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Animals living in complex social environments are often able to flexibly adjust their behaviour to different social contexts. Yet, flexible responses require time and energy and can be constrained by a variety of mechanisms. Here we asked whether the socio ecological environment experienced by the facultative cleaning goby Elacatinus prochilos influenced their interest in cleaning interactions. Elacatinus prochilos can be found in two socio ecological contexts: inhabiting cleaning stations, where they live in pairs and feed by removing ectoparasites from larger marine organisms (cleaners), or barrel sponges, where they live in dominance hierarchies and rarely engage in cleaning interactions (sponge-dwellers). We tested individuals used in a previous experiment with controlled social structures (small groups with dominance hierarchies) and feeding conditions (better access to food to dominants) to better understand which factors influence cleaning activity in facultative species. We found that dominance significantly decreased cleaning activity in male (and potentially female) sponge-dwellers. We hypothesize that long-term hormone-mediated or time allocation constraints may explain reduced cleaning activity in dominant male sponge-dwellers. While previous studies showed that E. prochilos can be socially flexible, here we showed that feeding flexibility might be restricted for dominant sponge-dwellers. This highlights the importance of testing the flexibility of individuals in different social contexts to better understand the evolutionary importance of adaptive behavioural flexibility. • Facultative cleaning gobies can live in different social and ecological conditions. • We investigated how these differences influenced cleaning interactions. • Dominant individuals from barrel sponges rarely interacted with clients. • This indicates a potential trade-off between dominance and cleaning behaviours. [ABSTRACT FROM AUTHOR]

Social species often show intraspecific variation in the size and composition of their social units, i.e. the social organization. This variation represents a challenge for comparative studies trying to understand evolutionary transitions in social organization complexity. Most studies on intraspecific variation in social organization (IVSO) have focused on the environmental factors selecting for variation in social organization but the mechanisms underlying such variation remain relatively unexplored. Here we investigated the potential mechanisms giving rise to IVSO in the Caribbean cleaning goby Elacatinus prochilos. This small reef fish typically exhibits two types of social organization, pair living or group living, which not only differ in group size but also in feeding strategy and social behaviour. Pair-living gobies cooperatively feed on ectoparasites removed from larger visiting fish, while group-living gobies feed on microinvertebrates living inside sponges and aggressively defend the best feeding spots against each other. We tested how flexible the social organization, social structure and competition over a food source of E. prochilos are by experimentally manipulating group composition, habitat structure and feeding condition of pair-living and group-living wild adult gobies in laboratory. Despite having different original social organizations in the wild, both pair-living and group-living gobies formed dominance hierarchies and exhibited affiliative behaviours in our group manipulations. This indicates that IVSO in this species is not driven by pre-determined or early-developed mechanisms, but rather a consequence of flexibility in social organization, also known as social flexibility. • We investigated the mechanisms of social organization variation in E. prochilos. • We used a controlled group manipulation to test the behavioural flexibility of individuals. • Individuals exhibiting different social organizations in the wild converged in social behaviour. • This indicates that variable social organization is driven by social flexibility. [ABSTRACT FROM AUTHOR]