Got Seal Milk?

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Mr.other people’s milk is nutritious. Breast milk contains hormones, fats, antibodies and more than 200 sugars that promote the development of the immune system and reduce the risk of infections. These sugars (“oligosaccharides”) also help establish the newborn’s developing gut microbiome, which is crucial for health. This is why doctors consider breastfeeding the optimal approach to feeding infants.

A new study published in Natural communications shows that Atlantic gray seal milk makes breast milk look like weak tea.

When a human baby is born, its main task during the first few weeks is to lie down and grow. On the other hand, gray seal pups’ 17 days of nursing should prepare them to dive directly into the North Atlantic to begin earning a living. These baby seals can triple their birth weight in those 17 days, laying down the blubber they’ll need to navigate the frigid waters in search of food.

Researchers at the University of Gothenburg in Sweden studied the composition of the milk of five wild gray seals (Halichoerus grypus) on the Isle of May off the coast of Scotland. Knowing that the chemical composition of breast milk changes as the pup grows, they collected and stored five samples from each mother seal at regular intervals throughout their 17 days of lactation.

Read more: »When stress comes with your mother’s milk»

Harvesting milk from wild lactating seals was no easy feat; animals weighing more than 500 pounds had to be anesthetized, administered oxytocin to promote milk release, and followed with a preventative antibiotic.

But the time-series characterization of seal milk using mass spectrometry, a method that precisely determines the compounds present in a sample, was unprecedented. “For the first time, we were able to analyze milk sugars from a wild mammal throughout the lactation period,” Daniel Bojar, study co-author and bioinformatician from the University of Gothenburg, said in a statement.

Bojar and his colleagues discovered 332 distinct sugar molecules, called oligosaccharides, in seal milk, compared to about 250 in human breast milk. Two thirds of the seal’s oligosaccharides were previously unknown molecules. While the largest known sugars in breast milk measure 18 units, seal milk contained molecules of 28 units. As in humans, the composition of seal milk changed as the offspring grew, becoming more diverse during the later stages of lactation to rival the complexity of mother’s milk.

During development in lactating humans and seals, sugars bind to immune system proteins to regulate cellular responses to stressors such as pathogens. Seal milk molecules possessed powerful antibacterial properties, a useful trait for a marine animal.

The authors suggest that their work points to possible antipathogenic molecules that could be exploited for human medical purposes, perhaps even for human infant formula.

And the discovery of new sugar molecules in these seals’ milk may be just the tip of the mammalian lactation iceberg. “We’ve done this for 10 different mammals and we find unique sugar molecules each time,” Bojar added. “We’ll continue. We have milk from 20 other mammals in the freezer.”

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Main image: Waitblock / Wikimedia Commons

• Devin Reese

  Published on November 25, 2025

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.