Why can’t men breastfeed? Because they may spread germs, scientists say

There’s somewhere in the ballpark of 1,600 crore human nipples in the world right now. However, only half of these serve a purpose — a pitiable figure, to say the least. If men could breastfeed, not only would that help nourish the offspring — which is a tiring task by itself — but it would probably also take some pressure off women.

However, go to the lush rainforests of Malaysia, and a peculiar phenomenon can be observed: the Dayak fruit bat, a rare case where a male mammal nurses its young with its own milk. This intriguing exception begs the question: why don’t more male mammals, including humans, take part in breastfeeding? A novel theory developed by mathematicians suggests that it might all come down to germs.

Evolution is a real buzzkill

For decades, evolutionary biologists have pondered why male mammals generally don't participate in lactation. In the 1970s, a prevailing theory emerged: the uncertainty of paternity. Since male mammals can't always be sure they are the biological fathers, their evolutionary drive to invest heavily in offspring care, including breastfeeding, is diminished.

However, this explanation doesn't fully satisfy the curiosity surrounding male lactation, especially when considering species like Azara's owl monkeys. These primates exhibit extraordinary paternal care, with males undertaking 80-90% of childcare duties, yet still, only females nurse.

Nipping it in the bud

While biology might be the go-to science to answer such a question, a group of mathematicians decided to have a crack at it. This birthed an entirely new theory: male mammals refrain from breastfeeding to curb the spread of harmful microbes.

Breast milk is more than just nutrition; it’s a living substance teeming with bacteria, viruses, fungi, and their genetic material. This microbial ecosystem is vital for an infant’s health, aiding in digestion, disease protection, and overall development. However, the transmission of these microbes from both parents could double the chances of spreading harmful microbes.

The study’s mathematicians suggest that by restricting lactation to females, mammals effectively use a "microbial sieve," filtering out potentially harmful microbes while retaining beneficial ones. This mechanism prevents the doubling of microbe transmission that would occur if both parents were involved in breastfeeding.

This explanation would also explain the bizarre conundrum of Azara's owl monkeys. Despite their heavy childcare roles, males may not have evolved to nurse their young to reduce the likelihood of transmitting harmful microbes, posing a greater risk to the infant’s developing microbiome.

"While microbes are not inherently harmful or beneficial; it's their presence and abundance that dictate the overall health of this internal community,” explains study author Brennen Fagan. “A 'wrong actor' at the early point of an animal's life could change the microbiome at a pivotal moment."

The findings of this research have been published in Nature Communications and can be accessed here.