Another reason parental care is generally provided by females, rather than males, has to do with the certainty of parentage. For many animal species females retain the eggs inside their bodies until after they are fertilized. A female who cares for these eggs can be certain that they are hers, and not those of another female, so energy and time invested are well spent. Males have no such assurance, for precisely the same reason: if fertilization happens inside females then males run the risk that sperm from a rival male actually fertilizes the offspring. Expending resources for the care of unrelated offspring is not cost-effective. Consequently, because females have invested the most already, and because they generally have higher certainty of genetic parentage than their mates, selection favors the evolution of maternal care of offspring more often than it does paternal care.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 684