Sociality does not predict signal complexity in response to playback in apteronotid weakly electric fishes.

Evolution of signal structure should be influenced by the social environment in which signals are sent and received. The "social complexity hypothesis" for communicative complexity postulates that individuals living in complex social groups require complex signals to communicate effectively. This hypothesis has been supported in many vocalizing animals. In other systems and modalities, however, relationships between sociality and signal complexity are less clear. Weakly electric knifefishes represent a novel model for investigating the relationship between signal structure and social structure. South American apteronotid knifefish continuously produce quasi-sinusoidal electric organ discharges (EODs) from their tail to communicate species, sex, and status. Fish also rapidly modulate EOD frequency to produce chirps, which coordinate agonistic and courtship interactions. Sociality and chirp structure vary extensively across apteronotids. Here, we quantified chirp complexity and variation in chirp structure across six species that vary in group size: three territorial species (Apteronotus albifrons, Parapteronotus hasemani, and 'Apteronotus' bonapartii), two gregarious species (Adontosternarchus devenanzii and Adontosternarchus balaenops), and a semi-social species (Apteronotus leptorhynchus). Chirp complexity and variation in chirp structure differed substantially across species but were unrelated to sociality. Within the Apteronotus and Adontosternarchus species pairs, one species produced complex chirps, whereas the other produced relatively simple chirps. Thus, neither phylogenetic relatedness nor social structure explained variation in chirp structure or complexity in the species we examined.

Supplementary information: The online version contains supplementary material available at 10.1007/s00265-025-03619-y.