A transitional stage between the ergatoid and winged male morph in the ant Cardiocondyla obscurior

Males of the ant Cardiocondyla show a dispersal dimorphism of a winged and wingless morph. The loss of flight has lead to morphological reductions in the wingless (ergatoid) males and also affected body size, eye size and pigmentation. As ergatoid males mate exclusively inside the maternal nest, they underlie increased male-male competition and therefore have also evolved additional changes in behaviour and physiology: in contrast to winged males, ergatoid males are highly aggressive towards each other and their spermatogenesis is prolonged compared to all other hymenopteran males. In addition to these two male morphs, we found males with an intermediate appearance. These "intermorphic" males provide a transitional stage between normal males in most investigated morphological and physiological parameters. As they are produced extremely rarely and only in colonies that switch between pure ergatoid to mixed male production, we argue that they likely represent a developmental mistake. Parallels between the determination of male morphs and female castes (queen-worker dimorphism and worker polymorphism) might help to understand how the large potential of phenotypic plasticity in both sexes of social insects is realised during development.