Richard Collins: New research on why male mammals cannot supply milk to their young

Having two milk suppliers, rather than one, would seem to make survival sense. Nature seldom misses a trick. Has she made a mistake with this strange embargo?

A vixen suckling her three fox cubs in early morning light on Sherkin Island, Co Cork. Picture: Robbie Murphy/Provision

A man developed breasts, like the two breasts of a woman, and he nursed his son — Talmud (Shabbat 53b) 

The 19th-century explorer Alexander von Humboldt claimed that a man in a Venezuelan village nurtured his son for three months when his wife was ill. In 2002, a Sri Lankan husband, whose wife had died, is said to have breastfed his children.

Eve was prior to Adam; we begin life in the womb as females. If a Y chromosome is present, male features are triggered. Daddies have nipples which don’t normally secrete milk, but will do so in response to certain hormones. Newly born babies, of both sexes, can produce ‘neonatal milk’. 

Youths occasionally deliver ‘witches milk’, thought to be caused by a hormonal surge at the onset of puberty. Recovering concentration camp victims have been known to lactate; their damaged livers fail to break down milk-stimulating hormones. Darwin speculated that both sexes of early mammals lactated.

The prohibition on males supplying milk is almost universal among mammals. Of the 6,500 odd species known to Science, the Dayak fruit bat of Malaysia, and the Bismarck masked flying fox of Papua New Guinea, are the only ones whose males breastfeed their young.

It seems odd that others don’t do so. Irish foxes, for example, provision their partners and families, bringing food to the den. Why don’t they offer milk also? Having two milk suppliers, rather than one, would seem to make survival sense. The feeding burden could be shared during lean times, and the loss of the mother wouldn’t mean that babies starved to death. Nature seldom misses a trick. Has she made a mistake with this strange embargo?

The traditional answer to the question is that uncertainty as to paternity discourages male mammals from breastfeeding their young. Few mammal species are monogamous and, among those that are, cuckolding is common. A male must be sure that the offspring, to which he devotes precious resources, are his own. Instead, by using his energies to mate with additional partners, he is likely to increase the number of his offspring and promulgate his genes more successfully. But is this explanation entirely convincing? Mathematicians at the University of York don’t think so. In a paper just published, they offer a complementary explanation based on a model they have developed.

Digestion requires a gut community of bacteria viruses and fungi. These break down food prior to absorption. Transmission of microbes from mother to baby begins during birth and continues with the milk she supplies. Rogue organisms attempt to invade the new gut biome. 

The mother’s lactation system filters them out, ensuring that only beneficial ones are transmitted to the infant. It is imperative that destructive parasites don’t get a foothold in the biome at the crucial early stage of the baby’s development. If males were allowed to breastfeed, however, this protection would be compromised; the sieve the mother deploys would be unable to filter out microbes coming from him.

The whole human world is formed from the womb of mother — Abhijit Naskar.

• Brennan Fagan et al. Maternal transmission as a microbial symbiont sieve and the absence of lactation in male mammals. Nature Communications. 2024.