Genetic and environmental sources of behavioural individuality: a test of the standard model

Behavioural variation is typically assumed to arise from the combination and interaction of genetic and environmental variation. However, recent work with genetically identical individuals has found that substantial behavioural individuality is expressed even when genetic and environmental variation are negligible. This surprising result requires direct testing of our standard model for the sources of behavioural individuality. Here, we tested the standard model by comparing among-individual variation in highly inbred crickets versus outbred crickets. Comparing inbred and outbred lines allows for direct testing of the standard model by contrasting the magnitude of among-individual variation in a uniform versus varied genetic background. We found substantial and significant differences in among-individual variances, with among-individual variances being roughly three times greater in outbred versus inbred crickets (posterior probability, p = 0.974). Repeatability was also significantly different between inbred and outbred crickets (0.15 versus 0.41, respectively; p = 0.984). This result supports the standard model and suggests that the surprising expression of behavioural variation in clonal and parthenogenic species may represent an important but unique pathway for the expression of behavioural individuality.