Male golden snub-nosed monkeys with wider faces relative to facial height are more aggressive and more likely to win fights, according to a new study that offers fresh insights into how primates may use physical traits to signal dominance.

The study, recently published in the journal Animal Behaviour, was based on two years and eight months of fieldwork in the Qinling Mountains of central China. Zhang Pei, an associate professor at Northwest University and first author of the study, recorded more than 1,600 conflict events involving 15 dominant males and measured the facial width-to-height ratio, or fWHR, of 126 monkeys.

The results showed that adult males with a higher fWHR initiated significantly more aggressive interactions and won a greater proportion of their fights, even after the researchers accounted for confounding factors such as body weight and harem group size. Other factors, including age and the advantage of striking first, also predicted the outcome of fights.

However, the study found no evidence that males deliberately targeted rivals with narrower faces. This suggests that fWHR may function primarily as a signal of the bearer's own fighting ability, rather than as a cue that rivals use to assess opponents and avoid costly conflicts.

"Our results indicate that facial width-to-height ratio serves as an honest signal of aggressiveness and fighting ability in male golden snub-nosed monkeys. However, we still need to determine whether this facial feature actually deters potential challengers before a contest begins," the paper said.

The study also documented a developmental reversal in sexual dimorphism. Subadult females had wider faces than males of the same age, but in adulthood, males had a significantly higher fWHR than females. The shift coincides with the onset of intense male-male competition for mating opportunities.

"We speculate that the link between facial width-to-height ratio and aggressiveness and fighting ability is not unique to a particular species, but is likely a general pattern across primates," Zhang told China Science Daily in a report published on Saturday.

Golden snub-nosed monkeys live in a complex multilevel society in which multiple family units travel and forage together as a large breeding band, often followed by all-male groups seeking to take over individual units. Given the high stakes of male competition, researchers suggest that signals linked to facial structure may have evolved to help reduce costly physical fights.

The researchers noted that about half of the offspring are not sired by the resident male, further intensifying competition among males for mating opportunities.

However, the team found no direct evidence that resident males more frequently initiated aggression toward adult males with a smaller fWHR.

Further research is therefore needed to determine the extent to which fWHR serves as an honest signal that can deter potential opponents and reduce conflict before physical contests begin.

The study was partially supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China and the Shaanxi Fundamental Science Research Project for Chemistry and Biology.

Contact the writers at wangsongsong@chinadaily.com.cn