Facts on Background Colour

http://jeb.biologists.org/cgi/content/full/205/16/2535

In salmonid fish, a darker skin colour has been suggested to signal social subordination. Substratum colour is another factor affecting skin pigmentation in fish; in the present experiment, juvenile Arctic charr (Salvelinus alpinus) were acclimated and allowed to interact in pairs for 5 days on a pale or dark background colour. Skin darkness was quantified prior to and following social interaction. Furthermore, agonistic behaviour and skin darkness were quantified, together with plasma levels of cortisol, adrenocorticotropin (ACTH) and -melanocyte-stimulating hormone (-MSH), and brain levels of monoamines and monoamine metabolites. The results show that fish interacting on a white background were more aggressive than those interacting on a black background. Social subordination resulted in skin darkening in fish kept on a white background, but not in fish kept on a black background. Furthermore, subordinate fish on a white background showed an elevation of brain norepinephric activity, an effect not seen in subordinate fish on a black background. Subordinate fish on both white and black backgrounds showed a similar activation of the brain serotonergic system and the hypothalamic—pituitary—interrenal axis. These results support the suggestion that skin darkening in subordinates acts as a social signal announcing social submission.

The hypothesis that skin darkening serves as a social signal in Arctic charr, acting to reduce unnecessary fights and energy loss in an established dominance hierarchy (O'Connor et al., 1999; Höglund et al., 2000). Fish on a white background showed a brighter skin colour and in these pairs the dominant fish performed more aggressive acts than did dominant fish on a black background. This observation may be explained by a dark fish representing less of a threat, and thus eliciting less aggression, than a pale conspecific. Higher levels of aggression resulted in a more intense social stress, as indicated by elevated brain norepinephrine activity, in subordinate fish kept on a white background. The more intense stress experienced by subordinates kept on the white background may explain why socially induced skin darkening in subordinate fish was observed only on this background. However, another explanation could be that fish acclimated to a black background colour had already reached a maximum level of skin darkening.