Abstract
Results from the first study of sexual selection in a member of the family Callionymidae are presented. Included is an ethogram for the spotted dragonet, Diplogrammus pauciradiatus, focusing on motor patterns observed during agonistic and courtship encounters and on sexual dimorphism. Males obtain longer bodies and longer first dorsal fins than females. Males also obtain black pigment on the lower jaw (mouth bar) and orange pigmentation on the head, two color patterns that were not observed in females. Both courtship and agonistic encounters involve lateral displays in which all fins are extended, but body posture and pigmentation patterns differ between the two types of display. Polygynous spawning occurs in the water column during the hour before dark, and pelagic eggs are released. Observations of laboratory colonies revealed that males form straight-line dominance hierarchies, and the most dominant males encountered females more often and obtained more spawns than all other males combined. Male dominance experiments revealed that standard length, first dorsal fin-length, possession of a mouth bar, and possession of orange pigments were all positively correlated with higher social status. Female choice experiments demonstrated a female preference for males with longer dorsal fins, but no preferences for males possessing the other traits were detected. When female choice was eliminated during male access experiments, the dominant males monopolized access to females only during the evening (time of spawning) and not during the afternoon. Field studies revealed that the population in Biscayne Bay, Florida, has a male:female ratio that is female biased (1:1.6). Males had a random distribution in the grassbeds sampled. Females had a clustered distribution in these same grassbeds, possibly providing groups to which dominant males can control access. These results all indicate that male-male competition is most responsible for the evolution of sexually dimorphic traits in males of D. pauciradiatus, with the possible exception of the elongated first dorsal fin. This is the first study to show a color pattern (mouth bar, orange pigmentation) in males of any species to influence only male-male competition and not female choice.