Behavioural individuality in clonal fish arises despite near-identical rearing conditions

Abstract

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.

Figure 1. Schematic of experimental design.

In the 0-day treatment, directly after birth, genetically identical individuals were isolated (grey arrows) and housed in the same tightly regulated environmental conditions. In the 7-day and the 28-day treatments, genetically identical individuals were reared with increasing durations of social experiences (black arrows) and were then isolated (grey arrows) in the same environments. In all treatments, we assessed the behavioural phenotype of the individuals after 7 weeks.

Figure 2. Individual behaviour in an open-field test over four repeated observations.

(a,b) show examples of a less active and a highly active individual, respectively; shown are heat map outputs produced by EthoVision software for each of the four trials. (c) Individual activity levels for the three treatments are shown. Each line represents one individual with the predicted intercept and slope from the models (N=31 for 0-day and 7-day treatments and N=32 for 28-day treatment). In all three treatments, we observe substantial among-individual differences in behaviour that are repeatable. Neither the level of among-individual variation, nor the amount of total behavioural variation differed between our three treatments (Table 1).