It is known for a great many species of animals living in groups that, at the moment of a sudden change in the social structure of the group and immediately after this, especially in situations where there is a change in the alpha male in the group, massive infant mortality occurs. In some cases, the progeny are killed by the new head of the harem, but in other cases it seems that their mothers kill or abandon the offspring. The Bruce effect is the best-known example of such a phenomenon; this is encountered in mice and other species of rodents (Bruce 1959). If we remove the original male from a pregnant mouse and replace it by a foreign male, the female usually very rapidly aborts.
These and related phenomena apparently accompany intersexual competition. From the viewpoint of the male, it is extremely important to be the biological father of the greatest possible number of offspring in the population. Any behavior that contributes to this goal is selectionally advantageous and will apparently become fixed during evolution. The male can best shorten the time until the individual females become pregnant with him by removing all foreign offspring as soon as possible. From the viewpoint of the female, such an approach is, of course, extremely disadvantageous, because she has already invested a great deal in the offspring. However, she mostly has no effective defense against this male strategy. To begin with, the females of a great many species are weaker than the males and, in addition, an offspring can be killed very quickly, while defense of an offspring against killing requires constant alertness and care. This asymmetry means that the females generally lose the battle for the young in advance, so that it is fundamentally more evolutionarily advantageous for her to cooperate with the male and to rapidly get rid of her own young or embryos.
It should be stated that there are a number of other explanations for confirmation of the Bruce effect, including unilateral, pheromone-mediated manipulation on the part of the male or hidden (post-copulation) female choice – preferential conception with the winning – and thus genetically better partner (Storey 1994).