Scientists have been puzzled for years about why male mammals have nipples and everything they need to feed their young with milk, but do not lactate. Even despite the presence of mammary glands, it is not produced. The reason is the absence of hormonal triggers that trigger lactation. But what does this have to do with? Various versions have been put forward regarding this matter. For example, back in the 70s, evolutionary scientists suggested that males lack confidence in biological paternity. Associated with this is a weaker evolutionary drive to care for offspring, at least through breastfeeding. However, staff at the University of York offered a different version.
Scientists have explained why in mammals, females feed their young, not males. Photo source: iflscience.com
Why do males lack milk
As a rule, the mother takes care of the offspring, or most of it. Therefore, it is logical that she is also responsible for breastfeeding. However, there are mammals in which fathers take care of their offspring. For example, this applies to owl monkeys. They can be called one of the most devoted fathers among primates, as they take on 80-90% of childcare. However, despite this, males do not feed their offspring with their own milk; they pass the cubs on to their partner for this purpose.
This refutes the version that the lack of lactation in males is associated with less care for the offspring. Having learned about these monkeys, staff at the University of York suggested that there was another evolutionary reason, and it lies on the surface.
Male owl monkeys care for their offspring more than females, but cannot feed their young with milk. Photo source: zoogalaktika.ru
Most likely, it's all about bacteria. When a mother breastfeeds her baby, she not only provides him with nutrition, but also transmits microorganisms that are present in her body, that is, bacteria, fungi and viruses. This is very important for the child, since the child’s intestinal microbiome begins to form.
But what will happen if two parents start transmitting their bacteria to the child as a result of breastfeeding? According to scientists, in this case the risk of the spread of harmful microbes in populations doubles. Thus, maternal lactation prevents the spread of harmful bacteria, or at least reduces this risk. Scientists write about this in their article published in the journal Nature Communications.
Breastfeeding offspring exclusively by females helps prevent the spread of harmful bacteria throughout the population. Photo source: zoo-galereya.ru
How intestinal bacteria affected the lactation of males
The baby begins to receive microbes from his mother at the time of birth, and maybe even earlier. Therefore, when a baby is nursed by its mother, it does not receive much more new bacteria than it already has. If the child is fed by the father, he will receive a completely new «portion» bacteria. A healthy microbiome is important for population health.
Microflora protects animals from disease by helping to digest food, and generally affects health and even the rate of aging. Thus, the absence of male lactation makes sense from an evolutionary perspective. According to the researchers, this theory fits with known strategies that mammals have evolved to prevent the spread of potentially harmful elements.
For example, in humans, mitochondrial DNA is passed on only from the mother. This mechanism is similar to a “filter”, but at the gene level, which prevents the spread of harmful mutations. Moreover, the prevalence of monogamous relationships among many species is also one of the strategies aimed at minimizing the spread of sexually transmitted infections. However, according to some scientists, monogamy is also a kind of protection of the species from incestuous relationships, which we discussed earlier.
Male carnivorous bats have not lost the ability to lactate. Photo source: animals.pibig.info
Some males can feed their offspring with milk
Considering that male mammals have everything necessary to feed their young, it can be assumed that the ancient ancestors of modern animals, in including humans, could feed their offspring with milk, regardless of gender. However, during the course of evolution, males have lost this ability, but not all of them.
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For some reason, male carnivorous bats continue to feed their offspring with milk. Moreover, the formation and secretion of milk from the mammary glands of males has been repeatedly recorded in many domestic animals – cats, goats, guinea pigs. True, unlike bats, their location is not regular.