Why Evolution Selects One Parent Over Another for Raising Babies

Parenting responsibility is rarely equal, and that's true across species. Now, scientists have taken a closer look to find out why that is, and why babies are only largely cared for by one parent.

Patterns of parenting in nature range from care by one parent only, to male/female biased care to care by both parents. Parental care actually involves one of the fundamental conflicts of interest between the sexes; care by either partner is beneficial to both partners, provided that the care is costly only to the caring individual. This means that each partner does best in a situation where most of the care is provided by the other partner.

Differences in care can be explained by differences in the costs and benefits of caring with two factors currently considered to be the key drivers: certainty of parentage and sexual selection. In this case, the researchers used a modeling approach to show that even in the complete absence of these factors, substantial differences in care are expected.

Sex differences in both the ability to care and levels of care are prone to spontaneous evolve as a result of the sexual conflict of interest over parental care and co-evolutionary interaction between levels of care and ability to care.

"While the coevolution of care and the ability to care thus predicts strong sex differences in care to emerge, it does not predict which sex is more likely to care," said John McNamara, one of the researchers, in a news release. "However, once other factors that give rise to even the slightest differences in the cost and benefits of care between the sexes, such as differences in certainty of parentage, are taken into account, a clear directionality emerges. The sex with the lower cost or higher benefit of care evolves to both be more able to care and to provide much higher levels of care than the other sex."

The findings are published in the journal Proceedings of the National Academy of the Royal Society B.

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