If there's one thing giant pandas can't live without, it's bamboo. These burly bears munch through anywhere from 20 to 40 pounds of it a day, with some scarfing down 80 pounds in that window. And in the wild, giant pandas spend at least half of every day looking for bamboo. In fact, it's so important to pandas, it may even influence the iconic markings on their coat.

So what happens when you have two giant pandas and run out of bamboo in the middle of a pandemic?

Unfortunately, for one Canadian zoo, it means a sad return to sender. The Calgary Zoo has announced that the giant pandas in its care — Er Shun and Da Mao — are heading back to China.

"We believe the best and safest place for Er Shun and Da Mao to be during these challenging and unprecedented times is where bamboo is abundant and easy to access," zoo president and CEO Clément Lanthier explained in a press release this week. "This was an incredibly difficult decision to make but the health and well-being of the animals we love and care for always comes first."

The bears had been on loan to Canadian zoos since 2013, as part of a 10-year pact. But with much of the world locked down and flights canceled amid the COVID-19 outbreak, bamboo has become increasingly hard to come by in Canada.

Up until now, the zoo has been getting it shipped fresh directly from China, an option that's no longer available during these viral times.

"The Calgary Zoo team has worked tirelessly with alternate bamboo suppliers to find a way to keep the giant pandas fed, despite misdirected shipments, slower than acceptable delivery times causing some poor quality bamboo that the giant pandas won't eat, and concern with limited supplies," the zoo noted in the release.

How to feed a panda

Strangely enough, bamboo doesn't pack much in the way of nutrients. High in indigestible fiber and low in protein. To complicate things, unlike most herbivores, pandas don't have a long digestive tract to break down plants. Nor do they have the four-chambered stomach of a cow. Instead, pandas are built like meat-eaters — just a simple stomach and a short digestive tract.

So how do they thrive on a strictly bamboo diet?

Bei Bei will depart the Zoo for China on November 19,2019 as part of the zoo's cooperative breeding agreement Giant panda Bei Bei eats bamboo at Smithsonian's National Zoo. (Photo: JIM WATSON/AFP/Getty Images)

Research suggests it's all about the three most basic nutritional building blocks for mammals: nitrogen, phosphorus, and calcium. In China, where giant pandas live — and where they're currently considered a vulnerable species — different kinds of bamboo provide different nutrients. Pandas get their nitrogen and phosphorus by consuming young wood bamboo shoots. When they're low on calcium, they forage for arrow bamboo shoots, which are rich with the essential mineral. This explains why pandas spend so much of their lives seeking out bamboo. It also explains why zoos have a hard time keeping up with their dietary needs.

And that's before you factor in a global pandemic.

"Er Shun and Da Mao will be deeply missed by staff, volunteers, donors and visitors from around the world," the Calgary Zoo noted in the release. "Knowing a second wave of COVID-19 is likely, and the bamboo supply chain challenges will continue to negatively impact the zoo's ability to bring bamboo to the giant pandas, the Calgary Zoo feels it's critical to move the beloved giant pandas back to China, where there are abundant local sources of bamboo, as soon as possible."

The zoo also noted that people won't be able to personally bid the bears a fond farewell. The facilities are temporarily closed. And, of course, pandemic or not, a giant panda naturally requires a little social distancing.

You can, however, relive those happier days when Er Shun gave birth to a pair of bouncing baby cubs during her time at the Toronto Zoo. Watch those cubs frolicking with mom in the video below:

Pandemic forces Canada to send giant pandas back to China
The Calgary Zoo can't get enough fresh bamboo for Er Shun and Da Mao, so the giant pandas are heading back to China.