Human activities cause rapid eutrophication and increased water turbidity in aquatic ecosystems, but their effects on fish communication and colour signals remain of debate. In particular, melanin-based coloration in fish has been understudied, because it was believed to be less costly to display than carotenoid-based colours. Here, we measured the phenotypic divergence of melanin-based coloration in 17 populations of gudgeon (Gobio occitaniae) along a turbidity gradient. We also tested the short-term plasticity of coloration using a reciprocal transplant experiment. We found strong variability in melanin-based coloration along the turbidity gradient: interpopulation divergence in coloration was higher than predicted by genetic drift, and fish were paler with increasing levels of turbidity. Finally, a reciprocal transplant experiment revealed that fish transplanted into more turbid habitats expressed a paler melanin-based coloration, suggesting that melanin-based coloration was highly plastic in the short term. Overall, our results suggest that eutrophication in human-altered rivers can rapidly alter melanin-based coloration, with potential consequences for fish visual communication and sexual selection.

Explaining the evolution of primate social organization has been fundamental to understand human sociality and social evolution more broadly. It has often been suggested that the ancestor of all primates was solitary and that other forms of social organization evolved later. However, previous research included the assumption that many understudied primate species were solitary, then finding transitions to more complex social systems being driven by various life history traits and ecological factors. Here we show that when intra-specific variation is accounted for, the ancestral social organization of primates was variable, with the most common social organization being pair-living but with approximatively 15-20% of social units of the ancestral population deviating from this pattern by being solitary living. We built a detailed database from primary field studies quantifying the number of individuals (social units) expressing different social organizations in each population. We used Bayesian phylogenetic models to infer the probability of each social organization, conditional on several socio-ecological predictors, in ancestral populations. Body size and activity patterns had large effects on transitions between types of social organizations. Our results challenge the assumption that ancestral primates were solitary and that pair-living evolved afterwards. Moreover, our results emphasize the importance of focusing on field data and accounting for intra-specific variation. Pair-living is evolutionary ancient, likely caused by reproductive benefits such as access to partners and reduced intra-sexual competition, with more complex social structure (pair-bonding) and care systems (biparental and allo-parental care) evolving later.

Freshwater ecosystems are highly impacted by human activities, but the effects of human‐induced perturbations on fish phenotypic divergence remain poorly understood, especially in riverine systems. In this study, we tested the effects of urbanisation and eutrophication on several morphological and physiological traits in 17 gudgeon populations of Gobio occitaniae from South of France. We first demonstrated that eutrophication gradient was associated with changes in morphology and energy reserves, while urbanisation was associated with changes in body condition. More specifically, fish from highly eutrophic sites had deeper bodies and larger jaws, and a higher lipid content in their muscles. Urban fish had a higher body condition compared to their rural counterparts. The phenotypic divergence (Pst) among sites was significantly higher than the neutral genetic differentiation (Fst), suggesting that these morphological and physiological differences cannot be explained by neutral genetic drift alone. This study thus suggests that eutrophication and urbanisation are major drivers of phenotypic divergence in riverine fish populations, and calls for further experimental studies investigating the ecological and evolutionary effects of human activities on riverine fish populations.