Have you ever wondered why male mammals don’t produce milk for their young? Well, a new mathematical model may explain it, and it has a lot to do with microbes.
The absence of male lactation in mammals has perplexed scientists for years. There is currently no universally satisfying reason why males should not produce milk. For one thing, genetically male mammals have both the means for producing milk, mammary tissues, and the potential to lactate. But they generally lack sufficient levels of the hormonal triggers that make it happen – hormones like prolactin, for instance, are actually downregulated in males.
During the 1970s, evolutionary theorists suggested that the near absence of lactating males could be explained by paternal uncertainty. Basically, males can’t be sure they are the biological father of their offspring, so there is less of an evolutionary drive to invest in their care, especially through breastfeeding.
However, mathematicians from the University of York believe there may be a different reason why males don’t breastfeed their young. According to a new mathematical model, the absence of lactating males may be driven by the microbes that live in breast milk. When a parent breastfeeds their child, they are doing more than just providing food. The act also transfers the parent’s microorganisms – including bacteria, fungi, and viruses – that are present in their body. This is vital for establishing the infant’s gut microbiome.
However, problems start to appear if both parents transmit their microbiomes through this process. If both fed their infants milk, then there is a greater chance of harmful microbes spreading through mammalian populations. Maternal-only lactation stops this from happening, as it serves as a kind of “sieve”, preventing the infants from receiving too many nasty microbes.
“This ecosystem plays a crucial role in health including by helping to protect animals against disease, helping to digest food and in many other ways we are only just discovering”, Brennen Fagan working at the Leverhulme Centre for Anthropocene Biodiversity and the Mathematics Department at the University of York, explained in a statement.
“While microbes are not inherently harmful or beneficial; it’s their presence and abundance that dictate the overall health of this internal community. A ‘wrong actor’ at the early point of an animal’s life could change the microbiome at a pivotal moment.”
Fagan and colleagues became fascinated with this topic after they learned about Azara’s owl monkeys.
“They turn previous assumptions about why males don’t breastfeed upside down because they are the most devoted dads in the primate world: They do 80–90 percent of childcare and only hand their babies back to their female partners for nursing”, George Constable, from the Department of Mathematics at the University of Work, added.
“When both parents are involved in feeding, the chance of a microbe being passed along and getting an initial foothold in a population is essentially doubled. So our theory suggests selection against the transmission of harmful microbes through mammary milk could be an additional selection pressure against male lactation.”
The mathematical model reveals the benefits of getting breastfed by one parent, but it also demonstrates why this would make sense from an evolutionary perspective. That’s because the infant has already received microbes from its mother during birth, and maybe even while in the womb.
“This theory fits with a pattern of strategies mammals have adopted in an evolutionary bid to limit the spread of potentially harmful elements. Notably, in humans mitochondrial DNA is exclusively passed down from the mother,” Constable explained.
“This mechanism serves as a natural filter, maintaining genetic integrity by suppressing the proliferation of detrimental mutations. Additionally, the prevalence of monogamous relationships among certain species has been suggested as an adaptive response aimed at minimizing the transmission of sexually transmitted infections (STIs).”
Although the research is useful for understanding the evolution of breastfeeding in mammals, the team stresses that it should not be seen as the basis for any societal judgment about the act in humans.
As Fagan noted, “Our model is very much focused on the long-term evolution of the animal kingdom. The model does not tell us about individual families making individual choices on how to safely feed their children, especially not for humans in the modern world.”
“Our hypothesis fills a gap in evolutionary theory and is concerned with selection pressures on mammals at population level and over very long periods of time spanning multiple generations.”
The paper is published in the journal Nature Communications.
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