Familiarity reduces aggression but does not modify acoustic communication in pairs of Nile tilapia (Oreochromis niloticus) and black-chinned tilapia (Sarotherodon melanotheron).

Reproduction involves multiple complex behaviours, and the effects of familiarity on such social interactions are seldom described in fish. This is particularly true for sound production and communication within aggressive or non-aggressive context. This study explores the effects of a common garden rearing without parental care of two closely related cichlid species (Nile tilapia Oreochromis niloticus and black-chinned tilapia Sarotherodon melanotheron) on their sound production features and social interactions. After 9 months in common garden rearing, from embryonic stage to first maturity, sound production and associated behaviours were recorded on specimens of the two species in intraspecific and interspecific pairings. The authors found that fish were able to produce the same kind of sounds as those recorded in similar context for their parents. Drum sounds were associated to chasing, lateral attack and courtship in O. niloticus and only to fleeing or avoidance in S. melanotheron. Specific grunts were produced in chasing, after biting and in nest building by O. niloticus, and specific rolling sounds were associated to courtship in S. melanotheron. Sound production and behaviours were not correlated to sex steroid levels, but the number of sounds recorded in aggressive context was correlated to dominance in O. niloticus. The authors conclude that one generation of common garden rearing does not modify sound features, which remain specific and innate in the two cichlids. Despite the familiarity, O. niloticus remained dominant on S. melanotheron, but the aggressiveness between the two species decreased.

Acoustic signals produced by Nile tilapia Oreochromis niloticus and black-chinned tilapia Sarotherodon melanotheron during intra- and interspecific pairings.

We characterised, for the first-time, the sound production of black-chinned tilapia Sarotherodon melanotheron and show differences with that of Nile tilapia Oreochromis niloticus in a hybridization pairing context. Although both species were able to produce drum sounds, they showed different acoustic features. Drum sounds were produced in aggressive (chasing or lateral attack) and non-aggressive (courtship) contexts by O. niloticus but only in aggressive situations (fleeing or avoidance) by S. melanotheron. The second type of sounds produced by O. niloticus were grunts, produced in both aggressive (chasing and after biting) and non-aggressive contexts (nest building). The second type of sound produced by S. melanotheron was a rolling sound, produced only during courtship. Each species was able to produce common sounds (drum) and species-specific sounds (grunts and rolling). This implies that species can communicate without being able to understand each other because the sounds emitted may probably have different significance. Drumming corresponded only to aggressivity in S. melanotheron, whereas this was not true for O. niloticus. 11-ketotestosterone (11-kt) levels were significantly higher in male O. niloticus than male S. melanotheron, but there was no significant correlation between 11-kt or estradiol concentrations and the number of sounds produced in aggressive or non-aggressive behavioural contexts in either species. During interspecies interactions, O. niloticus drum sounds are likely considered to be aggressive by S. melanotheron and could potentially constitute a reproductive barrier between the two species.