Local mate competition, variable fecundity and information use in a parasitoid

Experiments with sex allocation in parasitic wasps offer excellent opportunities for testing how the way in which organisms process information about their environment influences behaviour. If mating takes place in temporary patches, where only a small number of females produce offspring, then sex allocation theory predicts a female-biased sex ratio. When females lay different numbers of offspring in a patch, females that produce relatively fewer offspring should lay a less female-biased, or even male-biased, sex ratio. Recent theoretical models have predicted that the exact form of this relationship depends upon whether females know only their own clutch size (self knowledge) or also the clutch sizes laid by the other females on the patch (complete knowledge). We tested the predictions of these models by examining sex allocation when two females of the parasitoid wasp Nasonia vitripennis oviposited simultaneously on a patch. The offspring sex ratio (proportion of males) produced by a female was: (1) negatively correlated with the number of offspring that she laid; and (2) positively correlated with the body size of the other female on the patch. Larger females matured more eggs and laid more offspring in the experimental patch. This suggests that, as predicted by the complete knowledge model, the offspring sex ratio laid by a female became more female biased as she laid a greater proportion of the total offspring laid on the patch. Furthermore, females use the body size of other females to assess the clutch sizes that these will lay. Copyright 1998 The Association for the Study of Animal Behaviour