Caring for offspring is unequal between the sexes in many animal species and a new study suggests evolution is the culprit.

Making babies is one of the fundamental conflicts of interest between the sexes. Care by either partner is beneficial to both partners as it increases the health and survival prospects of the common young, while providing care is costly only to the caring individual. As a result, each partner does best in a situation where most of the care is provided by the other partner--an outcome that is clearly impossible.

Small initial differences which predispose one sex to care more are exaggerated once the ability to care evolves and as a result, one sex evolves attributes - such as mammary glands in female mammals or increased brain size in some fish - that enhance the ability to care. Then this sex does most or all of the care. Patterns of parenting in nature range from care by one parent only (seen in many mammals), to male/female biased care, to care by both parents (seen in many birds).

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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 (which decreases the benefit of care for the less certain sex - usually the male) and sexual selection (which increases the cost of care for the sex that can mate again faster).

Professor John McNamara, of the University of Bristol's School of Mathematics, and Dr. Max Wolf of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, used a modeling approach to show in Proceedings of the Royal Society B that even in the complete absence of these factors, substantial differences in care are to be expected. They found that sex differences in both the ability to care and levels of care are prone to spontaneously evolve as a result of the sexual conflict of interest over parental care and the 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. 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," McNamara says.   

Article: 'Sexual conflict over parental care promotes the evolution of sex differences in care and the ability to care', John M. McNamara, Max Wolf, Proceedings of the Royal Society B