Mammals that share their milk produce more offspring over the course of the year – and the benefits are even greater than those seen when nursing mothers are supported in other ways, such as being provided abundant food or parenting help.
The findings suggest that, across mammalian species, from rodents to primates, shared nursing benefits the entire community, says Paola Cerrito at New York University.
“We’re in a world of sharing now – sharing cars, sharing our homes on Airbnb when we’re not using them – where everyone wins,” says Cerrito. “And it’s a similar concept with sharing milk.”
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Cerrito’s grandmother used to tell her about women helping feed other women’s babies in Italy during World War II, “at a time when they were all malnourished”, she says.
Curious about this phenomenon, Cerrito and her colleague Jeffrey Spear, also at New York University, created models to assess the effects of different kinds of parental help on the reproductive performance of wild and domesticated mammals.
They used previously published data on 1806 species representing all orders of placental mammals. They looked at offspring weight – relative to maternal mass – and litter sizes, and also collected data on parent behaviour. Specifically they looked for published evidence of shared parenting and shared nursing. They also considered the effects of domestication – which generally means mothers receive shelter and ample food.
The two researchers found that both domestication and shared parenting – including infant-carrying by the fathers – had a positive effect on litter size and offspring weight. In particular, over the course of a year, domesticated species produced 68.2 per cent greater offspring weight than wild species.
But the greatest effect was related to shared nursing, says Cerrito. Species in which this behaviour is seen produced, over the course of a year, 83.1 per cent greater offspring weight than in species where infants are fed by their mothers alone.
Female mammals that suckle another’s young have more offspring
For both domestication and shared nursing, this increase in offspring weight typically translated as more – rather than bigger – offspring.
The findings suggest that even domesticated females reach what the researcher call a “physiological cap” as to the maximum amount of milk they can produce, because milk production requires a high amount of metabolic energy.
“All milk fed to an infant must first be metabolised by a lactating female,” Cerrito says. “[Milk-sharing] species are able to bypass that cap by spreading the metabolic demands across multiple females.”
The findings also provide insight into the biological benefits of the cultural practice of wet nursing in human societies, adds Spear. “Our research has shown that when mammals [share milk], it helps maximise total reproductive output,” he says. “In human terms, that means more infants surviving.”
Alexandre Roulin at the University of Lausanne, Switzerland, agrees. “I was pleasantly surprised. It was remarkable that the effect would be so pronounced,” he says. “It’s really surprising that the effect is greater than domestication.”
The findings of this “very cool study” are somewhat surprising for Sandra Heldstab at the University of Zurich, Switzerland. Along with other researchers in previous studies, Heldstab previously found no reproductive benefits of milk-sharing – technically known as allonursing.
The scientists generally assumed that was because, in species that allonurse, the infants still go to their own mothers’ teats more than 80 per cent of the time. “Allonursing therefore was expected to be more of an importance to strengthen social bonds, for a better immune response, or to act against infanticide,” she says.
The new study challenges that theory, she says, and “brings allonursing back into play concerning positive effects” on reproduction. She encourages more research that might tease out the specific effects depending on how much milk is actually shared in individual species and cases.