Gray seals have milk more complex than human breast milk

Gray seal pups drink milk packed with 332 types of complex sugar molecules. Humans, it turns out, have been overestimating our own biological complexity.

For decades, human breast milk held the title of most intricate mammal milk known to science. But a study published in Nature Communications in November found that Atlantic gray seals produce milk with roughly 33 percent more types of oligosaccharides—those complex sugars—than we do. Around 166 of the 240 sugar structures researchers analyzed had never been identified in any animal's milk before.

The research team, led by biochemist Daniel Bojar at the University of Gothenburg, collected milk samples from five Atlantic gray seal mothers on the Isle of May in Scotland. They tracked the composition across the animals' roughly two-and-a-half-week lactation periods. What they found was striking: seal milk contains around 332 unique oligosaccharides compared to approximately 250 in human milk. Some of these molecules are built from up to 28 sugar units—ten more than the largest oligosaccharides we've found in human milk.

Why does this matter. Those sugar molecules aren't random biological noise. They're specifically designed by evolution to help seal pups develop healthy microbiomes—the communities of bacteria that live in their guts and influence everything from immunity to digestion. The discovery suggests we've been looking in the wrong places for inspiration on how to support infant health.

"The study highlights the untapped biomedical potential hidden in understudied wild species," Bojar said. His team has now analyzed milk from ten previously uncharacterized mammal species, finding unique sugar molecules in every single one. They have samples from another 20 mammals waiting in the freezer.

The practical applications could be significant. Researchers believe these newly discovered oligosaccharides might eventually be synthesized as supplements to boost babies' immune systems or support gut health in adults. Some of the molecules in seal milk appear to protect young seals from harmful bacteria—a function that could translate into human health benefits if we can understand and replicate it.

Sabrina Spicer, a biochemist at Vanderbilt University who wasn't involved in the research, said the findings have "completely upended this idea that human milk was the pinnacle of complexity." That's not a blow to human biology so much as an invitation: if we've been wrong about milk, what else are we missing by only studying humans and domesticated animals.

The next phase involves understanding not just what these sugar molecules are, but what they actually do—work that could reshape how we think about infant nutrition and immune development.