Finding and deciding upon a potential mate is a difficult decision for many female animals. Females are justifiably cautious – the wrong choice could cost them a whole year of successful breeding. In many animals,including insects, females are attracted to a mate by his song. Male grasshoppers (Chorthippus sp.) advertise to females using a species-specific song, where each male's song is a variation upon the species-specific theme. These songs potentially signal male quality and so it may be important for female grasshoppers to discriminate between males to ensure they choose the most suitable mate.
Machens and colleagues wanted to find out if the female grasshopper auditory system can discriminate between fine differences in male songs. To discriminate between males, a female's auditory system must be able to encode the differences between male songs. Differences between male songs could be present in single auditory nerve fibres (sensory neurones) or in the combined activity of many auditory fibres. To test this, they played songs from different males to female grasshoppers whilst recording the responses of single auditory fibres. They asked whether the responses of single auditory fibres are sufficient to allow discrimination between males.
The team found that male songs generated precise spike trains in female auditory nerve fibres, so they decided to test whether these spike trains were sufficient to allow other neurones (auditory interneurones) in the female nervous system to discriminate between male songs. When a spike is transmitted from an auditory nerve fibre to an interneurone via a synapse, it is converted to a graded change in the membrane potential of the interneurone (a graded synaptic input). Knowing that interneurones are thought to perform the discrimination between songs, Machens and colleagues suspected that the inputs to these neurones might be a better measure of a female's ability to discriminate between males. Since they didn't know exactly which interneurones were responsible for discrimination, they designed a mathematical model to convert the auditory fibre spike trains into synaptic inputs. They suspected that the duration of each synaptic input would affect the resolution of the interneurone responses and, therefore, whether a female could distinguish between two very similar songs, so they varied input duration in their model to determine which was best for discrimination. Long or short input durations did not allow reliable discrimination between male songs, whereas those with intermediate durations did. Discrimination was also improved as the length of the songs increased.
From these experiments, Machens and colleagues concluded that there is sufficient information within single auditory fibres to allow females to distinguish reliably between males, even on the basis of the fine structure of their songs. This is surprising since it might be expected that spike trains from many auditory fibres (there are ∼50 receptors per ear) would have to be combined to enable females to distinguish between male songs.
Although Machens and colleagues show that females could potentially discriminate between male songs based on spike trains from single auditory fibres, this relies upon auditory interneurone properties. However, real auditory interneurones may have quite different properties from the model interneurones and clearly need to be identified. Future studies are sure to provide more fascinating insights into mechanisms underlying discrimination and mate choice found not only in grasshoppers but also in frogs, birds and mammals.
