in a culture medium mimicking the situation in the bird’s gut. The focal worm (middle compartment) can move freely between all compartments, while the stimulus worms are restricted to side compartments that overlap the middle compartment. Scores were assigned according to the focal worm’s position. “0” was considered neutral, 1 & 2 (or −1 & -2 respectively) for a tendency to the side of one of the stimulus worms. If there was a minimum of 25 % overlap between the focal worm and one of the stimulus worms, we assigned the score “3” or “-3”, as this was a position where mating was possible