Breastfeeding is the best way to feed an infant. Not only does it supply the proper nutrition, but it also supports the development of the immune system, provides healthy microbiome, reduces risk of diseases, and brings down overall mortality (1). There are also benefits resulting from the ability of breastfeeding to suppress maternal ovarian function and to delay the next pregnancy. Birth spacing is an important predictor of infant mortality, maternal health, and, in an evolutionary perspective, maternal reproductive success.

Todd and Lerch (2) document a significant decline in the ability of breastfeeding to cause suppression of the ovarian function and thus to reduce the chance of the next pregnancy. In breastfeeding mothers from 84 low- and middle-income countries, the duration of lactational amenorrhea (i.e., the length of time without menstrual cycles after having given birth) strongly decreased between 1975 and 2009. For example, lactational amenorrhea in Bangladesh decreased from 15.2 to 7.7 mo during the span of about 40 y. In some countries, thanks to substantial public health efforts in recent years, women extended their breastfeeding, but despite this, the timing of cycle resumption did not change! Even though it sounds like a mystery, it is not, from an evolutionary vantage point.

In Ethiopian villages, womens daily walking routine, due to installation of water pumps, shrank from about 3 h to 15 min. However, in response to this innovation, the risk of conceiving in a given month rose more than threefold (3). In the rural Gambia, increases in energy intake by nutritional supplementation provided to women during pregnancy and lactation were related to changes in the levels of prolactin and sex hormones, and shortened the time until next pregnancy (4). In Guatemala, women who, prenatally and during first 3 y of life, received supplement with higher energy and protein,

1Email: jasienska{at}post.harvard.edu.

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Public health needs evolutionary thinking - pnas.org