Color as an interspecific badge of status: a comparative test

Animals as diverse as cephalopods, insects, fish, and mammals signal their
social dominance to conspecifics to avoid costly fights. Even though
between-species fights may be equally costly, the extent to which
dominance signals are used between species is unknown. Here, we test the
hypothesis that differences in color are associated with dominance between
closely related species that aggressively interact over resources,
examining between-species variation in colors that are used in
within-species badges of status (black, white, and carotenoid coloration)
in a comparative analysis of diverse species of birds. We found that
dominant species have more black, on average, than subordinate species,
particularly in regions important for aggressive signaling (face, throat,
and bill). In addition, dominant species were more likely to have more
black in comparisons in which the dominant species was not larger than the
subordinate species, suggesting a greater importance of black as a signal
when other signals of dominance (size) are missing. Carotenoid colors
(i.e., red, pink, yellow, and orange) were not generally associated with
dominance across all species but may signal dominance in some taxonomic
groups. White appeared to have opposing functions in dominance signaling:
white was associated with dominance in species in which black was also
associated with dominance, but was associated with subordinance in species
in which carotenoid-based dominance signals may be used. Overall, these
results provide new evidence that colors may function broadly as signals
of dominance among competing species. Such signals could help to mediate
aggressive interactions among species, thereby reducing some costs of
co-occurrence and facilitating coexistence in nature.