Asked by: Akin Adeyemo, by email
Natural selection has favoured mothers that provide a great deal of care for their young because, in mammals, the cost of reproduction is relatively high. Carrying the developing foetuses around inside your body, instead of laying eggs, places a greater strain on the female and restricts the size of each litter, so it makes sense to devote a lot of time after birth to protect this investment.
But if food is limited, caring for all the young equally might result in all of them being malnourished, and possibly the mother as well. In this case, natural selection might favour the mother that selectively abandons or culls the weakest or youngest of her children to ensure that she can adequately provide for the rest.
Merino sheep that give birth to twins will often move off after the second one is born, abandoning it to die. The Merino breed originates from Spain and North Africa and this might be an evolutionary response to the low protein value of grazing in that region.