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Allomothering, or non-maternal infant care, can be performed by males such as a child’s father, non-reproductive males in polyandrous systems, or older siblings interested in abetting their own genetic material via their siblings (Theory of Inclusive Fitness).
Vervets, cebus monkeys, squirrel monkeys, and macaques are all known for allomothering performed by females not closely related to the pair. These alloparents help by carrying the infant, providing food, and guarding the infant from predators. Cebus monkey females have even been known to nurse infants not their own when looking after them (cf. wet nurse).
Jane Lancaster noted the reproductive benefits for primates as k-strategists in learning to be better mothers. Her learning-to-mother hypothesis postulates that primate females with no children of their own participate in allomothering, and evidence from studies by Sarah Hrdy and Lynn Fairbanks shows that females without offspring "tried to allomother more frequently than what you'd expected based on their proportion of the group's population, while parous females tried it much less than expected from their population in the group."
The theory is supported by evidence of the success of allomothering as a learning technique. "Lynn Fairbanks studied vervets and found that first-time mothers with high alloparenting experience raised 100% of their first offspring to maturity, but mothers with low experience had less than a 50% survival rate of their first infant."
An infant's birthmother, in a climate of allomothering, gains time relieved from parental duties, allowing her to forage more and reproduce more quickly. It also improves the chances for her infant to be adopted should she die.