Hrdy's PhD thesis tested the hypothesis that overcrowding causes infanticide in langur colonies. She went to Mount Abu in India to study Hanuman langurs and concluded that infanticide was independent of overcrowding—it was possibly an evolutionary tactic: When an outside male takes over a group, he usually proceeds to kill all infants. This postulated tactic would be very advantageous to the male langurs who practiced infanticide. Turnover in a langur tribe occurs approximately every 27 months. The male who is taking over has a very small window of opportunity to pass on his genes. If the females are nursing infants, it's likely that they won't ovulate for another year. Killing their dependent infants makes the females once again receptive to mating.