Female lions with cubs show various direct responses to immigrating males, including defense of their cubs or avoidance of the new males. Despite these responses, male replacement in the females' pride results in considerable cub mortality. Those females that remain in the pride and mate with the new males show low fertility in the first few months after a takeover of their pride. At the same time, however, females show heightened sexual activity, being more active in initiating copulations and seeking a greater number of mating partners. These two factors appear to elicit competition between male coalitions for control of the pride, with the result that larger coalitions eventually become resident. We suggest that this is adaptive because a female needs protection from male harassment of her cubs for over 2 yr in order to rear her cubs successfully, and only large male coalitions are likely to remain in a pride for more than 2 yr. We present a simple model that specifies one set of conditions under which a female will improve her lifetime reproductive success by showing temporary periods of infertility, but attracting a larger coalition.