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Infanticide
Infanticide is the culling of juveniles that aren't sired by the new male that has taken over a group or pride. This type of behaviour is shown in many species of animals, but is especially seen in lions. But why it is advantageous for a lion to kill cubs that aren't his when he takes over the pride? The females could quite easily turn on him if he attacks their offspring.
It doesn't seem likely that this behaviour has evolved by natural selection, it could possibly have evolved by means of social pathology in order to prevent over crowding. V. C. Wynne-Edwards says that this happens for the good of the group, otherwise the group would become over crowded and then eventually the extinction of the group would occur due to all the critical resources being used. He argued that this lead to evolutionary change among different groups and the existence of self sacrificing individuals. With the multiple allele idea it was easier to see that there were selective pressures acting on individual in a group, which would have more evolutionary impact.
This idea was not strongly believed G. C Williams wrote a book called Adaptation and Natural Selection in which he disagreed with Wynne-Edwards ideas, he said it was all because of the survival of multiple alleles being present in the species otherwise there would be two types of males: infanticidal ones, which would have shortened lives, or other ones which live longer and reproduce more.
However, this is not such a convincing idea. Sarah Hrdy believes that it is quite possibly due to sexual competition between males. As it might increase the lion's reproductive success because if the cubs are dead then the females will want to mate again; so he gains descendants and his genes therefore enter the gene pool. This is a more plausible idea because if the new male does not kill the cubs that are under 9 months old then it will take 2yrs before he is able to mate with the lionesses. If they lost their babies due to infanticide it was found that the females conceived within 4.5months of the new male taking over the pride.(Pusey and Packer 1987 and 1994)
This sort of behaviour should only happen with abnormal/ unusually high population sizes. Unfortunately this is not the case, as seen with langur populations that are untouched by humans and with moderate population sizes. So which theory is correct???
Sexual competition is definitely part of it. But is cannibalism? (Male lions kill cubs up to 9 months old) They can replenish energy lost after ousting the old male, but in Langur communities this act of cannibalism is not seen when new males just simple join a group.
It is most likely that the new male is showing a form of selfishness and kills the young so that he benefits by getting his genes into the gene pool quicker.
