The World Health Organization (WHO) recommends feeding breast milk along with complementary foods until age 2. The Atlantic gray seal (scientific name Halichoerus grypus) nurses its pups for only 17 days. Instead of a long nursing period, its milk is packed with more nutrients.
A team led by Prof. Daniel Bojar of the Department of Chemistry and Molecular Biology at the University of Gothenburg in Sweden said in Nature Communications on the 25th (local time) that "the milk of Atlantic gray seals has about 33% more types of sugar molecules than human breast milk." The team said studying the sugars in seal milk could help develop better infant formula.
◇ Boosting pathogen-fighting capacity with large sugar molecules
Mammalian milk contains various sugars along with fat and protein. These sugars play different roles. For example, lactose, which is relatively abundant in humans and cows, provides energy. In particular, oligosaccharides composed of 3 to 10 basic monosaccharides block disease-causing viruses and bacteria, and form the initial colonies of gut microbes to promote the development of the stomach and intestines.
Gray seal pups must activate their digestive systems by nursing for 17 days and build an immune system that can protect them from diseases and risk factors they may encounter in the North Atlantic. Scientists suspected that to accomplish such tasks in a short time, the mother's milk would be extremely finely tuned. The Swedish team proved this hypothesis true.
Bojar said, "Human breast milk contains up to 250 types of oligosaccharides, but gray seal milk was found to have as many as 332," adding, "Two-thirds of these were completely new substances." The team said some of the seal's sugar molecules had an unprecedented size composed of 28 monosaccharides, far exceeding the largest sugar unit known in breast milk (18).
Although sugars in animal milk play roles that important, they have not been properly studied except in humans. That is because it is difficult from the outset to collect milk from mothers intent on protecting their young. Bojar said, "For the first time, we were able to analyze the oligosaccharides secreted in the milk of a wild mammal over the nursing period."
◇ Oligosaccharides also change to match rapid growth
Patrick Pomeroy of the Sea Mammal Research Unit at the University of St. Andrews in the United Kingdom sedated five wild gray seals living off the Scottish coast and collected milk several times during the nursing period. After training artificial intelligence (AI) with mass spectrometry results for 240 newly identified oligosaccharides, the composition of seal milk was found to change throughout the nursing period, much like human breast milk.
Bojar said, "When we tested sugar molecules newly discovered in seals on human immune cells, we confirmed that they can modulate cellular responses to various threats," adding, "This suggests that wild marine mammals exposed to extreme environmental stress and high external risks have evolved more complex milk than humans to protect their rapidly growing young." The team also confirmed that several seal oligosaccharides have strong antimicrobial capacity against pathogenic bacteria.
Russ Hovey, a professor in the Department of Animal Science at the University of California, Davis (UC Davis), told the New York Times that day, "This discovery has broad potential applications," adding, "Milk in mammals is far more complex, valuable, precious and diverse than people think."
The team said future work could discover new bioactive compounds in mammalian milk that help infant nutrition, infection management and immune system strengthening. That could open the door to developing formula that functions better for babies than breast milk. Adults could also use it to maintain gastrointestinal health.
Bojar said, "Our team was the first in the world to use mass spectrometry to conduct an in-depth analysis of new oligosaccharides in mammals," adding, "We have analyzed 10 mammalian species so far and discovered unique sugar molecules every time." The team is already collecting and analyzing the milk of 20 more mammalian species.
◇ Studying mammalian evolution through oligosaccharide analysis
Analyzing oligosaccharides also aids evolutionary research. Mammals, which include humans, secrete milk from mammary glands to raise their young. This is the most distinguishing feature that sets them apart from other kinds of animals. The reason whales are mammals and not fish also lies in milk.
Mammals first appeared about 200 million years ago from oviparous reptiles that laid eggs. Over time, most mammals shifted from laying eggs to viviparity, giving birth to young that have grown to some extent in the mother's womb, and raised them by nursing. Then what came first, nursing or viviparity?
A team at the University of Lausanne in Switzerland reported in 2008 in the journal PLOS Biology that genetic changes enabling nursing drove the evolution of mammals from reptiles. Some mammals are oviparous. Platypuses and echidnas are.
The researchers analyzed lactation- and oviparity-related genes in humans, opossums, dogs and platypuses. The analysis found that the genes that produce proteins in milk originated in the common ancestor of mammals that lived between 200 million and 310 million years ago.
Meanwhile, the gene that produces a protein called vitellogenin, essential for developing in eggs, was present in all four mammals studied, but it was nonfunctional due to mutations except in the platypus. This gene last lost function between 70 million and 30 million years ago.
This means mammals began nursing before the placenta evolved. Mammals started nursing before they stopped laying eggs, and as there was no longer a need to supply nutrients to young via eggs, they ceased laying them. In mammals, nursing came before eggs.
References
Nature Communications (2025), DOI: https://doi.org/10.1038/s41467-025-66075-2
PLOS Biology (2008), DOI: https://doi.org/10.1371/journal.pbio.0060063