While studying her langurs in India, Hrdy observed males from outside the group routinely killing unweaned infants, as part of a troop takeover. This infanticidal behaviour, she realized, is a toxic side effect of sexual selection and male competition for mates. Rather than his having to wait two to three years for the female to wean another male’s baby before being available to mate again, by infanticide the new leader forces the bereaved mother into oestrus, making her readily available for fertilization. As a defence against infanticide, Hrdy theorized, females are driven to have sex with
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