Scientists in China have observed for the first time that Sichuan golden monkey has a ＂wet nurse＂： reducing the death rate in severe winter.

A starving baby, if the mother has other conditions such as insufficient milk, will usually be fed by other lactating mothers instead. This kind of behavior in humans is popularly called "wet nurse feeding", and scientifically it is defined as allomaternalbreastfeeding.

In fact, scientists have observed similar phenomena not only in humans, but also in rodents and primates. A recent study by scientists in China has further expanded the scope: the research team led by Zuo Fu, a professor at the School of Life Science and Technology of Central South University of Forestry and Technology, and Li Ming, head of the primate ecology research group of the Institute of Zoology, Chinese Academy of Sciences, obtained the evidence of the old world monkey’s maternal lactation for the first time. The research results were published on February 21st, Beijing time, in Science Progress, a sub-magazine of Science.

Breastfeeding behavior of Sichuan golden monkey. The pictures in this paper are provided by the research team.

In an interview with The Paper reporter (www.thepaper.cn), Zuo Fu said, "This is the first time to report that regular partheno-lactation exists in Old World monkeys, which has expanded the distribution of partheno-lactation in primates and made it appear in different groups such as proto-monkeys, New World monkeys, Old World monkeys and even humans."

Higher primates, that is, anthropophagus, include New World monkeys distributed in South America and Old World monkeys distributed in Eurasia (Old World). Previously, there was no report about the maternal lactation behavior of Old World monkeys. According to Zuo Fu, the research team started this new research in 2012 and observed it continuously for five years. The research was carried out in Dalongtan Research Base of Shennongjia National Park in Hubei Province.

The research base has a group of about 90 golden snub-nosed monkey. Sichuan golden monkey is endemic to China, with a golden fur. They are distributed in the forests at the junction of Hubei, Sichuan, Shaanxi and Gansu, and distributed in Minshan, Qionglai, Daxueshan, Xiaoliangshan, Qinling, Shennongjia and the northern slope of Motianling.

"We began to study the monkey group in 2006, and the researchers were able to identify all individuals over 3 years old in the group." Zuo Fu said that the birth of Sichuan golden monkey is basically concentrated in the period from March to May every year. Because the coat color and body shape of baby monkeys are similar, it is not easy to identify them, so there is no concern about whether there is maternal breastfeeding at first. "Until the spring of 2011, after accidentally finding a female monkey feeding two baby monkeys at the same time, the research team carefully observed the breastfeeding behavior of Sichuan golden monkeys in the group and found that the breastfeeding behavior of different mothers was more common."

This latest paper shows that in the field observation conducted by Zuo Fu et al. for five years in a row and after five farrowing seasons, 40 (87%) of 46 baby monkeys of Sichuan golden monkey were breast-fed by one or more non-mother monkeys, and 22 (48%) of 46 baby monkeys were breast-fed by at least two other mothers. This behavior of maternal breastfeeding mainly occurs in the first three months after the baby monkey is born.

The research team also found that 6 of the 46 baby monkeys did not receive maternal breastfeeding at all, and 4 of them died in the cold winter. In comparison, among the 40 baby monkeys that were breast-fed by at least one non-mother monkey, a total of 6 eventually died, and the mortality rate was greatly reduced.

It is worth noting that Sichuan golden monkey generally lives in temperate forests with high altitude, and it is extremely cold in winter for five months, and the food supply also has strong seasonal changes. Partheno-breastfeeding behavior may help baby monkeys fight against unfavorable living environment.

Zuo Fu said, "The behavior of maternal breastfeeding may provide energy support for the rapid development of baby monkeys after birth, so that the baby monkeys can feed in advance to achieve a good development state before the severe winter, and can successfully survive the winter of extreme low temperature and food shortage. This behavior may also provide energy support for the rapid development of the brain, which makes the brain capacity of golden monkeys relatively large."

Generally speaking, the behavior of maternal breastfeeding only occurs within three months or less after the baby monkey is born, and it almost stops when the baby monkey begins to eat natural food.

In addition, the research team also found that the maternal breastfeeding behavior in Sichuan golden monkey still follows certain principles. Baby monkeys can not suck milk in all non-maternal monkeys: the maternal breastfeeding behavior of Sichuan golden monkey only occurs between females who are related or breastfeeding each other.

It is mentioned in the paper that kinship-usually the "grandmother" or "aunt" of baby monkeys, or reciprocity plays an important role in maternal breastfeeding. About 90% female monkeys (25 out of 28) will breast-feed the cubs of other female monkeys who have breast-fed their own baby monkeys in the current or next year, which is similar to "reward".

Zuo Fu believes that the maternal breastfeeding behavior occurs between females who are related or cooperate with each other, and the mother will allow other females to contact the baby monkeys in the early stage of development, which is necessary for the infant-mother-maternal care relationship in the early stage of human evolution, so this study also provides a new perspective for understanding human evolution.