The impact of age and egg-laying cycle on female grasshoppers' preference functions for acoustic signals.

Female responsiveness and the shape of preference functions for male signal traits are important determinants for male mating success. We observed the responsiveness and the selectivity of virgin grasshopper females (Chorthippus biguttulus L.) for different features of males' acoustic signals throughout their life span to detect possible influences of age on the females' preference functions. In particular, we explored the hypothesis that the females may become less selective with increasing age and, therefore, would start to accept songs that are normally rejected. Such an age effect could relieve the selection pressure on male signal traits. In the majority of animals tested, the general responsiveness decreased with age although a few individuals exhibited an opposite trend. Contrary to the above expectation, there was no indication of a loss of selectivity in older females or an increased acceptance of normally unattractive song models. The timing within the oviposition cycle had a strong effect on responsiveness: near oviposition the general responsiveness increased and with it also the half width of the preference functions. However, highly unattractive song models remained unattractive also near oviposition.

Relative weighting of acoustic information during mating decisions in grasshoppers indicates signatures of sexual selection.

The decision with whom to mate is crucial in determining an individual's fitness and is often based on the evaluation of visual or acoustic displays produced during courtship. Accordingly, the algorithms for evaluating such courtship signals are shaped by sexual selection and should reflect the expected benefits and costs of mating: signals bearing heterospecific features should be rapidly rejected, since mating would produce no fertile offspring, while signals resembling conspecific ones should be weighted proportional to mate quality. We test these hypotheses in females of the grasshopper Chorthippus biguttulus who assess males by their song, which is produced as a sequence of subunits with species and individual specific signatures. We present mixed sequences of subunits with conflicting cues and use a computational model of decision-making to infer how sensory information is weighted and integrated over the song. Consistent with our hypothesis, females do weight sensory cues according to the expected fitness benefits/costs: heterospecific subunits are weighted particularly negatively and lead to a rejection of the male early in the song. Conspecific subunits are weighted moderately, permitting a more complete evaluation of the full song. However, there exists an overall negative bias against mating, possible causes of which are discussed.