Exploring Social Dominance in Wild Diademed Sifakas (Propithecus diadema): Females Are Dominant, but It Is Subtle and the Benefits Are Not Clear

Rarely observed in mammals, female dominance is documented in several of Madagascar's lemurs. Although dominance affects many aspects of primates' lives, studies have largely focused on dyadic agonistic interactions to characterise relationships. We explored the power structure of three diademed sifaka groups (Propithecus diadema) at Tsinjoarivo during the lean season (July-August, 325 h) using social behaviours, group leadership, displacements and feeding outcomes. Two groups had a hierarchy dominated by the breeding female, while the highest rank was held by the breeding male in the third; in dyadic interactions, breeding females dominated males in all groups. Inconsistencies in hierarchies suggest that groups vary, with rank related to kinship ties of breeders. Aggression and grooming were rare; adult females received aggression at lower frequencies than males. Group movements were led more by females and followed more by males, and female feeding priority was evident in displacements during feeding. However, males and females did not differ in feeding outcomes, as expected (particularly in the lean season) if female dominance (and/or male deference) serves to ensure better access for females. This unexpected pattern (female dominance despite rare aggression, clear female leadership and displacement, yet no observable benefit in grooming or feeding outcomes) defies easy explanation, and reinforces the fact that studies examining female power in lemurs should take a multifaceted approach. Further study is needed to understand this pattern, the physiological and reproductive consequences of female dominance (e.g. detecting subtler variation in food quality or intake rates) and exactly how (and when) the benefits of female dominance are manifested.

Vocal correlates of individual sooty mangabey travel speed and direction

Many group-living animals coordinate movements with acoustic signals, but so far most studies have focused on how group movements are initiated. In this study, we investigated movement patterns of wild sooty mangabeys (Cercocebus atys), a mostly terrestrial, forest-dwelling primate. We provide quantitative results showing that vocalization rates of mangabey subgroups, but not of focal individuals, correlated with focal individuals’ current movement patterns. More interestingly, vocal behaviour predicted whether individuals changed future speed, and possibly future travel direction. The role of vocalizations as a potential mechanism for the regulation of group movement was further highlighted by interaction effects that include subgroup size and the quality of poly-specific associations. Collectively, our results suggest that primate vocal behaviour can function beyond travel initiation in coordination and regulation of group movements.