Discrimination of individual mating songs as a first step towards sexual selection

Christian K. Machens, H. Schuetze, M.B. Stemmler, B. Ronacher and Andreas V.M. Herz

Humboldt University Berlin

Sexual selection is a major force driving the evolution of communication signals. Usually it is the females that show preferences for certain "qualities" of the signals produced by different conspecific males. For sexual selection to be effective, the animals - in our case grasshoppers - must perform a subtle signal analysis to distinguish conspecific mating songs. This requires that certain differences in the songs be preserved in the neural responses of the auditory periphery. Based on recorded spike trains of single auditory receptors, we investigated three related questions: (1) How well and how fast can different songs be discriminated? (2) What spike timing precision is required for this task? (3) Which song features are most important for discrimination?

For discrimination to work, spike trains elicited from multiple presentations of the same song must be more similar to each other than spike trains elicited from different songs. We therefore computed the similarity of each two spike trains by various methods, all dependent on a temporal scale factor that gauges the precision of spike timing. Conventional cluster analysis allows one to group the spike trains and assign them to one of the stimuli. The correctness of this assignment can be evaluated by the mutual information, giving a measure of the discrimination performance.

The following results were obtained: (1) Conspecific songs can be discriminated after a few hundred milliseconds. (2) Discrimination works best if spikes are evaluated at time scales of 10 msec. (3) Although the length of repetitive subunits helps in discriminating the songs, sufficient information must be contained in the structure of these subunits. Thus spike trains of auditory receptors carry enough information to enable a representation of the songs at high resolution.