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.

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