Questions I have about the Cotton-top Tamarin

After researching the Cotton-top Tamarin I still would like to know why females lead the groups while most species of primates have a male lead the group. I would also like to know the closest relatives of a Cotton-top Tamarin and their branch in the evolutionary tree. While I was researching I had a hard time finding information on the evolution of the Cotton-top Tamarin. If time a money were not and issue I would talk to and expert in primates or Cotton-top Tamarins and ask them if they know the answers to my question. I would go to a expert because talking to a person is usually clearer and easier find what you want to know than on the internet.