International audienceThe theory of sexual selection explains sexual dimorphism in ornaments used in mate choice. Mutual mate choice is a form of sexual selection that might explain sexually monomorphic ornamental traits. Under mutual mate choice, both sexes select partners based on the same ornament. We tested the mutual mate choice hypothesis in a mutually ornamented seabird, the king penguin (Aptenodytes patagonicus), through observations ofthe pair formation process in the field. Penguins that were ready to mate formed displaying pairs at the edge of the colony. Some of these pairsmoved into the breeding colony and produced an egg (definitive pairs), while other pairs separated and often switched to another potential partner (temporary pairs). Colored ornaments were quantified using color vision modeling. We predicted that birds would mate assortatively by theirelaborate ornamental traits (specifically, colors of beak spots, auricular patches of feathers, and breast patch of feathers). We also predicted thatdefinitive pairs would exhibit more elaborate ornaments than temporary pairs. The mutual mate choice hypothesis was supported by assortativepairing for color of the beak spots, but not for color or size of the auricular patches or for the color of the breast patch. An alternative hypothesis wasalso consistent with our results that female choice for a male ornamental trait and superior female condition associated with the same trait pro-duced assortative pairing patterns. More UV- and yellow-colored beak spots for females in definitive than temporary pairs supported the femalechoice hypothesis over the mutual mate choice hypothesis, but previous experimental results from altered beak spot colors supported the latter.Evidence to date thus supports both the mutual mate choice and female choice hypotheses