The nonapeptide arginine vasotocin (AVT) and its mammalian homologue arginine 
vasopressin play a key role in the regulation of social behaviour across 
vertebrates. In teleost fishes, three AVT neuronal populations have been 
described in the preoptic area (POA): the parvocellular (pPOA), the 
magnocellular (mPOA) and the gigantocellular (gPOA). Neurons from each of these 
areas project both to the pituitary and to other brain regions, where AVT is 
supposed to regulate neural circuits underlying social behaviour. However, in 
the fish species studied so far, there is considerable variation in which AVT 
neuronal populations are involved in behavioural modulation and in the direction 
of the effect. In this study, the association between AVT neuronal phenotypes 
and social status was investigated in the Mozambique tilapia (Oreochromis 
mossambicus). This species is an African female mouth-brooding cichlid fish in 
which males form breeding aggregations in which dominant males establish 
territories and subordinate males to act as floaters. With respect to sex 
differences in AVT neuronal phenotypes, females have a larger number of AVT 
neurons in the pPOA and mPOA. Within males, AVT appeared associated with social 
subordination, as indicated by the larger cell body areas of AVT neurons in mPOA 
and gPOA nuclei of non-territorial males. There were also positive correlations 
between submissive behaviour and the soma size of AVT cells in all three nuclei 
and AVT cell number in the mPOA. In summary, the results provide evidence for an 
involvement of AVT in the modulation of social behaviour in tilapia, but it was 
not possible to identify specific roles for specific AVT neuronal populations. 
The results presented here also contrast with those previously published for 
another cichlid species with a similar mating system, which highlights the 
species-specific nature of the pattern of association between AVT and social 
behaviour even within the same taxonomic family.