Not all company is comforting: metabolic consequences of conspecific presence depend on species’ social system in fish

Abstract

Social animals may reduce their overall metabolic demand through group living due to a “calming effect”. However, it remains unclear whether the metabolic response of individuals to group members varies depending on the social system, and how individual sociability influences this response. We measured the metabolic rates of the territorial three-striped dwarf cichlid (Apistogramma trifasciata) and the shoaling cardinal tetra (Paracheirodon axelrodi) in the presence or absence of conspecifics, examining the relationship between individual sociability and metabolic rates, with a focus on how sociability influences the metabolic response to conspecific presence. Territorial cichlids exhibited increased standard metabolic rate (SMR) and routine metabolic rate (RMR) in response to the presence of conspecifics, while shoaling tetras showed unaltered RMR or SMR. We found no evidence linking individual sociability to metabolic rates, nor any indication that individual sociability influences metabolic responses to grouping in either species. The contrasting metabolic responses to grouping in these species demonstrate that the energetic consequences of social proximity depend on the species’ evolved social structure. Given the lack of effect of individual sociability on metabolic rates, we suggest that sociability and maintenance metabolism may evolve independently, without underlying genetic covariation. This study highlights the intricate relationship between group living and individual energy expenditure, indicating that a species' social system plays a significant role in determining the extent to which the calming effect serves as an adaptive element of group living.