Grey seals produce milk with hundreds of never-seen sugars

Milk carries stories about growth, survival, and adaptation. Some of these stories stay hidden until someone decides to look closely. Grey seals offer one such case. Their pups face bitter seas from the moment they leave the shore.

The mothers respond with milk packed with energy and unusual sugars. New research now shows how remarkable this milk actually is, and why it deserves attention far beyond marine biology.

Scientists once assumed that human milk stood alone in complexity. Grey seals challenge that idea in a striking way. Their sugars are not only numerous but also chemically inventive, forming patterns rarely seen in mammals.

Complexity of seal milk

Grey seal pups drink for only seventeen days. Every hour counts. The mother must deliver protection, fuel, and support for the pup’s immune system. This pressure produces a milk that surprised researchers once they measured it.

“Our analysis shows that grey seal milk is extraordinary. We identified 332 different sugar molecules, or sugars, compared to about 250 in breast milk. Two-thirds were completely unknown previously,” said Daniel Bojar, senior lecturer in bioinformatics at the University of Gothenburg.

“Some of these molecules had a previously unseen size of 28 sugar units, which exceeds the largest known sugar units in breast milk, which are 18 units in size.”

Some sugars carry long repeating chains. Others include sulfate groups that shift how immune cells respond. Several show rare linkages that scientists have not recorded in other species.

Together, these features create a landscape of sugars that looks closer to human milk than to most domestic mammals.

Shifting milk sugar patterns

The research team studied the same seals across the whole nursing period. That allowed them to watch the milk change in real time.

Early milk looks different from late milk. The first days bring sugars linked with fast defense. Later days show an increase in larger and more decorated molecules.

“For the first time, we have been able to analyse the milk sugars in the milk of a wild mammal throughout the entire lactation period. This is important because the composition of the milk changes as the calf gets older,” said Bojar.

These shifts happen quickly because the nursing window is so short. Yet the pattern resembles human milk, which also evolves over time.

Grey seals compress that process into just over two weeks. The result is a coordinated system that meets each stage of pup growth with a new set of tools.

Complex milk to protect young seals

Some of these sugars bind to immune proteins. Others latch onto viral or bacterial surfaces. Each interaction nudges the immune system in a specific direction.

Scientists tested several newly identified sugars on human immune cells and saw clear changes in the way those cells behaved. The responses shifted depending on the type of activation signal the cells received.

“The sugars bind to various proteins found in, for example, the immune system. We have tested the newly discovered sugar molecules on human immune cells and found that they can regulate the cells’ response to various threats,” said Bojar.

“The findings suggest that wild marine mammals, which are exposed to extreme environmental stresses and high exposure to external dangers, have developed a complex milk to protect their rapidly growing young.”

In some cases, the sugars increased calming signals. In others, they reduced inflammatory markers.

A tiny structural change between two similar sugars created completely different responses. That level of precision suggests strong evolutionary pressure for fine control.

Seal milk fights harmful bacteria

Grey seal milk sugars also showed power against harmful bacteria. The team tested them on microbes known to form stubborn biofilms. These biofilms help bacteria resist treatment.

The sugars interfered with this process without slowing bacterial growth. That distinction matters, since it lowers the chance of resistance developing.

Some sugars blocked biofilms in staphylococci. Others blocked them in Klebsiella. These results point to natural molecules with real therapeutic promise.

Subtle differences in structure shaped their performance, revealing an entire vocabulary of biological signals hidden in marine milk.

Secrets of wild species

The study challenges older assumptions about non human mammals. Sugar diversity in grey seals matches and sometimes surpasses what appears in human milk.

Many of the newly identified structures appear nowhere else in scientific literature. That level of novelty suggests that wild species hold many more secrets.

“The study highlights the untapped biomedical potential hidden in understudied wild species. Our research group is the only one in the world to analyze milk sugars from these uncharacterized mammals using mass spectrometry in that depth,” said Bojar.

“We have done this for ten different mammals, and we find unique sugar molecules every time. We will continue. We have milk from another 20 mammals in the freezer.”

The broader message is simple. Nature is not short on ideas. The sugars in grey seal milk offer shapes, patterns, and functions that can guide safer infant formulas, immune support strategies, and approaches to infection control.

Their complexity comes from survival pressure, not from design. That makes them especially interesting for fields that need solutions rooted in biology rather than theory.

Future studies of wild milk

This expanded look at grey seal milk changes how we view early nutrition. It also shows that hidden chemical diversity exists in places we rarely study.

Each sugar carries small instructions. Some protect. Some signal. Some shape the gut environment. Together they form a system shaped by millions of years in cold, demanding waters.

As scientists continue to study wild milks, new structures will appear. Many will bring new insights. Some may inspire medical applications.

Grey seal milk stands as an early example of what waits beneath the surface when researchers decide to look closely.

The study is published in the journal Nature Communications.

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