"Those males with harems of females had much more brightly coloured lips compared to bachelor males that don't have access to females," Dr Grueter says.
"It looks like redder lips indicate higher male quality or status."
They also found that for most of the year, the lips of bachelors and those with multiple female admirers were relatively similar in colour.
Big changes came, however, during mating season, when the lips of younger or bachelor males grew paler, while their sexually successful pals adopted an even richer red.
