Aggression induced by ornament similarity is limited to females in a mutually ornamented fish, Betta brownorum

Abstract

Ornamentation found in both sexes, or mutual ornamentation, can be used as a threat signal towards rivals or to bias aggression towards those sharing preferences for mates or other resources. The widespread male bias in ornamentation suggests that sexual selection for these competitive functions is stronger in males, but these functions are rarely compared between the sexes. We investigated intrasexual contests in a mutually ornamented fish, Betta brownorum , that uniquely has both male bias and high intrasexual variation in the presence and size of side blotches. Neither blotch size nor body size predicted the winner in either sex. The frequency of lateral display increased with opponent body size in females. Physical attacks increased when facing similarly blotched opponents only in females. These results suggest that nonsexual social selection contributes more to the maintenance of blotches than sexual selection, as the latter predicts the threat signal and/or aggression bias mainly in the sex with greater ornamentation. Our study highlights that smaller female ornaments can have social functions that are distinct from those of male ornaments and may be partially maintained by social selection in many other systems. • A mutually ornamented fish, Betta brownorum , shows high ornament variation. • We found no advantage of large blotches in winning contests. • Similar blotch sizes between contestants increased physical attacks only in females. • Nonsexual social selection is suggested to be influential in female B. brownorum .