Collecting milk from a nursing seal is no easy task.
“If you can imagine 180 kilos of rather unhappy toothed and clawed beast protecting its single offspring that year, you get a vague idea of what you’re dealing with,” said Patrick Pomeroy, a biologist at the Sea Mammal Research Unit at the University of St. Andrews.
But with help from sedation and a vacuum pump, Dr. Pomeroy and his colleagues were able to acquire samples without too much fuss. They took about three tablespoons of milk from five seal mothers every three or four days, leaving plenty left over for baby seal.
With Dr. Pomeroy’s samples, other scientists have now worked out a surprising discovery: Seal milk is more chemically complex than the milk produced by humans, which was long thought to hold the creamy molecular crown among mammalian mother’s milks.
Milk, at its most basic level, is a very fancy kind of sweat, “but it’s much more functional than sweat,” said Daniel Bojar, a senior lecturer at the University of Gothenburg in Sweden and the study’s lead author. “It’s basically almost like a magical fluid,” providing energy and nutrition to a baby mammal while also laying the foundation for a healthy life.
Milk contains fat, protein and a variety of sugars. These different sugars have different jobs. For instance, the lactose that’s relatively abundant in human and cow milk provides energy to fuel the drinker’s metabolism. Longer chains of sugars, called oligosaccharides, are responsible for establishing the young mammal’s microbiome and for helping to ward off viruses and disease-causing bacteria.
Most of what scientists know about milk comes from humans and domesticated animals, but very little is known about the milk of most wild animals.
Dr. Bojar wanted to analyze the oligosaccharides present in seal milk, but he is a biochemist, not a professional seal milker. So, he enlisted the help of Dr. Pomeroy’s team.
They work with an Atlantic gray seal colony on a small island off the eastern coast of Scotland. Every fall, the mother seals come ashore to give birth to their pups. They stay there, without returning to the water to hunt, for about 18 days, until their pups are weaned.
“Mums come ashore pretty hefty, and they leave pretty lean,” Dr. Pomeroy said. Conversely, “the pup just balloons from being a tiny, little, scrawny bag of bones to being basically a spherical lump, well-padded-out and very fluffy.”
in 2013, Dr. Pomeroy and his colleagues were investigating how seal milk changes over the course of the seals’ 18-day lactation period. Leftover samples from that study were waiting in a freezer at the University of Glasgow when Dr. Bojar wanted to use them in 2022.
Liter of seal milk in hand, he set about characterizing the different oligosaccharide sugars present. “What we really want to know is how they are built up, the exact sugar tree that the molecule is composed of,” Dr. Bojar said. With a mass spectrometer, he and other scientists carried out tiny molecular collisions, and then based on the way the molecules broke apart, they ascertained how those larger structures must have been put together.
The researchers found that seal milk contains 332 different kinds of oligosaccharides — the most ever found in any kind of milk. Human milk contains, at most, roughly 250 oligosaccharides, and on average, around 100. The chemical structures of these seal oligosaccharides were also often more complex than those in human milk.
The complexity of seal milk sugars doesn’t mean that it’s better than any other kind of milk, merely that it has evolved to sustain the particular lifestyle of a seal. But because human milk has long been heralded by scientists as the peak of milk complexity, Dr. Bojar said it was “very satisfying to beat this allegation.”
Russ Hovey, a professor of animal science at the University of California, Davis, who was not involved with the paper, said these findings could have far-reaching applications.
The immune and microbiome benefits conferred by the sugars in seal milk might one day be used in human medicine. “From the standpoint of species conservation, we do have to appreciate and understand exactly how these species use lactation to survive in the first place and nourish their young,” Dr. Hovey said. “Milk is a lot more complicated and valuable and precious and diverse than a lot of people give it credit for.”
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