Dyeing the chests of socially naive male hamsters black, and thus enlarging and darkening their normal dark brown chest patches, gives such males an advantage in initial aggressive encounters with partners matched in weight and age. This effect disappears if such initial encounters occur in the dark, indicating the importance of visual cues. Video and film analysis demonstrated that, contrary to previous claims, the normal chest patches were not displayed in threat postures prior to fighting, but rather were displayed by losers of fights in an upright posture with clearly defensive functions. It is suggested that the chest patches may have evolved in conjunction with this defensive posture for use in both intra- and interspecific conflict; the possible reasons for the effectiveness of larger and darker chest patches in interspecific aggression are discussed.