The Mammal With the Most Complex Milk Might Not Be Humans, After All. The Atlantic Gray Seal Could Take That Title

A new study found 332 types of complex sugar molecules in the seals’ milk, some of which help protect young from harmful bacteria. The discovery might one day help improve human health by boosting babies’ immune systems

Margherita Bassi - Daily Correspondent

December 3, 2025 10:40 a.m.

A gray seal mother feeding a pup Patrick Pomeroy

Human breast milk has long reigned as the most complex known mammal milk. But another species might dethrone us: gray seals.

In a study published November 25 in the journal Nature Communications, researchers analyzed gray seal milk and found around 33 percent more types of complex sugar molecules than in human breast milk, many of which were previously unknown. The findings could pave the way for new methods to support human health.

Fundamentally, milk is a fancy sort of sweat, Daniel Bojar, a study co-author and biochemist at the University of Gothenburg in Sweden, tells the New York Times’ Kate Golembiewski. “But it’s much more functional than sweat,” he adds. “It’s basically almost like a magical fluid.”

One of milk’s magic ingredients is oligosaccharides. These chains of sugar molecules help protect babies from viruses and harmful bacteria, kickstart the body's microbiome and assist development of the gastrointestinal tract.

“Milk oligosaccharides are crucial for neonatal development and health in mammals,” Bojar and his colleagues write in the paper. “Yet most milk research focuses on humans, or on domesticated mammals that are poor in milk oligosaccharide complexity.”

Fun fact: Deep dive

Gray seals can dive to nearly 1,600 feet and stay underwater for up to an hour.

The team decided to study seals’ milk because the sugars in it mainly help pups develop their microbiomes, Bojar tells Chemical & Engineering News’ Brianna Barbu.

So, the researchers collected milk samples from five Atlantic gray seal moms on the Isle of May, a Scottish island. They gathered specimens at four timepoints throughout the animals’ lactation periods, which each lasted around two and a half weeks.

Molecular analysis of the milk revealed it contained 332 unique oligosaccharides, far more than the approximately 250 in human milk. The researchers investigated the structures of 240 types of sugar produced by seals, and discovered that around 166 had never been found in any animal’s milk. Some of those molecules consist of up to 28 sugar units, which is 10 more than the largest known human milk oligosaccharides.

Additionally, the team found that, like human breast milk, the composition of seal milk changes throughout the lactation period according to the pup’s needs.

An adult gray seal and a pup University of St. Andrews

The new work “has completely upended this idea that human milk was the pinnacle of complexity,” Sabrina Spicer, a biochemist and science communicator at Vanderbilt University who was not involved in the study, tells Chemical & Engineering News. While five seals from a single island is normally considered too small a sample size, the researchers were able to extract an impressive amount of information, she notes.

Bojar and his colleagues say that their study could lead to finding chemical compounds that can provide nutrients to infants, control infections and support the immune system, per a University of Gothenburg statement. For example, the newly discovered oligosaccharides in seal milk could one day be used as supplements to aid babies’ immune systems or to support adults’ gut health.

“The study highlights the untapped biomedical potential hidden in understudied wild species,” Bojar says in the statement, noting that his research group has now carried out in-depth milk analyses on ten previously uncharacterized mammal species.

“We find unique sugar molecules every time. We will continue. We have milk from another 20 mammals in the freezer,” he says.

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