A juvenile clown anemonefish (Amphiprion percula) in its anemone, Kimbe Bay, Papua New Guinea. Image credit: Lili Vizer (CCBY 4.0)
In many species, individuals live in social groups where access to essential resources, such as food, reproductive opportunities, and safe living spaces, is unequally distributed. Individuals with higher social status typically have priority access to these resources, creating dominance hierarchies across a wide range of taxa, from insects and fish to mammals.
Individuals occupying different social ranks often differ in their appearance and behaviour: dominants tend to be larger, more aggressive, and sometimes more ornamented, whereas subordinates are generally smaller and less aggressive. In many vertebrate dominance hierarchies, however, social rank is not fixed for life; instead, individuals can ascend the hierarchy over time. Such transitions require individuals to adjust their appearance, behaviour, and underlying physiology, i.e., their collective phenotype, in response to changing social status. Phenotype and social interactions can influence one another, creating a dynamic feedback loop rather than a simple, unidirectional relationship.
Despite the widespread occurrence of dominance hierarchies, how individuals coordinate phenotypic adjustments to assume and maintain different social ranks, and the physiological mechanisms underlying these changes, remains poorly understood. Vizer et al. tested the hypothesis that individuals coordinate growth, food intake, behaviour, and gene expression as they assume different social roles during dominance hierarchy formation. For their experiments, the researchers used clownfish (Amphiprion percula), a species with a well-characterised, size-based dominance hierarchy and social context-dependent growth trajectories.
Across 30 experimental replicates, Vizer et al. housed juvenile clownfish in size- and age-matched rival pairs or as solitary individuals. This design provided paired fish with equal initial opportunities to assume the dominant role, allowing social roles to emerge naturally through interactions. They quantified growth from measurements taken at the beginning and end of the experiment, monitored food intake and behaviour weekly over five weeks, and profiled whole-body gene expression at the end of the experiment.
Dominant fish grew twice as much as subordinate and solitary fish. This enhanced growth was accompanied by greater food intake, more aggressive behaviour, increased expression of genes associated with growth and bone formation, and reduced expression of satiety-associated genes. Conversely, subordinate fish grew less, consumed less food, and exhibited submissive behaviour, alongside reduced expression of genes associated with growth and bone formation and increased expression of satiety-associated genes. Notably, subordinate and solitary fish did not differ detectably in the phenotypic traits measured. Together, these findings reveal coordinated changes in growth, food intake, behaviour, and gene expression associated with the emergence of distinct social roles during dominance hierarchy formation.
The findings of Vizer et al. add to growing evidence that social environments and interactions shape individual phenotypes within dominance hierarchies, a pattern documented across diverse taxa, including fish, mole-rats, meerkats, and primates, with potential parallels in humans.
By linking social role to coordinated changes across behavioural, physiological, and molecular levels, this study provides a framework for investigating the bidirectional feedback between phenotype and social interactions. Understanding these feedback loops will help clarify how social experiences generate and reinforce context-dependent phenotypes, including socially regulated growth trajectories, across vertebrates.
