In the forests of the Shennongjia Mountains, between the Yangtze River and the Yellow River, at more than two thousand meters of altitude, the females of the flat golden langur have developed a solidary evolutionary adaptation. This species of monkey, endemic to this central region of China, suffers terrible temperatures of up to 14 degrees below zero in winter, causing the death of half of the offspring that are born in spring. "If infants have not reached a minimum level of development before the onset of winter, it is likely that they will not survive," primatologist Zuofu Xiang explains by e-mail. Therefore, these females suckle each one of the children of the group, to get a nourishing boost that guarantees the survival of all the offspring.
Zuofu, from the Institute of Zoology of the Chinese Academy of Sciences, is part of the team of scientists that has just documented this peculiarity of this endangered species. Based on more than eight years of observations on the nature of the offspring, their mothers and their reproductive history, the study provides the first evidence of this behavior in African or Asian primates. The so-called alomaternal lactation, breastfeeding other offspring, has been observed anecdotally in simias of some species of these continents, but this is the first time that it is observed as a systematic strategy adopted by the whole community.
It is a proven success strategy. Of the 46 monkeys that were born in a period of five years, only six were breastfed only by their mothers. Four of those six pups died before reaching one year, 66%. Meanwhile, of the other 40 received milk from at least one other female only six (15%) died, and two of them at the hands of a new alpha male who decided to kill them.
The monkeys that were observing Zuofu and his team form a group that varied between 62 and 82 individuals, divided into four social groups, each of these four controlled families by a male on whom several females who mate with him depend, according to published in the magazine Science Advances.
There were six females who refused to take care of other offspring. When they were mothers, their offspring were rejected by the others. And four of them died
When looking at the duration of this group breeding, the scientists observed that the offspring received milk from females other than their mother from the day of birth, which shows absolute trust between them. The blood ties between females of this species of golden monkeys are especially stable. And supportive breastfeeding was maintained until the third month of the baby, just before they start eating other solid foods after four months. Their biological mothers breastfeed until one year of age.
The reciprocity was almost total: 90% of the mothers whose offspring were breastfed by another female responded and took care of the son of that companion in the following months. There were six females, however, who rejected all the attempts of other offspring to suckle their breasts. The offspring of these six females were slammed when they wanted other mothers to breastfeed. And four of those little ones died.
Scientists say that in conditions of limited access to resources, this generosity with other offspring can negatively affect maternal health due to physiological and nutritional stress, reducing their chances of survival and future reproduction. So, why are they sacrificed in this way for the offspring of the others? When the researchers began to record the lactations that were crossed between different females of the group, when breastfeeding other offspring, the first thing that stood out was that they only take place within the same subgroup. That is, the females of the same harem take care of all the offspring of their family companions, but not of other families within the same group.
"Breast-fed pups receive more breastfeeding in total than if they only received milk from their mother," Zuofu confirms.
"The babies suckled by other mothers receive in total longer breastfeeding time than if they only received milk from their mother," confirms Zuofu. Thus, this solidarity between all of them ensures that all their creatures receive during the first three months of life that intake of additional energy necessary to achieve sufficient growth to reach the harsh winter period. In addition, scientists suggest that by giving more breast, these females could experience an increase in milk production that benefits their offspring and their relatives.
Energetic needs vary depending on the size of the litter, the size of the brain or the hardness of the environment, according to Zuofu. And precisely the golden monkeys have the highest adult brain volume of all the primates of their family and live in forests with long winters in which the night temperatures generally fall below 0 ° C, where the scarcity of resources can endanger survival. "These social and ecological features are conducive to the evolution of alomaternal upbringing," says the primatologist. "Therefore, we propose that this behavior may have arisen through natural selection when postnatal energy requirements are important and adverse environmental conditions provide an incentive for shared feeding," concludes Zuofu.