This story appears in the June 2017 issue of National Geographic magazine.
Of two hunky monkeys, which would you say has the most sex appeal?
That’s easier to answer if you’re a rhesus macaque (Macaca mulatta)—or Constance Dubuc, an evolutionary biologist at the University of Cambridge.
Since 2012 Dubuc and colleagues from New York University have studied more than 250 free-ranging rhesus macaques at a field site in the Caribbean. The goal: to learn how face color—which varies from pale pink to deep red in the species—affects reproductive success.
To isolate color’s role in attraction, Dubuc showed each rhesus test subject two photos of faces in different shades of red. She found that dark red faces appeal strongly to females and somewhat to males—and she did so, in part, by tracking eye movements. “It’s the same as with humans,” she says. “If you see someone attractive in a bar or on the street, your eyes will linger a little longer.”
Researchers also logged the monkeys’ courtship acts by face color—and found that dark red–faced males got more propositions, from more females, than medium- or pale-colored males did.
In the best measure of reproductive success—number of offspring—dark red–faced females outdo paler ones. But for males, there’s a twist: To get more couplings, and thus more offspring, they must have dominance in their group as well as a dark red face, Dubuc says. “Color alone wouldn’t be enough.”