For male rhesus macaques hoping to woo a mate, it just might help to look a little macho. While researchers have long suspected that differing facial features among male and female primates of the same species may play a role in mate choice, the draw of'facial masculinity' has remained understudied outside of human subjects. To better understand how female monkeys perceive these traits, primatologists observed free-range rhesus macaques over a series of'looking-experiments.' The study revealed that the female macaques held their gaze longer when looking at more masculine faces – but, the researchers aren't exactly sure why. While researchers have long suspected that differing facial features among male and female primates of the same species may play a role in mate choice, the draw of'facial masculinity' has remained understudied outside of human subjects In the new study led by Penn State primatologist and biological anthropologist Kevin Rosenfield, researchers studied rhesus macaques on Cayo Santiago, just off the east coast of Puerto Rico. The team observed the reactions of 107 female macaques presented with pairs of images of male faces.