Senthilvel K. S. S. Nathan

The uniquely enlarged noses of male proboscis monkeys are prominent adornments, and a sexually selected male trait. A recent study showed significant correlations among nose, body, and testis sizes and clear associations between nose size and the number of females in a male’s harem. However, to date, the analyses of other common male traits, i.e., canines, are lacking. Whereas male nose size had a positive correlation with body size, we unexpectedly found a negative correlation between body and canine sizes. We explain this by an interaction between sexual and natural selection. Larger noses in males may interfere with the use of canines, thereby reducing their effectiveness as weapons. Additionally, longer canines are opposed by natural selection because the larger gape it imposes upon its bearer reduces foraging efficiency, particularly in folivores. This unique case of decoupling of body and canine size reveals that large canines carry an ecological cost.

The uniquely enlarged noses of male proboscis monkeys (Nasalis larvatus) are prominent adornments, and a sexually selected male masculine trait, as recently revealed by the significant correlations among nose, body, and testis sizes and the clear association between nose size and the number of females in a male’s harem. However, till date, the analyses of other common male traits, i.e., canines, are lacking. Whereas male nose size had a positive correlation with body size, we unexpectedly found a negative correlation between body and canine sizes. Similar relationship between nose and body size was observed in females, whereas only a weak correlation was noted between canine and body size. We explain this by an interaction between sexual and natural selection. Larger noses in males may interfere with the use of canines, thereby reducing their effectiveness as weapons. However, longer canines are opposed by natural selection because they impose a larger gape upon its bearer and reduce foraging efficiency, particularly in folivores. The difference between the sexes regarding the regression intercept for body size vs. canine size was significant, although the difference of the slopes was insignificant, consistent with the hypothesis that the nose interferes with the effectiveness of canines as weapons in males but not in females. Moreover, a mathematical simulation supported our prediction. This unique case of decoupling of body and canine size reveals that large canines carry an ecological cost. It also raises interesting questions regarding the developmental mechanism that produces a negative correlation among adult body, nose size, and canine size.

Multidimensional approaches support audiovisually coevolved large noses on the unique society of proboscis monkeys.