In most primates males are bigger in their body size than females. Consequently, by default males have been assumed to be dominant over all females, with the exception of bonobos (where co-dominance was apparent), squirrel monkeys (with interspecific variation in co-dominance, male dominance, and female dominance; e.g. [2]) and certain lemurs (with clear female dominance). Since 2000 we have investigated the consequences for social structure of the self-reinforcing effects of winning and losing fights, in general [2] and for the dominance relationships between the sexes [3,4]. We have shown, first in a series of computer models that the degree of female dominance over males will depend not only on body size but also on the self-reinforcing effects of winning and losing fights [4]. Here we showed that the degree of dominance by females over males was higher in species with aggression that was fiercer rather than milder and, if aggression was fierce, that female dominance increased with the percentage of males in the group. This happens because a high percentage of males in a group implies that males will be defeated more often by other males and thus some males become low in rank. Particularly over low-ranking males, females may become dominant. We confirmed empirically that female dominance is greater in fiercely aggressive macaques (despotic species) than in mildly aggressive ones (egalitarian species) [4], and that female dominance increased with the percentage of males in the group, not only in primates [4], but also in humans [5]. To obtain a better understanding how general these phenomena are in primates and how they come about, we need empirical data particularly on species where female dominance is observed in varying degrees. This is the case in capuchin monkeys [6,7]. Therefore, we asked Prof. dr. P. Izar to visit us. She will bring empirical data on several groups of capuchins observed under natural conditions in Brazil. These groups differ in sex-ratio of adults. She observed qualitatively that intensity of aggression and female dominance differs between groups and between sites and sub-species. We will together analyze her data with our standard methods. This will result in a scientific paper and a conference talk. Hopefully it will also result in a long-term cooperation in which we learn more about the benefits of dominance over males for female capuchins. It may be beneficial, for instance, for exerting female choice, by rejecting copulation attempts or for forming coalitions against males. Such a study is of great importance for increasing our understanding not only on behavior of capuchins, but also, for emancipatory issues in humans. References [1] S. Boinski, K. Sughrue, L. Selvaggi, R. Quatrone, M. Henry, and S. Cropp, Behaviour 139, 227 (2002). [2] C. K. Hemelrijk, P Roy Soc Lond B Bio 266, 361 (1999). [3] C. K. Hemelrijk, J. Wantia, and M. Daetwyler, Behaviour 140, 1247 (2003). [4] C. K. Hemelrijk, J. Wantia, and K. Isler, PLoS ONE 3, e2678 (2008). [5] K. Stroebe, B. A. Nijstad, and C. K. Hemelrijk, Social Psychological and Personality Science 1-10 (2016). [6] K. Izawa, Primates 21, 443 (1980). [7] M. Verderane, P. Izar, E. Visalberghi, and D. Fragaszy, Behaviour 150, 659 - 689 (2013).