A team of scientists led by Prof. Li Ming at the Institute of Zoology, Chinese Academy of Sciences, found widespread allomaternal nursing behavior in an Old World monkey, the golden snub-nosed monkey. The findings were published in an article entitled "Routine allomaternal nursing in a free-ranging Old World monkey" in Science Advances on Feb 20, 2019 (DOI: 10.1126/sciadv.aav0499).

The evolution of lactation in metatherianand eutherian mammals has resulted in a large degree of nutritional and developmental dependency between a female and her offspring. Milk production is energetically costly for mothers who need to synthesize and provide nutrients, hormones, vitamins, and immune compounds.Therefore,lactating females should be reluctant to invest time or energy into nursing others’ offspring (allomaternal nursing). While regular allomaternal nursing has been documented in a number of rodent and carnivore species, as well as in some prosimians, New World monkeys and humans, it is not common in Old World monkeys and apes.

 
Based on more than eight years of field observations of infants and their mothers as well as their reproductive histories at Shennongjia National Park, Central China, the study provides the first evidence of regular allomaternal nursing in golden snub-nosed monkeys (Fig 1) and expands the taxonomic distribution of this behaviour in primates to Old World Monkeys.

The study found that most infants of golden snub-nosed monkeys were allonursed by one or two additional adult females who were not their mother. Allomaternal nursing was largely confined to the first three months of an infant’s life, and principally occurred between related females who reciprocally nursed each other’s offspring, therefore, the study supported the kin selection and reciprocity hypotheses and provide an evolutionary explanation for allomaternal nursing in golden snub-nosed monkeys (Fig 2). In contrast, their study provide no support for the misdirected parental care hypothesis which had previously been argued to explain allomaternal nursing in primates.
Allomaternal nursing also enhanced infant survivorship and did not have a negative impact on the future reproductive success of allonursers. Therefore, considering that similar social and ecological traits typify all primate species with allomaternal nursing (including humans), the researchers propose that allomaternal nursing may have arisen through natural selection when heavy postnatal energetic requirements and harsh or unpredictable environmental conditions placed a premium on shared provisioning.

Reciprocity and relatedness played a significant role in the maintenance of allomaternal nursing in these primates. Mothers permitted other females to take their infants as early as their first day of life and let them carry and groom their infants. These female bonds may be mediated through kinship and common residence in the same social unit, as well as other forms of social behaviors (i.e. grooming) that promote a set of affiliative and permissive relationships, which are required to develop infant-mother-allomaternal caregiver relationship. Such relationships also are crucial in human social interactions. Therefore, the study expands the taxonomic distribution of allomaternal nursing and provide fresh insights into the possible factors driving evolution of allomaternal nursing behavior in primates, including humans.

Science Advances: Xiang et al. Routine allomaternal nursing in a free-ranging Old World monkey

http://advances.sciencemag.org/content/5/2/eaav0499

Fig 1 A mother is simultaneously suckling two infants in the same social unit.

Fig 2 Factors affecting the extent of allomaternal nursing (allonursing): (A) increased allomaternal nursing within mother-daughter dyads, (B) positive relationship between mother's and allonurser's pattern of reciprocal allomaternal nursing (C) no difference in allomaternal nursing proportions between females with an unweaned infant (approximately one year of age) and females with a neonate (< 6 months of age), (D) no difference in allomaternal nursing proportions between primiparous and multiparous mothers. (***, p < 0.001; **, p < 0.01; ns, no significance, p > 0.05).