Courtship and mate discrimination within and between species of Timema walking-sticks

The evolution of premating isolation via divergence in mating behaviour has been strongly implicated in the process of speciation. Timema walking-sticks show weak to moderate reproductive isolation between populations of the same species on different host plants, and high levels of isolation between species. In this paper we conducted experimental studies of within-species and between-species mating behaviour in Timema to address two central issues pertaining to the influence of behaviour on speciation: (1) how divergence in courtship influences reproductive isolation within and between species, and (2) whether the same or different traits mediate premating isolation within and between species. Mating behaviour involves three phases: pairing (whereby the male climbs onto the female's dorsal surface), courtship (which involves leg and antenna waving), and copulation. We found that courtship was qualitatively similar across the genus, but there were statistically significant quantitative differences in leg and antenna waving frequencies between Timema species. However, no-choice trials within and between species showed that discrimination within species is a function of whether males will court females after pairing, while discrimination between species is a function of whether males and females will pair. Because in both cases mate discrimination occurs before courtship is performed, we infer that courtship does not directly influence reproductive isolation in Timema. Moreover, because within-species and between-species discrimination take place at different stages in the mating sequence, intraspecific mate discrimination and species recognition appear to represent distinct processes. These findings suggest that, at least in Timema, speciation may involve the accumulation or replacement of mate discrimination mechanisms used within species.