Some species are monogamous, with both males and females providing parental care. Evolution can favor monogamy if males have more offspring by helping with child care than if they abandon their offspring to seek more matings. In many birds, for example, two full-time parents are required: when one goes off to forage, the other incubates the eggs. But monogamous species are not that common in the wild. Only 2 percent of all mammal species, for instance, have this type of mating system.
