Behavioural individuality is thought to be caused by differences in genes and/or environmental conditions. Therefore, if these sources of variation are removed, individuals are predicted to develop similar phenotypes lacking repeatable individual variation. Moreover, even among genetically identical individuals, direct social interactions are predicted to be a powerful factor shaping the development of individuality. We use tightly controlled ontogenetic experiments with clonal fish, the Amazon molly (Poecilia formosa), to test whether near-identical rearing conditions and lack of social contact dampen individuality. In sharp contrast to our predictions, we find that (i) substantial individual variation in behaviour emerges among genetically identical individuals isolated directly after birth into highly standardized environments and (ii) increasing levels of social experience during ontogeny do not affect levels of individual behavioural variation. In contrast to the current research paradigm, which focuses on genes and/or environmental drivers, our findings suggest that individuality might be an inevitable and potentially unpredictable outcome of development.

Changes after Mar 9, 2018: We discovered that we had wrongly assigned body size data to individual IDs in the original publication. This occurred when we joined the body size and behavioral data files: the ID values in each file were sorted differently thus joining the body size data to the wrong IDs. We retained all our original data files and so could trace back the source of this error and correct it. Uploaded data sets are now containing the corrected body size data, an R-code of our analysis as well as a pdf of the results obtained in R.