In a West African cichlid fish, the more aggressive, polygynous red males have bigger oxytocin‑producing brain cells than the less aggressive yellow males, but the total number of oxytocin cells is the same. This hints that oxytocin neuron size, not cell count, might help shape different social behaviors in these fish.

Alternative male morphs are found in many species of fishes. These morphs often differ in suites of social behaviours, such as aggression and territoriality, associated with alternative reproductive tactics. Such consistent morph-typical behavioural profiles suggest common differences in underlying neuroendocrine mechanisms. The nonapeptide hormones oxytocin and vasotocin are linked to a wide range of behaviours, including aggression, mating behaviour and social attraction. These behaviours often differ between male morphs, suggesting that nonapeptides may mediate behavioural variation between morphs. We compared two morphs of a West African cichlid, Pelvicachromis pulcher, to test for differences in nonapeptide neuronal phenotypes. P. pulcher exhibit at least four distinct male colour morphs, of which the red and yellow morphs are the most common and have the best-described social and reproductive behaviours. Red males tend towards breeding as harem holders, whereas yellow males breed monogamously or act as satellite males on the territories of red males. Here, we examine nonapeptide-producing neurons in the preoptic area of the hypothalamus in the red and yellow morphs. We found that the more aggressive polygynous red morph has larger parvocellular oxytocin and vasotocin-producing neurons, as well as larger gigantocellular oxytocin neurons, compared to the yellow morph. We did not find any association between the number of oxytocin or vasotocin cells and male morph. Our results suggest that the production of oxytocin and vasotocin in the preoptic area may play a role in determining morph-typical behaviour among P. pulcher males.