Status-dependence and morphological trade-offs in the expression of a sexually selected character in the mite, Sancassania berlesei

In the male dimorphic mite Sancassania berlesei, fighter males kill rivals with a pair of armoured legs whereas scrambler males are benign with unmodified legs. In an adaptive response mediated by colony pheromones, fighter expression increases at low colony density. Under the status-dependent evolutionarily stable strategy (ESS) model we expected heavier final instar nymphs to become fighters. This was supported in group reared nymphs. In individually reared nymphs fighter expression was experimentally suppressed using two concentrations of colony pheromone. Here, male morph expression again depended on tritonymphal body mass and contact is therefore unnecessary for individuals to judge their status. Fighter suppression was greater in the higher pheromone treatment, but morph determination remained status dependent. The weight and length of fighters was lower than scramblers of same-weight final instar nymphs, indicating a developmental trade-off, and a cost not recouped at the adult stage.