Animal behavior: Female mammals that suckle another’s young have more offspring

Piglets suckling from a sow

Shutterstock / Thuwanan Krueabudda

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 with 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 the second world war, “at a time when they were all malnourished”, she says.

Curious about this phenomenon, she 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 at litter sizes, and also collected data on parent behavior. Specifically, the researchers looked for published evidence of shared parenting and shared nursing. They also considered the effects of domestication, in which mothers generally receive shelter and ample food.

The pair 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 biggest effect was related to shared nursing, says Cerrito. Species in which this behavior is seen produced 83.1 per cent greater offspring weight over the course of a year than those where infants are fed by their mothers alone.

For both domestication and shared nursing, this increase in offspring weight typically translated as more, rather than bigger, young.

The findings suggest that even domesticated females reach what the researchers call a “physiological cap” as to the maximum amount of milk they can produce, because this process requires a high amount of metabolic energy.

“All milk fed to an infant must first be metabolized by a lactating female,” says Cerrito. “[Milk-sharing] species are able to bypass that cap by spreading the metabolic demands across multiple females.”

The research also provides insight into the biological benefits of the cultural practice of wet nursing in human societies, says Spear. “Our research has shown that when mammals [share milk], it helps maximize 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 [of shared nursing] 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 to Sandra Heldstab at the University of Zurich, Switzerland. Along with other researchers in earlier studies, Heldstab previously found no reproductive benefits of milk-sharing, technically known as allonursing.

Scientists generally assumed that was because infants in species that allonse 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 idea and “brings allonursing back into play concerning positive effects” on reproduction, says Heldstab. She encourages more research that might tease out the specific effects depending on how much milk is actually shared in individual species and cases.

Journal reference: PNASDOI: 10.1073/pnas.2114674119

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