The scientists observed that the female chimps keep quiet during sex!  This is a strategy to keep the other females from knowing.  They try to mate the best males.  The chimp population calls also found that the females are interested in having sex with a large number of mates.  They want to find the strongest male and confuse the paternity so that their offspring are taken care by all of them! 

The strategy is to make the alpha male, the highest ranking male think that he is the father of the child.  The study was made by Simon Townsend, a researcher at University of St. Andrews who is an evolutionary psychologist.  He thinks that if the paternity is confused, the chimp thinks that their offsprings will be taken care of.