Social and spatial conflict drive resident aggression towards outsiders in a group-living fish

Group-living animals often experience within-group competition for
resources like shelter and space, as well as for social status. Because of
this conflict, residents may aggressively resist joining attempts by new
members. Here we asked whether different forms of competition mediate this
response, specifically competition over i) shelter abundance, ii) spatial
position within groups, and iii) social or sexual roles. We performed
experiments on wild groups of Neolamprologus multifasciatus cichlids in
Lake Tanganyika, either increasing or decreasing the number of shelters
(empty snail shells) within their territories. We predicted that increases
in resource abundance would reduce conflict and lower the aggression of
residents towards presented conspecifics, while decreases in resources
would increase aggression. We explored the effects of social conflict and
spatial arrangement by introducing same or opposite sex conspecifics, at
greater or lesser distances from resident subterritories. We found that
changing the abundance of shells had no detectable effect on the responses
of residents to presented conspecifics. Rather, aggression was strongly
sex-dependent, with male residents almost exclusively aggressing presented
males, and female residents almost exclusively aggressing presented
females. For females, this aggression was influenced by the spatial
distances between the presented conspecific and the resident female
sub-territory, with aggression scaling with proximity. In contrast,
presentation distance did not influence resident males, which were
aggressive to all presented males regardless of location. Overall, our
results show that group residents respond to presented conspecifics
differently depending on the type of competitive threat these potential
joiners pose.