New mathematical model explains lack of breastfeeding in male mammals

The Malaysian rainforest is home to the only known wild male mammal that produces milk, making the Dayak fruit bat extremely rare, even though most male mammals still have the ability to lactate.

In the 1970s, evolutionary theorists hypothesized that paternity uncertainty may explain why males rarely lactate, even though their offspring may benefit from the additional nutrition: Because male mammals cannot be sure whether they are the biological father, they have less evolutionary incentive to invest heavily in caring for their offspring, including breastfeeding.

Mathematicians at the University of York have proposed a complementary perspective. Nature Communicationssuggest that the reason male mammals don't breastfeed may be due to the rich population of microbes that live in breast milk and play an important role in shaping the infant's gut microbiota.

This theory demonstrates that harmful microorganisms can spread through mammalian populations through the transmission of milk microbiota from both parents. Exclusive maternal lactation halts this phenomenon by restricting the transmission of milk microbiota to the female, effectively acting as a sieve, leaving only those microorganisms with beneficial effects.

One of the study authors, Dr George Constable, from the School of Mathematics at the University of York, said: “When we read about Azara's owl monkeys we were fascinated by the subject as it overturns previous hypotheses about why males don't breastfeed, because they are the most dedicated fathers in the primate kingdom. Males take on 80-90% of the childcare burden and only return the babies to their female partners for nursing.”

“When both parents are involved in lactation, the chances of a microbe being passed on and gaining an initial foothold in the population are effectively doubled. Our theory therefore suggests that selection against the transmission of harmful microbes via breast milk could be an additional selective pressure for lactation in males.”

First author of the study, Dr Brennan Fagan, from the Leverholm Anthropocene Biodiversity Centre and School of Mathematics at the University of York, added: “Breast milk is a living substance that plays a key role in shaping the mammalian gut microbiome – a complex ecosystem of bacteria, viruses, fungi and their genetic material. This ecosystem plays a vital role in health by protecting animals from disease, helping them digest food, and in many other ways that we are only just beginning to discover.”

“Microbes are neither inherently harmful nor beneficial, but their presence and abundance determine the overall health of this internal community. 'Wrong actors' early in an animal's life can alter the microbiome at critical moments.”

Although the mathematical model highlights the benefits of being nourished by only one parent, the researchers say it makes evolutionary sense for the mother to provide nutrition, because microbial transmission is inevitable during birth, and possibly even in the womb.

Dr Constable added: “This theory fits with a pattern of strategies that mammals have adopted throughout evolution to limit the spread of potentially harmful elements. In particular, human mitochondrial DNA is inherited exclusively from the mother. This mechanism acts as a natural filter, maintaining genetic integrity by suppressing the proliferation of harmful mutations. Furthermore, the widespread practice of monogamous relationships among certain species has been suggested to be an adaptive response aimed at minimizing the transmission of sexually transmitted infections (STIs).”

The researchers caution that their hypothesis is not intended to serve as a basis for judgment about different methods of feeding human infants.

Dr Fagan added: “Our model is very much focused on the long-term evolution of the animal kingdom. It doesn't tell us anything about the individual choices that individual families make about how to safely feed their young, and especially not for humans in the modern world.”

“Our hypothesis fills a gap in evolutionary theory and concerns selective pressures on mammals at the population level and over very long periods of time across multiple generations.”