As panda baby grows, mom’s milk changes

When a panda baby is born, it looks more like a naked mole-rat (minus the teeth) than an adult panda. It’s hairless, as small as a stick of butter and lacks the distinctive black-and-white coloring of its species. But in the weeks that follow its birth, the baby grows, gains hair and black spots and soon reaches what I call the “adorable stuffed animal” phase of life.

It turns out, though, that the baby panda isn’t the only thing that is changing during those early weeks. Mom’s milk is, too, a new study finds. And that changing diet may help to prepare baby for the bamboo diet it will eat later in life.

It’s common in kangaroos and other marsupials, which give birth to tiny babies that develop inside a pouch, for a mother’s milk to change dramatically as the young animal grows. In placental mammals, though, which tend to give birth to babies that are more fully developed, such changes are not common. But pandas give birth to such underdeveloped babies — just one one-thousandth the size of mom — that researchers suspected that a panda mom’s milk undergoes changes similar to those seen in marsupials.

Kate Griffiths of the University of Glasgow in Scotland and colleagues analyzed milk collected from six mama pandas at the Chengdu Research Base of Giant Panda Breeding in China. The team started by comparing the panda milk to that of several other species, including dolphins, elephants, grizzly bears, polar bears and an anonymous woman from Scotland. Elephant milk was the oddest of the bunch, with little similarity to that of any other species analyzed. But pandas proved their right to the name “bear” as their milk was most similar to that of grizzlies and polar bears.

But panda milk exhibited many changes  over the first few weeks after birth, the researchers report October 21 in Royal Society Open Science . After a panda is born, the milk it receives is rich in immunoglobulins, proteins that help protect the infant against pathogens and are thought to control the baby’s gut microbiome . That “may be particularly relevant to giant pandas in their progression from a milk-based to a predominantly vegetarian diet despite an anatomically carnivorous digestive system,” the researchers write.

As those immunoglobulin proteins tail off, others take their place, such as caseins, which provide a source of amino acids, as well as proteins that transfer vitamins and fats. The types of sugars changed over the weeks, with mom producing some forms of saccharide polymers that can’t be digested by baby but are thought to prevent bacteria from colonizing the gut.

The concentration of lactose — the milk sugar that causes problems for many adult humans — quickly declined in the milk. It is thought that panda cubs can’t digest lactose after about three weeks of age, so the decline in lactose makes sense. But the finding shows why hand-rearing cubs on cow milk, which has a lot of lactose, is a bad idea, the researchers note.

The team also found one odd chemical in the milk in the first week or two — phenol sulfate. The researchers think that the chemical is a by-product of mom starving herself in the first weeks after her cub is born. Without food entering her digestive tract, gut bacteria may be scavenging nitrogen from their own proteins and producing toxic phenols as a result. Gut cells may be turning that into nontoxic phenol sulfate, which then gets incorporated into mom’s milk. When she starts eating again, the chemical disappears.

Marsupial milk goes through even more dramatic changes than panda milk. But because mammal milk isn’t well studied, the researchers didn’t have a similar time series of milks from other placental mammals to which they could compare the panda milk. That means they can’t say for certain whether these changes are much slower than in other animals. However, the researchers suspect that panda milk changes more slowly, simply given how immature baby pandas are when they arrive and how much more growing they’ve got to do.