Despite the fact that their digestive system digests plants poorly, Pandas are still quite happy living off shoots and leaves, thanks partially to their lazy life style. Pandas evolved from bears that eat both plants and meat, and started on their bamboo-only diet about 2 million years ago. Although pandas have lost their taste for meat, they have not developed a herbivores-like digestive system, nor have they obtained a gut microbiota compatible with their new diet. So how do they get enough energy out of bamboos? By spending 14 hours a day eating and moving as little as possible!
In fact, pandas are so good at being lazy that they typically use about 38% of the calories used by a similarly sized mammal. It’s not just low physical activity, they also have reduced-sized vital organs, and panda-unique mutations that result in decreased levels of thyroid hormones. So it is ok to be lazy, you just need to be incredibly good at it!
Pandas and gut bacteria: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4442137/
Pandas and low energy expenditure: http://www.ncbi.nlm.nih.gov/pubmed/26160943