What happens when you apply eyeliner to a cichlid, specifically a Neolamprologus pulcher? According to some researchers at McMaster University in Ontario, Canada, you can make the fish appear more or less threatening to an unfamiliar conspecific.
In experiments to gauge visual threat signals, the researchers used waterproof eyeliner to both enhance and diminish the intensity of facial bars on territory “intruder” N. pulchers. Rates of aggression were then measured against those fish whose bars had been enhanced and those whose bars had been diminished. This experiment also measured the degree of same sex aggression, where the researchers predicted that a conspecific of the same gender would pose a more direct threat to the dominance hierarchy.
What was the conclusion? That facial bar intensity can affect territory defense levels. Specifically, lesser facial bar intensity resulted in greater aggression under certain circumstances. This sounds a bit counterintuitive, but it suggests that a less dominant appearing fish may be treated more harshly by a same gender conspecific. Sort of like bullies are more likely to pick on the weak.
My summary is very over simplified because I didn’t include all of the variables involved. For example, the experiments included both breeder and helper specimens, which behaved differently within the experimental manipulation. Remember, N. pulcher are cooperatively breeding species. See my previous two posts about cooperatively breeding cichlids – Who’s watching the kids? and Who’s watching the kids, pt. 2.
Cuthbert, B.M. and Balshine, S. “Visual threat signals influence social interactions in a cooperatively breeding fish.” Animal Behavior. 2019;151:177–184. https://doi.org/10.1016/j.anbehav.2019.03.018