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Description: How early life experiences are stored on a molecular level and affect behavioural phenotypes later in life is not well understood. In fish, reproductive phenotypes are often easily discernible and frequently depend on previous life experiences. DNA methylation is an epigenetic mechanism which is both sensitive to environmental conditions and stable across cell divisions. In this study, we therefore investigate whether DNA methylation mediates early life experiences and predetermines the territorial male reproductive phenotype in the round goby, Neogobius melanostomus. We investigate early life predisposition by growth back-calculations and then study DNA methylation by MBD-Seq in the round goby hypothalamus as the brain region controlling vertebrate reproductive behaviour. We find that the territorial reproductive phenotype is linked to a high growth rate in the first year of life. Hypothalamic DNA methylation patterns, however, reflect the current behavioural status independently of early life experiences. Together, our data suggest a non-predetermination scenario in which indeterminate males progress to a non-territorial status in the spawning season, and in which some males then assume a specialized territorial phenotype if current conditions are favourable.